Understanding Acoustic Emission Testing- Reading I

Understanding Acoustic Emission Testing, AET- Reading 1

My Pre-exam ASNT Self Study Notes 3rd September 2015

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E&P Applications

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Concrete Offshore structure

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Wind Energy Offshore structure

Refinery Applications

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E&P Applications

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Refinery Applications

http://wins-ndt.com/oil-chem/spherical-tanks/ Charlie Chong/ Fion Zhang

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http://www.smt.sandvik.com/en/search/?q=stress+corrosion+cracking Charlie Chong/ Fion Zhang

The Magical Book of Neutron Radiography

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ASNT Certification Guide NDT Level III / PdM Level III AE - Acoustic Emission Testing Length: 4 hours Questions: 135 1 Principles and Theory • Characteristics of acoustic emission testing • Materials and deformation • Sources of acoustic emission • Wave propagation • Attenuation • Kaiser and Felicity effects, and Felicity ratio • Terminology (refer to acoustic emission glossary, ASTM 1316)

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2 Equipment and Materials

• Signal conditioning

• Transducing processes

• Signal detection

• Sensors

• Signal processing

• Sensor attachments

• Source location

• Sensor utilization

• Advanced signal processing

• Simulated acoustic emission sources

• Acoustic emission test systems

• Cables

• Accessory materials • Factors affecting test equipment selection

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3 Techniques

4 Interpretation and Evaluation

• Equipment calibration and set up for

• Data interpretation

test

• Data evaluation

• Establishing loading procedures

• Reports

• Precautions against noise

5 Procedures

• Special test procedures

6 Safety and Health

• Data displays

7 Applications • Laboratory studies (materialcharacterization) • Structural applications

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Reference Catalog Number NDT Handbook, Second Edition: Volume 5, Acoustic Emission Testing 130 Acoustic Emission: Techniques and Applications 752

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Fion Zhang at Shanghai 3rd September 2015

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http://meilishouxihu.blog.163.com/

Greek Alphabet

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http://greekhouseoffonts.com/

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Video on - Leak Detection on Buried Water Piping using Acoustic Emission

â– https://www.youtube.com/watch?v=9kq6JxIJDik

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Contents: AE Codes and Standards ■ ASTM ■ ASME V 1. Reading 01- www.geocities.ws/raobpc/AET.html 2. Reading 02- Sidney Mindess University of British Columbia Chapter 16: Acoustic Emission Methods 3. Reading 03- AET ndt-ed.org 4. Reading 04- Terms & Definitions ASTM E1316 5. Reading 05- Q&A 25 items 6. Reading 06- High Strength Steel- TWIP Steel 7. Reading 07- AET- optimum solution for leakage detection of water pipeline 8. Others reading.

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ASME V Article Numbers: Gen RT Nil UT UT PT MT ET Visual LT AE AE AE Qualif.

ACFM

Article 1 Article 2 Article 3 Article 4 for welds Article 5 for materials Article 6 Article 7 Article 8 Article 9 Article 10 Article 11 (FRP) Article 12 (Metallic) Article 13 (Continuous) Article 14 Article 15

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ASTM Standards E569 - 07 Standard Practice for Acoustic Emission Monitoring of Structures During Controlled Stimulation E650 – 97 (2007) Standard Guide for Mounting Piezoelectric Acoustic Emission Sensors E749 - 07 Standard Practice for Acoustic Emission Monitoring During Continuous Welding E750 - 04 Standard Practice for Characterizing Acoustic Emission Instrumentation E751 - 07 Standard Practice for Acoustic Emission Monitoring During Resistance Spot-Welding

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ASTM Standards E976 - 05 Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response E1067 - 07 Standard Practice for Acoustic Emission Examination of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels E1106 - 07 Standard Test Method for Primary Calibration of Acoustic Emission Sensors E1118 - 05 Standard Practice for Acoustic Emission Examination of Reinforced Thermosetting Resin Pipe (RTRP) E1139 - 07 Standard Practice for Continuous Monitoring of Acoustic Emission from Metal Pressure Boundaries

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ASTM Standards E1211 - 07 Standard Practice for Leak Detection and Location Using SurfaceMounted Acoustic Emission Sensors E1419 - 09 Standard Practice for Examination of Seamless, Gas-Filled, Pressure Vessels Using Acoustic Emission E1495 - 02 (2007) Standard Guide for Acousto-Ultrasonic Assessment of Composites, Laminates, and Bonded Joints E1736 - 05 Standard Practice for Acousto-Ultrasonic Assessment of FilamentWound Pressure Vessels

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ASTM Standards E1781 - 08 Standard Practice for Secondary Calibration of Acoustic Emission Sensors E1888 /E1888M – 07 Standard Practice for Acoustic Emission Examination of Pressurized Containers Made of Fiberglass Reinforced Plastic with Balsa Wood Cores E1930 – 07 Standard Practice for Examination of Liquid-Filled Atmospheric and Low-Pressure Metal Storage Tanks Using Acoustic Emission E1932 - 07 Standard Guide for Acoustic Emission Examination of Small Parts E2075 – 05 Standard Practice for Verifying the Consistency of AE-Sensor Response Using an Acrylic Rod

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ASTM Standards E2076 - 05 Standard Test Method for Examination of Fiberglass Reinforced Plastic Fan Blades Using Acoustic Emission E2191 - 08 Standard Practice for Examination of Gas-Filled Filament-Wound Composite Pressure Vessels Using Acoustic Emission E2374 - 04 Standard Guide for Acoustic Emission System Performance Verification E2478 - 06a Standard Practice for Determining Damage-Based Design Stress for Fiberglass Reinforced Plastic (FRP) Materials Using Acoustic Emission E2598 - 07 Standard Practice for Acoustic Emission Examination of Cast Iron Yankee and Steam Heated Paper Dryers

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Typical AET Signal

https://dspace.lib.cranfield.ac.uk/bitstream/1826/2196/1/Acoustic%20Emission%20Waveform%20Changes%202006.pdf Charlie Chong/ Fion Zhang

Typical AET Signal

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Study Note 1: AET http://www.geocities.ws/raobpc/AET.html

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http://www.geocities.ws/raobpc/AET.html

What is AE Acoustic emission is the technical term for the noise emitted by materials and structures when they are subjected to stress. Types of stresses can be (1) mechanical, (2) thermal or (3) chemical. This emission is caused by the rapid release of energy within a material due to events such as crack initiation and growth, crack opening and closure, dislocation movement, twinning, and phase transformation in monolithic materials and fiber breakage and fibermatrix debonding in composites.

The subsequent extension occurring under an applied stress generates transient elastic waves which propagate through the solid to the surface where they can be detected by one or more sensors. The sensor is a transducer that converts the mechanical wave into an electrical signal (piezoelectric) . In this way information about the existence and location (triangulation by multi-transducers) of possible sources is obtained. Acoustic emission may be described as the "sound" emanating from regions of localized deformation within a material.

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http://www.geocities.ws/raobpc/AET.html

Until about 1973, acoustic emission technology was primarily employed in the non-destructive testing of such structures as pipelines, heat exchangers, storage tanks, pressure vessels, and coolant circuits of nuclear reactor plants.

However, this technique was soon applied to the detection of defects in rotating equipment bearings.

Applications: Static subjects Dynamic subjects

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http://www.geocities.ws/raobpc/AET.html

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Acoustic Emission Acoustic Emission (AE) refers to generation of transient elastic waves 瞬间弹 性波 during rapid release of energy from localized sources within a material.

The source of these emissions in metals is closely associated with the dislocation movement accompanying plastic deformation and with the initiation and extension of cracks in a structure under stress. 应力作用下, 结 构中的裂纹萌生/扩展(塑性变形)造成的位错运动.这位错运动会引发瞬间的弹 性波.

Other sources of AE are: melting, phase transformation, thermal stresses, cool down cracking and stress build up, twinning, fiber breakage and fibermatrix debonding in composites. 其他会引起瞬间的弹性波 的因素: 熔化,相变,热应力冷却裂纹和应力建立,孪晶,在复合材料中的纤维断裂和纤 维-基体界面脱粘

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http://www.geocities.ws/raobpc/AET.html

AE Technique The AE technique (AET) is based on the detection and conversion of high frequency elastic waves emanating from the source to electrical signals. This is accomplished by directly coupling piezoelectric transducers on the surface of the structure under test and loading the structure. The output of the piezoelectric sensors (during stimulus) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by suitable electronics. AET can non-destructively predict early failure of structures. Further, a whole structure can be monitored from a few locations and while the structure is in operation. AET is widely used in industries for detection of faults or leakage in pressure vessels, tanks, and piping systems and also for on-line monitoring welding and corrosion.

The difference between AET and other non-destructive testing (NDT) techniques is that AET detects activities inside materials, while other techniques attempt to examine the internal structures of materials by sending and receiving some form of energy.

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http://www.geocities.ws/raobpc/AET.html

Types of AET Acoustic emissions are broadly classified into two major types namely;  continuous type (associated with lattice dislocation)  burst type. (twinning, micro yielding, development of crack) The waveform of continuous type AE signal is similar to Gaussian random noise, but the amplitude varies with acoustic emission activity. In metals and alloys, this form of emission is considered to be associated with the motion of dislocations. Burst type emissions are short duration pulses and are associated with discrete release of high amplitude strain energy. In metals, the burst type emissions are generated by twinning, micro yielding, development of cracks.  Continuos type (Gaussian random noise) → Motion of dislocation,  Burst type (discrete high amplitude strain energy) → twinning, micro yielding, development of cracks

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http://www.geocities.ws/raobpc/AET.html

What is Normal (Gaussian) distribution In probability theory, the normal (or Gaussian) distribution is a very common continuous probability distribution. Normal distributions are important in statistics and are often used in the natural and social sciences to represent real-valued random variables whose distributions are not known.[1][2] The normal distribution is remarkably useful because of the central limit theorem. In its most general form, under mild conditions, it states that averages of random variables independently drawn from independent distributions are normally distributed. Physical quantities that are expected to be the sum of many independent processes (such as measurement errors) often have distributions that are nearly normal.[3] Moreover, many results and methods (such as propagation of uncertainty and least squares parameter fitting) can be derived analytically in explicit form when the relevant variables are normally distributed.

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https://en.wikipedia.org/wiki/Normal_distribution

The normal distribution is sometimes informally called the bell curve.

However, many other distributions are bell-shaped (such as Cauchy's, Student's, and logistic). The terms Gaussian function and Gaussian bell curve are also ambiguous because they sometimes refer to multiples of the normal distribution that cannot be directly interpreted in terms of probabilities.

The probability density of the normal distribution is:

Hereμ is the mean or expectation of the distribution (and also its median and mode). The parameter σ is its standard deviation with its variance then σ2. A random variable with a Gaussian distribution is said to be normally distributed and is called a normal deviate.

If μ = 0 and σ = 1, the distribution is called the standard normal distribution or the unit normal distribution denoted by N(0,1) and a random variable with that distribution is a standard normal deviate.

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https://en.wikipedia.org/wiki/Normal_distribution

Probability density function for the normal distribution

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https://en.wikipedia.org/wiki/Normal_distribution

Cumulative distribution function of an acoustic emission

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https://en.wikipedia.org/wiki/Normal_distribution

Cumulative distribution function of an acoustic emission

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https://en.wikipedia.org/wiki/Normal_distribution

Discussion Subject: What is the difference between an Gaussian random noise and an engineering acoustic emission?

Answer: The waveform of continuous type AE signal is similar to Gaussian random noise, but the amplitude varies with acoustic emission activity.

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https://en.wikipedia.org/wiki/Normal_distribution

Crystal Twinning Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.

Twinning can often be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern.

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https://en.wikipedia.org/wiki/Crystal_twinning

Twin boundaries occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them. It is a highly symmetrical interface, often with one crystal the mirror image of the other; also, atoms are shared by the two crystals at regular intervals. This is also a much lowerenergy interface than the grain boundaries that form when crystals of arbitrary orientation grow together.

Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures. They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shapememory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel. In certain types of high strength steels, very fine deformation twins act as primary obstacles against dislocation motion. These steels are referred to as 'TWIP' steels, where TWIP stands for TWinning Induced Plasticity

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https://en.wikipedia.org/wiki/Crystal_twinning

What is Crystal Twinning Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner.

Crystal-A

Crystal-B

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https://en.wikipedia.org/wiki/Crystal_twinning

Crystal Twinning

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https://en.wikipedia.org/wiki/Crystal_twinning

Crystal Twinning

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https://en.wikipedia.org/wiki/Crystal_twinning

Fivefold twinning in a gold nano-particle (electron microscope image).

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https://en.wikipedia.org/wiki/Crystal_twinning

Crystal Twinning- Diagram of twinned crystals of Albite. On the more perfect cleavage, which is parallel to the basal plane (P), is a system of fine striations, parallel to the second cleavage (M).

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https://en.wikipedia.org/wiki/Crystal_twinning

Crystal Twinning- Martensitic Formation

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https://en.wikipedia.org/wiki/Crystal_twinning

AET Set-up

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http://www.geocities.ws/raobpc/AET.html

Continuous type- Gaussian random noise

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Continuous type

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Discrete Burst Type

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Discussion Subject: explains on the weak damages signal w.r.t the severe damage in term of the recorded peak signal.

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http://www.geocities.ws/raobpc/AET.html

Discrete Burst Type (Kaiser effect)

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Kaiser Effect Plastic deformation is the primary source of AE in loaded metallic structures.

An important feature affecting the AE during deformation of a material is ‘Kaiser Effect’, which states that additional AE occurs only when the stress level exceeds previous stress level. A similar effect for composites is termed as 'Falicity effect'. (?)

Comments: Kaiser effect- when the load is released and later applied, AE will not be emitted until the previous maximum is reaches.

Falicity effect- an effect that deviate from Kaiser effect

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Kaiser Effect- which states that additional AE occurs only when the stress level exceeds previous stress level. A similar effect for composites is termed as 'Falicity effect'. (?)

http://www.ndt.net/ndtaz/content.php?id=476 Charlie Chong/ Fion Zhang

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Felicity effect is an effect in acoustic emission that reduces Kaiser effect at high loads of material. Under Felicity effect the acoustic emission resumes before the previous maximum load was reached Felicity effect

Kaiser effect

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https://en.wikipedia.org/wiki/Felicity_effect

Basic AE history plot showing Kaiser effect (BCB), Felicity effect (DEF), and emission during hold (GH) 2

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Activity of AE Sources in Structural Loading AE signals generated under different loading patterns can provide valuable information concerning the structural integrity of a material. Load levels that have been previously exerted on a material do not produce AE activity. In other words, discontinuities created in a material do not expand or move until that former stress is exceeded. This phenomenon, known as the Kaiser Effect, can be seen in the load versus AE plot to the right. As the object is loaded, acoustic emission events accumulate (segment AB). When the load is removed and reapplied (segment BCB), AE events do not occur again until the load at point B is exceeded. As the load exerted on the material is increased again (BD), AE’s are generated and stop when the load is removed.

However, at point F, the applied load is high enough to cause significant emissions even though the previous maximum load (D) was not reached.

This phenomenon is known as the Felicity Effect. This effect can be quantified using the Felicity Ratio, which is the load where considerable AE resumes, divided by the maximum applied load (F/D).

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Kaiser Effect- The phenomenon, known as the Kaiser Effect, can be seen in the load versus AE plot to the right. As the object is loaded, acoustic emission events accumulate (segment AB). When the load is removed and reapplied (segment BCB), AE events do not occur again until the load at point B is exceeded

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Felicity Effect – the applied load is high enough to cause significant emissions even though the previous maximum load (D) was not reached. This phenomenon is known as the Felicity Effect.

(F) Felicity Ratio= F/D

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(D)

AE Parameters Various parameters used in AET include: AE burst, threshold, ring down count, cumulative counts, event duration, peak amplitude, rise time, energy and RMS voltage etc. Typical AE system consists of signal detection, amplification & enhancement, data acquisition, processing and analysis units.

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http://www.geocities.ws/raobpc/AET.html

AE Parameters Various parameters used in AET include:       

AE burst, threshold, ring down count, cumulative counts, event duration, peak amplitude, rise time, energy and RMS voltage etc.

Typical AE system consists of signal detection, amplification & enhancement, data acquisition, processing and analysis units.

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http://www.geocities.ws/raobpc/AET.html

Sensors / Source Location Identification The most commonly used sensors are resonance type piezoelectric transducers with proper couplants. In some applications where sensors cannot be fixed directly, waveguides are used. Sensors are calibrated for frequency response and sensitivity before any application. The AE technique captures the parameters and correlates with the defect formation and failures.

When more than one sensors is used,  AE source can be located based by measuring the signal’s arrival time to each sensor. By comparing the signal’s arrival time at different sensors, the source location can be calculated through triangulation 三角测量 and other methods.  AE sources are usually classified based on activity 活动力 and intensity 强 度. A source is considered to be active if its event count continues to increase with stimulus.  A source is considered to be critically active if the rate of change of its count or emission rate consistently increases with increasing stimulation 变化率随着刺激增加不断提高.

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AET Advantages AE testing is a powerful aid to materials testing and the study of deformation, fatigue crack growth, fracture, oxidation and corrosion. It gives an immediate indication of the response and behaviour of a material under stress, intimately connected with strength, damage and failure. A major advantage of AE testing is that it does not require access to the whole examination area. In large structures / vessels permanent sensors can be mounted for periodic inspection for leak detection and structural integrity monitoring.

Typical advantages of AE technique include: 1.

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high sensitivity, early and rapid detection of defects, leaks, cracks etc., on-line monitoring, location of defective regions, minimization of plant downtime for inspection, no need for scanning the whole structural surface and minor disturbance of insulation.

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http://www.geocities.ws/raobpc/AET.html

AET Limitations On the negative side;  AET requires stimulus. (process stimulus or externally test stimulus?)  AE technique can only (1) qualitatively estimate the damage and predict (2) how long the components will last. So,  other NDT methods are still needed for thorough examinations and for obtaining quantitative information.  Plant environments are usually very noisy and the AE signals are usually very weak. This situation calls for incorporation of signal discrimination and noise reduction methods. In this regard, (1) signal processing and (2) frequency domain analysis are expected to improve the situation.

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http://www.geocities.ws/raobpc/AET.html

A Few Typical Applications  Detection and location of leak paths in end-shield of reactors (frequency analysis)  Identification of leaking pressure tube in reactors  Condition monitoring of 17 m Horton sphere during hydro testing (24 sensors)  On-line monitoring of welding process and fuel end-cap welds  Monitoring stress corrosion cracking, fatigue crack growth  Studying plastic deformation behaviour and fracture of SS304, SS316, Inconel, PE-16 etc  Monitoring of oxidation process and spalling behaviour of metals and alloys

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http://www.geocities.ws/raobpc/AET.html

Acoustic Emission Testing applications are most suitable for: 1.

2.

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Aboveground Storage Tank Screening for Corrosion & Leaks Pressure Containment Vessels (Columns, Bullets, Cat Crackers) Horton Spheres & legs Fiberglass Reinforced Plastic Tanks and Piping Offshore Platform Monitoring Nuclear components inspection Tube Trailers Railroad tank cars Bridge Critical Members monitoring Pre- & Post-Stressed Concrete Beams Reactor Piping High Energy Seam Welded Hot Reheat Piping Systems in Power Plants.

On-Stream Monitoring Remote Long Term Monitoring

http://www.techcorr.com/services/Inspection-and-Testing/Acoustic-Emission-Testing.cfm

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Acoustic Emission Testing Advantages Compared to conventional inspection methods the advantages of the Acoustic Emission Testing technique are: • • • • •

Tank bottom Testing without removal of product.

Inspection of Insulated Piping & Vessels Real time monitoring during cool-down & start-ups Real Time Monitoring Saves Money Real Time Monitoring Improves Safety

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Tank AET

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End of Reading 1

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Study Note 2: Acoustic Emission Method Sidney Mindess University of British Columbia Chapter 16: Acoustic Emission Methods

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16 Acoustic Emission Methods

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Dam

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http://www.boomsbeat.com/articles/116/20140118/tianzi-mountains-china.htm

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Content: 16.1 16.2 16.3 16.4 16.5 16.6

Introduction Historical Background Theoretical Considerations Evaluation of Acoustic Emission Signals Instrumentation and Test Procedures Parameters Affecting Acoustic Emissions from Concrete The Kaiser Effect · Effect of Loading Devices · Signal Attenuation · Specimen Geometry · Type of aggregate ·Concrete Strength 16.7 Laboratory Studies of Acoustic Emission Fracture Mechanics Studies · Type of Cracks · Fracture Process Zone (Crack Source) Location · Strength vs. Acoustic Emission Relationships · Drying Shrinkage · Fiber Reinforced Cements and Concretes · High Alumina Cement · Thermal Cracking · Bond in Reinforced Concrete · Corrosion of Reinforcing Steel in Concrete 16.8 Field Studies of Acoustic Emission 16.9 Conclusions

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Foreword: Acoustic emission refers to the sounds, both audible and sub-audible (ultrasonic?, subsonic?) , that are generated when a material undergoes irreversible changes, such as those due to cracking.

Acoustic emissions (AE) from concrete have been studied for the past 30 years, and can provide useful information on concrete properties. This review deals with the parameters affecting acoustic emissions from concrete, including discussions of the Kaiser effect, specimen geometry, and concrete properties. There follows an extensive discussion of the use of AE to monitor cracking in concrete, whether due to: (1) externally applied loads, (2) drying shrinkage, or (3) thermal stresses.

AE studies on reinforced concrete are also described. While AE is very useful laboratory technique for the study of concrete properties, its use in the field remains problematic.

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16.1 Introduction It is common experience that the failure of a concrete specimen under load is accompanied by a considerable amount of audible noise. In certain circumstances, some audible noise is generated even before ultimate failure occurs. With very simple equipment- a microphone placed against the specimen, an amplifier, and an oscillograph — subaudible sounds can be detected at stress levels of perhaps 50% of the ultimate strength; with the sophisticated equipment available today, sound can be detected at much lower loads, in some cases below 10% of the ultimate strength. These sounds, both audible and subaudible, are referred to as acoustic emission. In general, acoustic emissions are defined as “the class of phenomena whereby transient 转瞬即逝的 elastic waves are generated by the rapid release of energy from localized sources within a material.” These waves propagate through the material, and their arrival at the surfaces can be detected by piezoelectric transducers.

Keywords: Audible & Sub-audible sounds

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Acoustic emissions, which occur in most materials, are caused by irreversible changes, such as (1) dislocation movement, (2) twinning, (3) phase transformations, (4) crack initiation, and propagation, (5) debonding between continuous and dispersed phases in composite materials, and so on.

In concrete, since the first three of these mechanisms do not occur, acoustic emission is due primarily to: 1. Cracking processes 2. Slip between concrete and steel reinforcement 3. Fracture or debonding of fibers in fiber-reinforced concrete

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16.2 Historical Background The initial published studies of acoustic emission phenomena, in the early 1940s, dealt with the problem of predicting rockbursts in mines; this technique is still very widely used in the field of rock mechanics, in both field and laboratory studies.

The first significant investigation of acoustic emission from metals (steel, zinc, aluminum, copper, and lead) was carried out by Kaiser. Among many other things, he observed what has since become known as the Kaiser effect: “the absence of detectable acoustic emission at a fixed sensitivity level, until previously applied stress levels are exceeded.�

While this effect is not present in all materials, it is a very important observation, and it will be referred to again later in this review. The first study of acoustic emission from concrete specimens under stress appears to have been carried out by RĂźsch, who noted that during cycles of loading and unloading below about 70 to 85% of the ultimate failure load, acoustic emissions were produced only when the previous maximum load was reached (the Kaiser effect).

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At about the same time, but independently, L’Hermite also measured acoustic emission from concrete, finding that a sharp increase in acoustic emission (magnitude or event count?) coincided with the point at which Poisson’s ratio also began to increase (i.e., at the onset of significant matrix cracking in the concrete).

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Poisson's ratio, named after SimĂŠon Poisson, is the negative ratio of transverse to axial strain. When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect.

Poisson's ratio Ńľ (nu) is a measure of this effect. The Poisson ratio is the fraction (or percent) of expansion divided by the fraction (or percent) of compression, for small values of these changes.

Conversely, if the material is stretched rather than compressed, it usually tends to contract in the directions transverse to the direction of stretching.

This is a common observation when a rubber band is stretched, when it becomes noticeably thinner. Again, the Poisson ratio will be the ratio of relative contraction to relative expansion, and will have the same value as above. In certain rare cases, a material will actually shrink in the transverse direction when compressed (or expand when stretched) which will yield a negative value of the Poisson ratio.

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https://en.wikipedia.org/wiki/Poisson%27s_ratio

Figure 1: A cube with sides of length L of an isotropic linearly elastic material subject to tension along the x axis, with a Poisson's ratio of 0.5. The green cube is unstrained, the red is expanded in the x direction by ∆L due to tension, and contracted in the y and z directions by ∆L'.

Poisson Ratio = ∆L‘/ ∆L

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https://en.wikipedia.org/wiki/Poisson%27s_ratio

In 1965, however, Robinson used more sensitive equipment to show that acoustic emission occurred at much lower load levels than had been reported earlier, and hence, could be used to monitor earlier microcracking (such as that involved in the growth of bond cracks in the interfacial region between cement and aggregate).

In 1970, Wells built a still more sensitive apparatus, with which he could monitor acoustic emissions in the frequency range from about 2 to 20 kHz. However, he was unable to obtain truly reproducible records for the various specimen types that he tested, probably due to the difficulties in eliminating external noise from the testing machine. Also in 1970, Green reported a much more extensive series of tests, recording acoustic emission frequencies up to 100 kHz. Green was the first to show clearly that acoustic emissions from concrete are related to failure processes within the material; using source location techniques, he was also able to determine the locations of defects. It was this work that indicated that acoustic emissions could be used as an early warning of failure. Green also noted the Kaiser effect, which suggested to him that acoustic emission techniques could be used to indicate the previous maximum stress to which the concrete had been subjected. As we will see below, however, a true Kaiser effect appears not to exist for concrete.

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Green also noted the Kaiser effect, which suggested to him that acoustic emission techniques could be used to indicate the previous maximum stress to which the concrete had been subjected. As we will see below, however, a true Kaiser effect appears not to exist for concrete.

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Nevertheless, even after this pioneering work, progress in applying acoustic emission techniques remains slow. An extensive review by Diederichs et al. (et al means: and others), covers the literature on acoustic emissions from concrete up to 1983. However, as late as 1976, Malhotra noted that there was little published data in this area, and that “acoustic emission methods are in their infancy.� Even in January, 1988, a thorough computer-aided search of the literature found only some 90 papers dealing with acoustic emissions from concrete over about the previous 10 years; while this is almost certainly not a complete list, it does indicate that there is much work to be carried out before acoustic emission monitoring becomes a common technique for testing concrete. Indeed, there are still no standard test methods which have even been suggested for this purpose.

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16.3 Theoretical Considerations When an acoustic emission event occurs at a source with the material, due to (1) inelastic deformation or (2) to cracking, the stress waves travel directly from the source to the receiver as body waves. Surface waves may then arise from mode conversion. When the stress waves arrive at the receiver, the transducer responds to the surface motions that occur.

It should be noted that the signal captured by the recording device may be affected by: â– â– â–

the nature of the stress pulse generated by the source, the geometry of the test specimen, and the characteristics of the receiver,

making it difficult to interpret the recorded waveforms.

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Two basic types of acoustic emission signals can be generated (Figure 16.1):  Continuous emission is “a qualitative description of the sustained signal level produced by rapidly occurring acoustic emission events.” These are generated by events such as plastic deformations in metals, which occur in a reasonably continuous manner.  Burst emission is “a qualitative description of the discrete signal related to an individual emission event occurring within the material,” such as that which may occur during crack growth or fracture in brittle materials.

These burst signals are characteristic of the acoustic emission events resulting from the loading of cementitious materials.

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FIGURE 16.1 The two basic types of acoustic emission signals. (A) Continuous emission. (B) Burst emission.

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16.4 Evaluation of Acoustic Emission Signals A typical acoustic emission signal from concrete is shown in Figure 16.2.12 However, when such acoustic events are examined in much greater detail, as shown in Figure 16.3, the complexity of the signal becomes even more apparent; the scatter in noise, shown in Figure 16.3, makes it difficult to determine exactly the time of arrival of the signal; this means that very sophisticated equipment must be used to get the most information out of the acoustic emission signals. In addition, to obtain reasonable sensitivity, the acoustic emission signals must be amplified. In concrete, typically, system gains in the range of 80 to 100 decibels (dB) are used.

Comments: 20log (I/Io) = 80, (I/Io) = 10000 20log(I/Io) = 100, (I/Io) = 100000

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FIGURE 16.2 A typical acoustic emission signal from concrete. (From Berthelot, J.M. et al., private communication, 1987. With permission.)

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FIGURE 16.3 Typical view of an acoustic emission event as displayed in an oscilloscope screen. (Adapted from Maji, A. and Shah, S.P., Exp. Mech., 26, 1, 1988, p. 27.)

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FIGURE 16.3 Typical view of an acoustic emission event as displayed in an oscilloscope screen. (Adapted from Maji, A. and Shah, S.P., Exp. Mech., 26, 1, 1988, p. 27.)

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There are a number of different ways in which acoustic emission signals may be evaluated.

■ Acoustic Emission Counting (ring-down counting) This is the simplest way in which an acoustic emission event may be characterized. It is “the number of times the acoustic emission signal exceeds a preset threshold during any selected portion of a test,” and is illustrated in Figure 16.4. A monitoring system may record:

FIGURE 16.4 The principle of acoustic emission counting (ring-down counting).

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1. The total number of counts (e.g., 13 counts in Figure 16.4). Since the shape of a burst emission is generally a damped sinusoid, pulses of higher amplitude will generate more counts.

2. The count rate. This is the number of counts per unit of time; it is particularly useful when very large numbers of counts are recorded.

3. The mean pulse amplitude. This may be determined by using a root-mean square meter, and is an indication of the amount of energy being dissipated.

Clearly, the information obtained using this method of analysis depends upon both the gain and the threshold setting. Ring-down counting is affected greatly by the characteristics of the transducer, and the geometry of the test specimen (which may cause internal reflections) and may not be indicative of the nature of the acoustic emission event. In addition, there is no obvious way of determining the amount of energy released by a single event, or the total number of separate acoustic events giving rise to the counts.

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■Event counting — Circuitry is available which counts each acoustic emission event only once, by recognizing the end of each burst emission in terms of a predetermined length of time since the last count (i.e., since the most recent crossing of the threshold). In Figure 16.4, for instance, the number of events is three. This method records the number of events, which may be very important, but provides no information about the amplitudes involved.

since the most recent crossing of the threshold Charlie Chong/ Fion Zhang

■Rise time — This is the interval between the time of first occurrence of signals above the level of the background noise and the time at which the maximum amplitude is reached. This may assist in determining the type of damage mechanism.

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■ Signal duration — This is the duration of a single acoustic emission event; this too may be related to the type of damage mechanism.

■ Amplitude distribution — This provides the distribution of peak amplitudes. This may assist in identifying the sources of the emission events that are occurring.

■ Frequency analysis — This refers to the frequency spectrum of individual acoustic emission events. This technique, generally requiring a fast Fourier transformation analysis of the acoustic emission waves, may help discriminate between different types of events. Unfortunately, a frequency analysis may sometimes simply be a function of the response of the transducer, and thus reveal little of the true nature of the pulse.

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Energy analysis — This is an indication of the energy released by an acoustic emission event; it may be measured in a number of ways, depending on the equipment, but it is essentially the area under the amplitude vs. time curve (Figure 16.4) for each burst. Alternatively, the area under the envelope of the amplitude vs. time curve may be measured for each burst.

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Defect location — By using a number of transducers to monitor acoustic emission events, and determining the time differences between the detection of each event at different transducer positions, the location of the acoustic emission event may be determined by using triangulation techniques. Work

by Maji and Shah, for instance, has indicated that this technique may be accurate to within about 5 mm.

Analysis of the wave-form— Most recently, it has been suggested that an elaborate signals processing technique (deconvolution -反褶积) applied to the wave-form of an acoustic emission event can provide information regarding the volume, orientation, and type of microcrack. Ideally, since all of these methods of data analysis provide different information, one would wish to measure them all. However, this is neither necessary nor economically feasible. In the discussion that follows, it will become clear that the more elaborate methods of analysis are useful in fundamental laboratory investigations, but may be inappropriate for practical applications.

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FIGURE 16.5 The main elements of a modern acoustic emission detection system.

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The Fourier transform- (Deconvolution -反褶积 of Frequency) The Fourier transform decomposes a function of time (a signal) into the frequencies that make it up, similarly to how a musical chord can be expressed as the amplitude (or loudness) of its constituent notes. The Fourier transform of a function of time itself is a complex-valued function of frequency, whose absolute value represents the amount of that frequency present in the original function, and whose complex argument is the phase offset of the basic sinusoid in that frequency. The Fourier transform is called the frequency domain representation of the original signal. The term Fourier transform refers to both the frequency domain representation and the mathematical operation that associates the frequency domain representation to a function of time.

The Fourier transform is not limited to functions of time, but in order to have a unified language, the domain of the original function is commonly referred to as the time domain. For many functions of practical interest one can define an operation that reverses this: the inverse Fourier transformation, also called Fourier synthesis, of a frequency domain representation combines the contributions of all the different frequencies to recover the original function of time.

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Fourier-Transform (FT) The Fourier theorem states that any waveform can be duplicated by the superposition of a series of sine and cosine waves. As an example, the following Fourier expansion of sine waves provides an approximation of a square wave. The three curves in the plot show the first one term (black line), four terms (blue line), and sixteen terms (red line) in the Fourier expansion.

As more terms are added the superposition of sine waves better matches a square wave.

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http://www.tissuegroup.chem.vt.edu/chem-ed/data/fourier.html

Fourier-Transform (FT) of Frequency To understand any complicated signal, one of the first step is to generate the Fourier transform of that signal. Fourier transform is a mathematical function that decomposes a time varying signal, as shown in figure to the right, into several sinusoidal waves.

These sinusoidal waves will have different frequency, amplitude and phases but when you add them all together, the original waveform is magically recreated. The fundamental idea here is complexity reduction by splitting a waveform into manageable chunks. For reasons that initially baffled me, the powers there be chose sinusoidal waves as this manageable chunk.

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https://ranabasheer.wordpress.com/2014/03/16/why-do-we-use-fourier-transform/

Signal Evaluation: Analysis of the wave-form

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http://sirius.mtm.kuleuven.be/Research/NDT/AcousticEmissions/index.html

Signal Evaluation: Acoustic Emission Counting (ring-down counting)

Ring-down count= 13

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Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

Raise time mV/Îźs

Signal duration Îźs

Event counts = 3 in unit time

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Signal Evaluation: Amplitude Distribution- Triangulation to locate source

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http://iopscience.iop.org/0964-1726/21/3/035009;jsessionid=DE0B79359A6ADDA1365CAC54ABA381A2.c2

Signal Evaluation: Amplitude Distribution- Triangulation to locate source

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http://iopscience.iop.org/0964-1726/21/3/035009;jsessionid=DE0B79359A6ADDA1365CAC54ABA381A2.c2

Signal Evaluation: Frequency analysis

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Signal Evaluation: Energy analysis- it is essentially the area under the amplitude vs. time curve Note: all areas under curves or only areas above threshold.

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Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

ring-down counting

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Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

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16.5 Instrumentation and Test Procedures Instrumentation (and, where necessary, the associated computer software) is available, from a number of different manufacturers, to carry out all of the methods of signal analysis described above. It might be added that advances in instrumentation have outpaced our understanding of the nature of the elastic waves resulting from microcracking in concrete. The main elements of a modern acoustic emission detection system are shown schematically in Figure 16.5.

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FIGURE 16.5 The main elements of a modern acoustic emission detection system.

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FIGURE 16.5 The main elements of a modern acoustic emission detection system.

Raw Display?

Selective Display?

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A brief description of the most important parts of this system is as follows: 1. Transducers: Piezoelectric transducers (generally made of lead zirconate titanate, PZT) are used to convert the surface displacements into electric signals. The voltage output from the transducers is directly proportional to the strain in the PZT, which depends in turn on the amplitude of the surface waves. Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100μV. There are basically two types of transducers. (a) Wide-band transducers are sensitive to acoustic events with frequency responses covering a wide range, often several hundred kHz. (b) Narrow-band transducers are restricted to a much narrower range of frequencies, using bandpass filters. Of course, the transducers must be properly coupled to the specimen, often using some form of silicone grease as the coupling medium.

Keywords:  Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100μV.  Wide band & Narrow Band

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Discussion Subject: A brief description of the most important parts of this system is as follows: 1.

Transducers: Piezoelectric transducers (generally made of lead zirconate titanate, PZT) are used to convert the surface displacements into electric signals. The voltage output from the transducers is directly proportional to the strain in the PZT, which depends in turn on the amplitude of the surface waves. Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100μV. There are basically two types of transducers. (a) Wide-band transducers are sensitive to acoustic events with frequency responses covering a wide range, often several hundred

(b) Narrow-band transducers are restricted to a much narrower range of frequencies, using bandpass filters. Of course, the transducers must be properly coupled to the specimen, often kHz.

using some form of silicone grease as the coupling medium.

Keywords:  Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100μV.  Wide band & Narrow Band

Question: Band pass (selective, High, Low?) as part of transducer constructions? Or post transducer electronic?

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PZT:- If the p.d or the stress is changing the resulting effect also changes. Therefore if an alternating potential difference with a frequency equal to the resonant frequency of the crystal is applied across it the crystal will oscillate. A number of crystalline materials show this effect – examples of these are quartz, barium titanate, lithium sulphate, lead metaniobate, lead zirconate titanate (PZT) and polyvinylidine difluoride.

Piezoelectric transducers can act as both as a transmitter and a detector of vibrations.

However there are certain conditions. The crystal must stop vibrating as soon as the alternating potential difference is switched off so that they can detect the reflected pulse. For this reason a piece of damping material with an acoustic impedance the same as that of the crystal is mounted at the back of the crystal. (See Figure 2).The transducer is made with a crystal that has a thickness of one half of the wavelength of the ultrasound, resonating at its fundamental frequency. A layer of gel is needed between the transducer and the body to get good acoustic coupling (see acoustic impedance).

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http://www.schoolphysics.co.uk/age16-19/Medical%20physics/text/Piezoelectric_transducer/index.html

The transducer is made with a crystal that has a thickness of one half of the wavelength of the ultrasound, resonating at its fundamental frequency.

Example: Frequency= 519Hz, Wavelength 位 = Speed/ frequency = 5890/519=11.35mm. The thickness of the transducer= 5.7mm approx.

s= 5890m/s

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http://www.olympus-ims.com/en/ndt-tutorials/thickness-gage/appendices-velocities/

AET Transducer In 0.1KHz~2.0KHz

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UT Transducers 2.0~5.0 MHz (≠AET Transducer)

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2. Preamplifier: Because of the low voltage output (≤100ΟV) , the leads from the transducer to the preamplifier must be as short as possible; often, the preamplifier is integrated within the transducer itself. Typically, the gain in the preamplifier is in the range 40 to 60 dB (x100, x1000). (Note: The decibel scale measures only relative amplitudes. Using this scale:

where V is the output amplitude and Vi is the input amplitude. That is, a gain of 40 dB will increase the input amplitude by a factor of 100; a gain of 60 dB will increase the input amplitude by a factor of 1000, and so on.)

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3. Passband filters: are used to suppress the acoustic emission signals that lie outside of the frequency range of interest. (high pass, low pass, selective pass) 4. The main amplifier: further amplifies the signals, typically with a gain of an additional 20 to 60 dB. 5. The threshold discriminator: is used to set the threshold voltage above which signals are counted (or analyze) .

The remainder of the electronic equipment depends upon the way in which the acoustic emission data are to be recorded, analyzed, and displayed.

Acoustic emission testing may be carried out in the laboratory or in the field.

Basically, one or more acoustic emission transducers are attached to the specimen. The specimen is then loaded slowly, and the resulting acoustic emissions are recorded.

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There are generally two (or more) categories of tests: 1. To use the acoustic emission signals to learn something about the internal structure of the material, and how structural changes (i.e., damage) occur during the process of loading. In this case, the specimens are generally loaded to failure.

2. To establish whether the material or the structure meet certain design or fabrication criteria. In this case, the load is increased only to some predetermined level (“proof � loading). The amount and nature of the acoustic emissions may be used to establish the integrity of the specimen or structure, and may also sometimes be used to predict the service life. (i.e., hydrostatic testing) 3. Inservice monitoring where the loadings are the service loading? (e.g., monitoring of crack growth in a inservice coke drum) 4. Other?

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16.6 Parameters Affecting Acoustic Emissions from Concrete 16.6.1 The Kaiser Effect The earliest acoustic emission studies of concrete, such as the work of R端sch, indicated that a true Kaiser effect (see above) exists for concrete; that is, acoustic emissions were found not to occur in concrete that had been unloaded until the previously applied maximum stress had been exceeded on reloading. This was true, however, only for stress levels below about 75 to 85% of the ultimate strength of the material; for higher stresses, acoustic emissions began again at stresses somewhat lower than the previous maximum stress.

Subsequently, a number of other investigators have also concluded that concrete exhibits a Kaiser effect, at least for stresses below the peak stress of the material. (felicity effect) Keypoints: For concrete This was true, however, only for stress levels below about 75 to 85% of the ultimate strength of the material

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Spooner and Dougill confirmed that this effect did not occur beyond the peak of the stress-strain curve (i.e., in the descending portion of the stress-strain curve), where acoustic emissions occurred again before the previous maximum strain was reached. It has also been suggested that a form of the Kaiser effect occurs as well for cyclic thermal stresses in concrete, and for drying and wetting cycles. On the other hand, Nielsen and Griffin have reported that the Kaiser effect is only a very temporary effect in concrete; with only a few hours of rest between loading cycles, acoustic emissions are again recorded during reloading to the previous maximum stress. They therefore concluded “that the Kaiser effect is not a reliable indicator of the loading history for plain concrete.� Thus, it is unlikely that the Kaiser effect could be used in practice to determine the previous maximum stress that a structural member has been subjected to.

Comments: The continual curing of concrete matrix repair the previous loading induced effects (microcracks, disbonding etc.) and return the concrete back to almost preloading condition.

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Kaiser Effect- Concrete

For concrete This was true, however, only for stress levels below about 75 to 85% of the ultimate strength of the material

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that this effect did not occur beyond the peak of the stressstrain curve (i.e., in the descending portion of the stressstrain curve), where acoustic emissions occurred again before the previous maximum strain was reached.

Spooner and Dougill conclusion on Kaiser Effect- Concrete: They therefore concluded “that the Kaiser effect is not a reliable indicator of the loading history for plain concrete.�

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16.6.2 Effect of Loading Devices As is well known, the end restraint of a compression specimen of concrete due to the friction between the ends of the specimen and the loading platens can have a considerable effect on the apparent strength of the concrete.

These differences are also reflected in the acoustic emissions measured when different types of loading devices are used. For instance, in compression testing with stiff steel platens, most of the acoustic emission appears at stresses beyond about half of the ultimate stress; with more flexible platens, such as brush platens, significant acoustic emission appears at about 20% of the ultimate stress. This undoubtedly reflects the different crack patterns that develop with different types of platens, but it nonetheless makes inter-laboratory comparisons, and indeed even studies on different specimen geometries within the same laboratory, very difficult.

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16.6.3 Signal Attenuation The elastic stress waves that are generated by cracking attenuate as they propagate through the concrete. Thus, large acoustic emission events that take place in the concrete far from a pick-up transducer may not exceed the threshold excitation voltage due to this attenuation, while much smaller events may be recorded if they occur close to the transducer. Very little information is available on acoustic emission attenuation rates in concrete. It has been shown that more mature cements show an increasing capacity to transmit acoustic emissions. Related to this, Mindess has suggested that the total counts to failure for concrete specimens in compression are much higher for older specimens, which may also be explained by the better transmission through older concretes.

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As a practical matter, the maximum distance between piezoelectric transducers, or between the transducers and the source of the acoustic emission event, should not be very large. Berthelot and Robert required an array of transducers arranged in a 40-cm square mesh to locate acoustic emission events reasonably accurately. They found that for ordinary concrete, with a fifth transducer placed in the center of the 40 x 40-cm square mesh, only about 40% of the events detected by the central transducer were also detected by the four transducers at the corners; with high strength concrete, this proportion increased to 60 to 70%. Rossi also found that a 40-cm square mesh was needed for a proper determination of acoustic emission events.

Although more distant events can, of course, be recorded, there is no way of knowing how many events are “lost� due to attenuation. This is an area that requires much more study.

16.6.4 Specimen Geometry It has been shown that smaller specimens appear to give rise to greater levels of acoustic emission than do larger ones. The reasons for this are not clear, although the observation may be related to the attenuation effect described above. After an acoustic emission event occurs, the stress waves not only travel from the source to the sensor, but also undergo (1) reflection, (2) diffraction, and (3) mode conversions within the material. The basic problem of wave propagation within a bounded solid certainly requires further study, but there have apparently been no comparative tests on different specimen geometries.

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16.6.5 Type of Aggregate It is not certain whether the mineralogy of the aggregate has any effect on acoustic emission. It has been reported that concretes with a smaller maximum aggregate size produce a greater number of acoustic emission counts than those with a larger aggregate size; however, the total energy released by the finer aggregate concrete is reduced. This is attributed to the observation that concretes made with smaller aggregates start to crack at lower stresses; in concretes with larger aggregate particles, on the other hand, individual acoustic events emit higher energies. For concretes made with lightweight aggregates, the total number of counts is also greater than for normal weight concrete, perhaps because of cracking occurring in the aggregates themselves.

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16.6.6 Concrete Strength It has been shown that the total number of counts to the maximum load is greater for higher strength concretes. However, as was mentioned earlier, for similar strength levels the total counts to failure appears to be much higher for older concretes.

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16.7 Laboratory Studies of Acoustic Emission By far the greatest number of acoustic emission studies of concrete have been carried out in the laboratory, and have been largely “theoretical� in nature: 1. To determine whether acoustic emission analysis could be applied to cementitious systems 2. To learn something about crack propagation in concrete

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16.7.1 Fracture Mechanics Studies A number of studies have shown that acoustic emission can be related to crack growth or fracture mechanics parameters in cements, mortars, and concretes. Evans et al. showed that acoustic emission could be correlated with crack velocity in mortars. Morita and Kato and Nadeau, Bennett, and Mindess were able to relate total acoustic emission counts to Kc (the fracture toughness). In addition, Lenain and Bunsell found that the number of emissions could be related to the sixth power of the stress intensity factor, K. (K6?) Izumi et al. showed that acoustic emissions could also be related to the strain energy release rate, G. In all cases, however, these correlations are purely empirical; no one has yet developed a fundamental relationship between acoustic emission events and fracture parameters, and it is unlikely that such a relationship exists.

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16.7.1 Fracture Mechanics Studies A number of studies have shown that acoustic emission can be related to crack growth or fracture mechanics parameters in cements, mortars, and concretes. Evans et al. showed that acoustic emission could be correlated with crack velocity in mortars.

Morita and Kato and Nadeau, Bennett, and Mindess were able to relate total acoustic emission counts to Kc (the fracture toughness). In addition, Lenain and Bunsell found that the number of emissions could be related to the sixth power of the stress intensity factor, K. (K6?) Izumi et al. showed that acoustic emissions could also be related to the strain energy release rate, G. In all cases, however, these correlations are purely empirical; no one has yet developed a fundamental relationship between acoustic emission events and fracture parameters,

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and it is unlikely that such a relationship exists.

16.7.2 Type of Cracks A number of attempts have been made to relate acoustic events of different frequencies, or of different energies, to different types of cracking in concrete.

For instance, Saeki et al., by looking at the energy levels of the acoustic emissions at different levels of loading, concluded that the first stage of cracking, due to the development of bond cracks between the cement paste and the aggregate, emitted high energy signals; the second stage, which they termed “crack arrest,” emitted low energy signals; the final stage, in which cracks extended through the mortar, was again associated with high energy acoustic events. Similarly, Tanigawa and Kobayashi used acoustic energies to distinguish the onset of “the proportional limit, the initiation stress and the critical stress.” On the other hand, Tanigawa et al. tried to relate the fracture type (pore closure, tensile cracking, and shear slip) to the power spectra and frequency components of the acoustic events. The difficulty with these and similar approaches is that they tried to relate differences in the recorded acoustic events to preconceived notions 先入为主的观念 of the nature of cracking in concrete; direct cause and effect relationships were never observed.

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16.7.3 Fracture Process Zone (Crack Source) Location Perhaps the greatest current interest in acoustic emission analysis is its use in locating fracture processes, and in monitoring the damage that concrete undergoes as cracks progress. Okada et al. showed that the location of crack sources obtained from differences in the arrival times of acoustic emissions was in good agreement with the observed fracture surface. At about the same time, Chhuy et al. and Lenain and Bunsell were able to determine the length of the damaged zone ahead of the tip of a propagating crack using onedimensional acoustic emission location techniques. In subsequent work, Chhuy et al., using more elaborate equipment and analytical techniques, were able to determine damage both before the initiation of a visible crack and after subsequent crack extension. Berthelot and Robert and Rossi used acoustic emission to monitor concrete damage as well.

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They found that, while the number of acoustic events showed the progression of damage both ahead and behind the crack front, this technique alone could not provide a quantitative description of the cracking. However, using more elaborate techniques, including amplitude analysis and measurements of signal duration, Berthelot and Robert concluded that “acoustic emission testing is practically the only technique which can provide a quantitative description of the progression in real time of concrete damage within test specimens.” Later, much more sophisticated signals processing techniques were applied to acoustic emission analysis.

In 1981, Michaels et al.15 and Niwa et al. developed deconvolution techniques 反褶积技术 to analyze acoustic waveforms, in order to provide a stress-time history of the source of an acoustic event. Similar deconvolution techniques were subsequently used by Maji and Shah to determine the volume, orientation and type of microcrack, as well as the source of the acoustic events. Such sophisticated techniques have the potential eventually to be used to provide a detailed picture of the fracture processes occurring within concrete specimens.

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16.7.4 Strength vs. Acoustic Emission Relationships Since concrete quality is most frequently characterized by its strength, many studies have been directed towards determining a relationship between acoustic emission activity and strength. For instance, Tanigawa and Kobayashi concluded that “the compressive strength of concrete can be approximately estimated by the accumulated AE counts at relatively low stress level.” Indeed, they suggested that acoustic emission techniques might provide a useful nondestructive test method for concrete strength. Earlier, Fertis had concluded that acoustic emissions could be used to determine not only strength, but also static and dynamic material behavior. Rebic, too, found that there is a relationship between the “critical” load at which the concrete begins to be damaged, which can be determined from acoustic emission measurements, and the ultimate strength; thus, acoustic emission analysis might be used as a predictor of concrete strength.

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Sadowska-Boczar et al. tried to quantify the strength vs. acoustic emission relationship using the equation:

Where: Fr is the rupture strength, Fp is the stress corresponding to the first acoustic emission signal, and a and b are constants for a given material and loading conditions.

Using this linear relationship, which they found to fit their data reasonably well, they suggested that the observation of acoustic emissions at low stresses would permit an estimation of strength, as well as providing some characterization of porosity and critical flaw size.

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Unfortunately, the routine use of acoustic emissions as an estimator of strength seems to be an unlikely prospect, in large part because of the scatter in the data, as has been noted by Fertis. As an example of the scatter in data.

Figure 16.6 indicates the variability in the strength vs. total acoustic emission counts relationship; the within-batch variability is even more severe, as shown in Figure 16.7.23

FIGURE 16.6 Logarithm of total acoustic emission counts vs. compressive strength of concrete cubes. (From Mindess, S., Int. J. Cem. Comp. Lightweight Concr., 4, 173, 1982. With permission.)

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FIGURE 16.7 Within-batch variability of total acoustic emission counts vs. applied compressive stress on concretecubes. (From Mindess, S., Int. J. Cem. Comp. Lightweight Concr., 4, 173, 1982. With permission.)

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16.7.5 Drying Shrinkage Acoustic emission has been used to try to monitor shrinkage in cement pastes and mortars. Nadeau et al. found that, in hardened pastes, the acoustic emission resulted from cracking due to the unequal shrinkage of the hydration products. Mortar gave less acoustic emission than hardened paste, suggesting that the fracture processes at the sand/cement paste interface are not an important source of acoustic emission. Jeong et al. also suggested that, in autoclaved aerated concrete, the acoustic emissions during drying could be related to microcracking. Again, however, it is unlikely that acoustic emission measurements will be able to be used as a means of predicting the shrinkage as a function of time.

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16.7.6 Fiber Reinforced Cements and Concretes A number of acoustic emission studies have been carried out on fiber reinforced cements and concretes. Lenain and Bunsell, in a study of asbestos cement, found that acoustic emissions resulted both from cracking of the matrix and fiber pullout.

They noted that the Kaiser effect was not found for this type of fiberreinforced composite, since on unloading of a specimen the partially pulled out fibers were damaged, and particles of cement attached to them were crushed, giving rise to acoustic emissions on unloading. Because these damaged fibers were then less able to resist crack growth, on subsequent reloading cracks started to propagate at lower stress levels than on the previous cycle, thus, giving off acoustic emissions below the previously achieved maximum load.

Akers and Garrett also studied asbestos cement; they found that acoustic emission monitoring could be used to detect the onset and development of prefailure cracking.

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However, they concluded that “there is no basis whatsoever for using amplitude discrimination in acoustic emission monitoring for distinguishing between the various failure modes which occur in this material.� On the other hand, Faninger et al. argued that in fiber-reinforced concrete the amplitude pattern of the acoustic emission signals did make it possible to distinguish whether fracture had occurred in the fibers or between them. Similarly, Jeong et al. stated that acoustic emission frequency analysis could distinguish between different micro-fracture mechanisms in fiber-reinforced autoclaved aerated concrete.

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Fiber Reinforced Cements and Concretes

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16.7.7 High Alumina Cement In concretes made with high alumina (calcium aluminate) cement, the conversion from CAH10 * to C3AH6* on prolonged aging can lead to a large increase in porosity and therefore a large decrease in strength. There has thus been considerable interest in finding a nondestructive technique to monitor high alumina cement concrete (HAC) members. Parkinson and Peters concluded that the conversion process itself is not a source of acoustic emission activity, since no acoustic emissions were generated during the accelerated conversion of pastes at the critical w/c ratio of 0.35. However, at the high w/c ratio of 0.65, conversion was accompanied by a high level of acoustic emission activity, due to the fracture processes taking place during conversion, associated perhaps with the liberation of excess water. Arrington and Evans suggested that the structural integrity of HAC could be evaluated from the shape of the acoustic emission vs. load plot, the emissions recorded while the specimens were held under a constant load, and the decay of emission activity with time. *Note that cement chemistry notation is being used: C= CaO; A= Al2O3; H= H2O. Charlie Chong/ Fion Zhang

Perhaps the most extensive series of tests on HAC, carried out at the Fulmer Research Institute in the U.K., was reported by Williams. Apart from observing that the Kaiser effect existed up to the point at which the beams cracked, some tentative suggestions were made for monitoring HAC beams with acoustic emissions: 1. If, on loading a beam, no acoustic emission is noted, then the applied load is still less than about 60% of the ultimate load; if acoustic emission occurs, then this percentage of the ultimate load has been exceeded.

If, upon unloading such a beam, further acoustic emission activity is recorded, then the beam is cracked. The amount of acoustic emission during this unloading could indicate the degree to which the cracking load had been exceeded.

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2. If a beam is under its service load, it would behave similarly on application of a superimposed load. The presence or absence of acoustic emissions during this further loading and unloading might indicate the condition of the beam.

3. If a beam under service load showed no acoustic emission activity during further loading, but did so at a later date when loaded to the same level, then the strength must have decreased during that time interval.

As well, Williams noted similar behavior on testing of ordinary prestressed concrete beams, and suggested that these techniques could be used to evaluate any type of concrete structure, as long as acoustic emissions not connected with beam damage could be eliminated.

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16.7.8 Thermal Cracking Relatively little work has been carried out on acoustic activity when concrete is subjected to high temperatures, such as those that may be encountered in fires. However, Hinrichsmeyher et al. carried out tests up to temperatures of 900째C. They claimed that acoustic emission analysis during heating enabled them to distinguish the different types of thermally induced cracking that occurred. They noted a thermal Kaiser effect in the temperature range 300 to 600째C, which might help in determining the maximum temperature reached in a previous heating cycle. The technique was even sensitive enough to record the acoustic emissions from the quartz inversion at 573째C.

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16.7.9 Bond in Reinforced Concrete A number of acoustic emission studies of debonding of reinforcing bars in reinforced concrete have been carried out. Kobayashi et al. tested simulated beam-column connections with a 90째 hooked reinforcing bar subjected to various cyclic loading histories. They found that the penetration of a surface crack down to the level of the bar gave rise to only one or two acoustic events; most acoustic emission signals were generated by the internal cracking around the bar due to fracture at the lugs (ribs) of the bars. Acoustic emission signals were able to indicate, with reasonable accuracy, the degree of debonding. They suggested that acoustic emission techniques could be used to determine the amount of bond deterioration in concrete structures during proof testing, or due to overloads. In addition, several studies of bond degradation at elevated temperatures have been carried out. Royles et al. studied simple pullout specimens at temperatures up to 800째C.

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They found that acoustic emissions were associated with the adhesive failure at the steel-concrete interface, followed by local crushing under the ribs of the reinforcing bars. They suggested that acoustic emissions could be used to identify the point of critical slip. In further work, Royles and Morley suggested that acoustic emission techniques might be useful in estimating the quality of the bond in reinforced concrete structures that had been subjected to fires.

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16.7.10 Corrosion of Reinforcing Steel in Concrete The deterioration of concrete due to corrosion of the reinforcing steel is a major problem, which is usually detected only after extreme cracking has already taken place. Weng et al. found that measurable levels of acoustic emission occurred even during the corrosion of unstressed reinforced concrete. They suggested that, at least in the laboratory, acoustic emission monitoring would assist in characterizing corrosion damage. In subsequent work, Dunn et al. developed a relationship between the observed damage and the resulting acoustic emissions. Damage could be detected in its early stages, and by a combination of total counts and amplitude measurements, the nature of the corrosion damage could be determined.

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Corrosion of Reinforcing Steel in Concrete

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Corrosion of Reinforcing Steel in Concrete

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16.8 Field Studies of Acoustic Emission As shown in the previous section, acoustic emission analysis has been used in the laboratory to study a wide range of problems. Unfortunately, its use in the field has been severely limited; only a very few papers on field application have appeared, and these are largely speculation on future possibilities. The way in which acoustic emission data might be used to provide information about the condition of a specimen or a structure has been described by Cole; his analysis may be summarized as follows: 1. Is there any acoustic emission at a certain load level? If no, then no damage is occurring under these conditions; if yes, then damage is occurring.

2. Is acoustic emission continuing while the load is held constant at the maximum load level? If no, no damage due to creep is occurring; if yes, creep damage is occurring. Further, if the count rate is increasing, then failure may occur fairly soon.

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3. Have high amplitude acoustic emissions events occurred? If no, individual fracture events have been relatively minor; if yes, major fracture events have occurred.

4. Does acoustic emission occur if the structure has been unloaded and is then reloaded to the previous maximum load? If no, there is no damage or crack propagation under low cycle fatigue; if yes, internal damage exists and the damage sites continue to spread even under low loads.

5. Does the acoustic emission occur only from a particular area? If no, the entire structure is being damaged; if yes, the damage is localized.

6. Is the acoustic emission in a local area very localized? if no, damage is dispersed over a significant area; if yes, there is a highly localized stress concentration causing the damage.

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16.9 Conclusions From the discussion above, it appears that acoustic emission techniques may be very useful in the laboratory to supplement other measurements of concrete properties. However, their use in the field remains problematic.

Many of the earlier studies held out high hopes for acoustic emission monitoring of structures. For instance, McCabe et al. suggested that, if a structure was loaded, the absence of acoustic emissions would indicate that it was safe under the existing load conditions; a low level of acoustic emissions would indicate that the structure should be monitored carefully, while a high level of acoustic emission could indicate that the structure was unsafe. But this is hardly a satisfactory approach, since it does not provide any help with quantitative analysis. In any event, even the sophisticated (and expensive) equipment now available still provides uncertain results when applied to structures, because of our lack of knowledge about the characteristics of acoustic emissions due to different causes, and because of the possibility of extraneous noise (vibration, loading devices, and so on).

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Another serious drawback is that acoustic emissions are only generated when the loads on a structure are increased, and this poses considerable practical problems. Thus, one must still conclude, with regret, that “acoustic emission analysis has not yet been well developed as a technique for the evaluation of phenomena taking place in concrete in structures.�

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Concrete Structures

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Concrete Structures

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Concrete Structures

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Concrete Structures- The Troll A platform

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Concrete Structures- The Troll A platform

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Concrete Structures- The Troll A platform

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Concrete Structures- The Troll A platform

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Concrete Structures- Draugen

End of Reading 2

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Study Note 3: Introduction to Acoustic Emission Testing http://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Intro.htm

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1.0 Introduction Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden redistribution of stress in a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors.

With the right equipment and setup, motions on the order of picometers (10-12 m) can be identified. Sources of AE vary from natural events like: 1.

2.

3.

4.

5.

6.

earthquakes and rock bursts to the initiation and growth of cracks, slip and dislocation movements, melting, twinning, and phase transformations

in metals. In composites, matrix cracking and fiber breakage and de-bonding contribute to acoustic emissions.

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AE’s have also been measured and recorded in polymers, wood, and concrete, among other materials. Detection and analysis of AE signals can supply valuable information regarding the origin and importance of a discontinuity in a material. Because of the versatility of Acoustic Emission Testing (AET), It has many industrial applications e.g.

1.

2.

3.

4.

5.

assessing structural integrity, detecting flaws, testing for leaks, or monitoring weld quality and is used extensively as a research tool.

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Twinning

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AET

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Acoustic Emission is unlike most other nondestructive testing (NDT) techniques in two regards. The first difference pertains to the origin of the signal. Instead of supplying energy to the object under examination, AET simply listens for the energy released by the object. AE tests are often performed on structures while in operation, as this provides adequate loading for propagating defects and triggering acoustic emissions.

The second difference is that AET deals with dynamic processes, or changes, in a material. This is particularly meaningful because only active features (e.g. crack growth) are highlighted. The ability to discern between developing and stagnant defects is significant. However, it is possible for flaws to go undetected altogether if the loading is not high enough to cause an acoustic event.

Furthermore, AE testing usually provides an immediate indication relating to the strength or risk of failure of a component. Other advantages of AET include fast and complete volumetric inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and clean a specimen.

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Unfortunately, AE systems can only qualitatively gauge how much damage is contained in a structure. In order to obtain quantitative results about size, depth, and overall acceptability of a part, other NDT methods (often ultrasonic testing) are necessary. Another drawback of AE stems 逆 from loud service environments which contribute extraneous noise to the signals. For successful applications, signal discrimination and noise reduction are crucial.

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2.0 A Brief History of AE Testing Although acoustic emissions can be created in a controlled environment, they can also occur naturally. Therefore, as a means of quality control, the origin of AE is hard to pinpoint. As early as 6,500 BC, potters were known to listen for audible sounds during the cooling of their ceramics, signifying structural failure. In metal working, the term "tin cry" (audible emissions produced by the mechanical twinning of pure tin during plastic deformation) was coined around 3,700 BC by tin smelters in Asia Minor. The first documented observations of AE appear to have been made in the 8th century by Arabian alchemist Jabir ibn Hayyan. In a book, Hayyan wrote that Jupiter (tin) gives off a ‘harsh sound’ when worked, while Mars (iron) ‘sounds much’ during forging. Many texts in the late 19th century referred to the audible emissions made by materials such as tin, iron, cadmium and zinc. One noteworthy correlation between different metals and their acoustic emissions came from Czochralski, who witnessed the relationship between tin and zinc cry and twinning. Later, Albert Portevin and Francois Le Chatelier observed AE emissions from a stressed Al-Cu-Mn (Aluminum-Copper-Manganese) alloy.

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The next 20 years brought further verification with the work of Robert Anderson (tensile testing of an aluminum alloy beyond its yield point), Erich Scheil (linked the formation of martensite in steel to audible noise), and Friedrich Forster, who with Scheil related an audible noise to the formation of martensite in high-nickel steel. Experimentation continued throughout the mid-1900’s, culminating in the PhD thesis written by Joseph Kaiser entitled "Results and Conclusions from Measurements of Sound in Metallic Materials under Tensile Stress.” Soon after becoming aware of Kaiser’s efforts, Bradford Schofield initiated the first research program in the United States to look at the materials engineering applications of AE. Fittingly, Kaiser’s research is generally recognized as the beginning of modern day acoustic emission testing.

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3.0 Theory - AE Sources As mentioned in the Introduction, acoustic emissions can result from the initiation and growth of cracks, slip and dislocation movements, twinning, or phase transformations in metals. In any case, AE’s originate with stress.

When a stress is exerted on a material, a strain is induced in the material as well. Depending on the magnitude of the stress and the properties of the material, an object may return to its original dimensions or be permanently deformed after the stress is removed. These two conditions are known as elastic and plastic deformation, respectively.

The most detectible acoustic emissions take place when a loaded material undergoes plastic deformation or when a material is loaded at or near its yield stress. On the microscopic level, as plastic deformation occurs, atomic planes slip past each other through the movement of dislocations. These atomicscale deformations release energy in the form of elastic waves which “can be thought of as naturally generated “ultrasound” traveling through the object.

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Crack: When cracks exist in a metal, the stress levels present in front of the crack tip can be several times higher than the surrounding area. Therefore, AE activity will also be observed when the material ahead of the crack tip undergoes plastic deformation (micro-yielding).

Fatigue Crack: Two sources of fatigue cracks also cause AE’s.

■ The first source is emissive particles (e.g. nonmetallic inclusions) at the origin of the crack tip. Since these particles are less ductile than the surrounding material, they tend to break more easily when the metal is strained, resulting in an AE signal.

■ The second source is the propagation of the crack tip that occurs through the movement of dislocations and small-scale cleavage produced by triaxial stresses.

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The amount of energy released by an acoustic emission and the amplitude of the waveform are related to the magnitude and velocity of the source event.

AE Amplitude: The amplitude of the emission is proportional (�) to the (a) velocity of crack propagation and the (b) amount of surface area created.

Large, discrete crack jumps will produce larger AE signals than cracks that propagate slowly over the same distance.

Detection and conversion of these elastic waves to electrical signals is the basis of AE testing. Analysis of these signals yield valuable information regarding the origin and importance of a discontinuity in a material. As discussed in the following section, specialized equipment is necessary to detect the wave energy and decipher which signals are meaningful.

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http://www.nature.com/nmat/journal/v10/n11/full/nmat3167.html

Activity of AE Sources in Structural Loading AE signals generated under different loading patterns can provide valuable information concerning the structural integrity of a material. Load levels that have been previously exerted on a material do not produce AE activity. In other words, discontinuities created in a material do not expand or move until that former stress is exceeded. This phenomenon, known as the Kaiser Effect, can be seen in the load versus AE plot to the right. As the object is loaded, acoustic emission events accumulate (segment AB). When the load is removed and reapplied (segment BCB), AE events do not occur again until the load at point B is exceeded. As the load exerted on the material is increased again (BD), AE’s are generated and stop when the load is removed.

However, at point F, the applied load is high enough to cause significant emissions even though the previous maximum load (D) was not reached.

This phenomenon is known as the Felicity Effect. This effect can be quantified using the Felicity Ratio, which is the load where considerable AE resumes, divided by the maximum applied load (F/D).

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Kaiser Effect: Load levels that have been previously exerted on a material do not produce AE activity. This phenomenon, known as the Kaiser Effect

Felicity Effect: The applied load is high enough to cause significant emissions even though the previous maximum load was not reached. This phenomenon is known as the Felicity Effect.

Felicity Ratio: Felicity Ratio, which is the load where considerable AE resumes, divided by the previous maximum applied load (F/D).

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Kaiser/Felicity effects

Felicity effect Felicity ratio = F/D Kaiser effect

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Knowledge of the Kaiser Effect and Felicity Effect can be used to determine if major structural defects are present. This can be achieved by applying constant loads (relative to the design loads exerted on the material) and “listening� to see if emissions continue to occur while the load is held. As shown in the figure, if AE signals continue to be detected during the holding of these loads (GH), it is likely that substantial structural defects are present.

In addition, a material may contain critical defects if an identical load is reapplied and AE signals continue to be detected. Another guideline governing AE’s is the Dunegan corollary, which states that if acoustic emissions are observed prior to a previous maximum load, some type of new damage must have occurred. (Note: Time dependent processes like corrosion and hydrogen embrittlement tend to render the Kaiser Effect useless) Dict: Corollary: something that results from something else.

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Dunegan corollary states that if acoustic emissions are observed prior to a previous maximum load, some type of new damage must have occurred. (Note: Time dependent processes like corrosion and hydrogen embrittlement tend to render the Kaiser Effect useless)

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Q. What is the Dunegan Corollary? a. It states that if acoustic emissions are observed prior to a previous maximum load, some type of new damage must have occurred. b. When the applied load is high enough to cause significant emissions even though the previous maximum load was not reached. c. Gauging signal arrival times or differences in the spectral content of true AE signals and background noise. d. the number of times a signal crosses a preset threshold Corollary: is a statement that follows readily from a previous statement.

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http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUXJ QZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET

Comments: Emissions are observed prior to a previous maximum load;  Felicity effect, (when the applied load is high enough)  Dunegan corollary, (when the load is less than the preceding load) Keywords:  Kaiser effect,  Felicity effect,  Dunegan corollary

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Noise The sensitivity of an acoustic emission system is often limited by the amount of background noise nearby. Noise in AE testing refers to any undesirable signals detected by the sensors. Examples of these signals include frictional sources (e.g. loose bolts or movable connectors that shift when exposed to wind loads) and impact sources (e.g. rain, flying objects or wind-driven dust) in bridges. Sources of noise may also be present in applications where the area being tested may be disturbed by mechanical vibrations (e.g. pumps).

To compensate for the effects of background noise, various procedures can be implemented. Some possible approaches involve fabricating special sensors with electronic gates for noise blocking, taking precautions to place sensors as far away as possible from noise sources, and electronic filtering (either using signal arrival times or differences in the spectral content of true AE signals and background noise).

Comments: ■ Spectral filtering ■ Time of flight filtering ■ Placement ■ Sensor with electronic gate?

Pseudo Sources In addition to the AE source mechanisms described above, pseudo source mechanisms produce AE signals that are detected by AE equipment.

Examples include liquefaction and solidification, friction in rotating bearings, solid-solid phase transformations, leaks, cavitation, and the realignment or growth of magnetic domains (See Barkhausen Effect).

Comments: Noise ≥ Pseudo Sources?

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Barkhausen Effect The Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed. Discovered by German physicist Heinrich Barkhausen in 1919, it is caused by rapid changes of size of magnetic domains (similarly magnetically oriented atoms in ferromagnetic materials). Barkhausen's work in acoustics and magnetism led to the discovery, which provided evidence that magnetization affects whole domains of a ferromagnetic material, rather than individual atoms alone. The Barkhausen effect is a series of sudden changes in the size and orientation of ferromagnetic domains, or microscopic clusters of aligned atomic magnets (spins), that occurs during a continuous process of magnetization or demagnetization. The Barkhausen effect offered direct evidence for the existence of ferromagnetic domains, which previously had been postulated theoretically. Heinrich Barkhausen discovered that a slow, smooth increase of a magnetic field applied to a piece of ferromagnetic material, such as iron, causes it to become magnetized, not continuously but in minute steps.

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https://en.wikipedia.org/wiki/Barkhausen_effect

Wave Propagation A primitive wave released at the AE source is illustrated in the figure right. The displacement waveform is a step-like function corresponding to the permanent change associated with the source process.

The analogous velocity and stress waveforms are essentially pulse-like. The width and height of the primitive pulse depend on the dynamics of the source process. Source processes such as microscopic crack jumps and precipitate fractures are usually completed in a fraction of a microsecond or a few microseconds, which explains why the pulse is short in duration. The amplitude and energy of the primitive pulse vary over an enormous range from submicroscopic dislocation movements to gross crack jumps.

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Primitive AE wave released at a source. The primitive wave is essentially a stress pulse corresponding to a permanent displacement of the material. The ordinate quantities refer to a point in the material.

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Waves radiates from the source in all directions, often having a strong directionality depending on the nature of the source process, as shown in the second figure. Rapid movement is necessary if a sizeable amount of the elastic energy liberated during deformation is to appear as an acoustic emission.

Angular dependence of acoustic emission radiated from a growing microcrack. Most of the energy is directed in the 90 and 270o directions, perpendicular to the crack surfaces.

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Most of the energy is directed in the 90ยบ and 270ยบ directions, perpendicular to the crack surfaces.

90ยบ

270ยบ Charlie Chong/ Fion Zhang

Angular dependence of acoustic emission radiated from a growing microcrack. Most of the energy is directed in the 90 and 270o directions, perpendicular to the crack surfaces.

As these primitive waves travel through a material, their form is changed considerably. Elastic wave source and elastic wave motion theories are being investigated to determine the complicated relationship between the AE source pulse and the corresponding movement at the detection site. The ultimate goal of studies of the interaction between elastic waves and material structure is to accurately develop a description of the source event from the output signal of a distant sensor.

However, most materials-oriented researchers and NDT inspectors are not concerned with the intricate knowledge of each source event. Instead, they are primarily interested in the broader, statistical aspects of AE. Because of this, they prefer to use narrow band (resonant) sensors which detect only a small portion of the broadband of frequencies emitted by an AE. These sensors are capable of measuring hundreds of signals each second, in contrast to the more expensive high-fidelity sensors used in source function analysis. More information on sensors will be discussed later in the Equipment section.

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The signal that is detected by a sensor is a combination of many parts of the waveform initially emitted. Acoustic emission source motion is completed in a few millionths of a second. As the AE leaves the source, the waveform travels in a spherically spreading pattern and is reflected off the boundaries of the object. Signals that are in phase with each other as they reach the sensor produce constructive interference which usually results in the highest peak of the waveform being detected. The typical time interval from when an AE wave reflects around the test piece (repeatedly exciting the sensor) until it decays, ranges from the order of 100 microseconds in a highly damped, nonmetallic material to tens of milliseconds in a lightly damped metallic material.

Decay Time: highly damped (intrinsic) , nonmetallic material → order of 100 microseconds (10-6 s) lightly damped metallic material → tens of milliseconds (10-3 s)

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Decay time

Decay Time: highly damped, nonmetallic material → order of 100 microseconds (s-6) lightly damped metallic material → tens of milliseconds (s-3)

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highly damped, nonmetallic material ~.0001 s lightly damped metallic material, ~.001 s.

Decay time

Decay Time: highly damped, nonmetallic material → order of 100 microseconds (10-6 s) lightly damped metallic material → tens of milliseconds (10-3 s)

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Attenuation The intensity of an AE signal detected by a sensor is considerably lower than the intensity that would have been observed in the close proximity of the source. This is due to attenuation.

There are three main causes of attenuation, (1) beginning with geometric spreading. As an AE spreads from its source in a plate-like material, its amplitude decays by 30% every time it doubles its distance from the source. In three-dimensional structures, the signal decays on the order of 50%. This can be traced back to the simple conservation of energy.

(2) Another cause of attenuation is material damping, as alluded 指出 to in the previous paragraph. While an AE wave passes through a material, its elastic and kinetic energies are absorbed and converted into heat. (σabs) (3) The third cause of attenuation is wave scattering. Geometric discontinuities (e.g. twin boundaries, nonmetallic inclusions, or grain boundaries) and structural boundaries both reflect some of the wave energy that was initially transmitted. (σscat)

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Attenuation: 1. Spread (30% for 2D, 50% for 3D for each doubling of distance from source), 2. Material damping, absorption.

3. Scattering (reflection & difrraction)

3

1

2 3

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Measurements of the effects of attenuation on an AE signal can be performed with a simple apparatus known as a Hsu-Nielson Source. This consists of a mechanical pencil with either 0.3 or 0.5 mm 2H lead that is passed through a cone-shaped Teflon shoe designed to place the lead in contact with the surface of a material at a 30 degree angle. When the pencil lead is pressed and broken against the material, it creates a small, local deformation that is relieved in the form of a stress wave, similar to the type of AE signal produced by a crack. By using this method, simulated AE sources can be created at various sites on a structure to determine the optimal position for the placement of sensors and to ensure that all areas of interest are within the detection range of the sensor or sensors.

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Teflon shoe

http://www.ndt.net/ndtaz/content.php?id=474

Wave Mode and Velocity As mentioned earlier, using AE inspection in conjunction with other NDE techniques can be an effective method in gauging the location and nature of defects. Since source locations are determined by the time required for the wave to travel through the material to a sensor, it is important that the velocity of the propagating waves be accurately calculated. This is not an easy task since wave propagation depends on the material in question and the wave mode being detected. For many applications, Lamb waves are of primary concern because they are able to give the best indication of wave propagation from a source whose distance from the sensor is larger than the thickness of the material. For additional information on Lamb waves, see the wave mode page in the Ultrasonic Inspection section.

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Lamb waves in acoustic emission testing Acoustic emission uses much lower frequencies than traditional ultrasonic testing, and the sensor is typically expected to detect active flaws at distances up to several meters. A large fraction of the structures customarily testing with acoustic emission are fabricated from steel plate - tanks, pressure vessels, pipes and so on. Lamb wave theory is therefore the prime theory for explaining the signal forms and propagation velocities that are observed when conducting acoustic emission testing. Substantial improvements in the accuracy of AE source location (a major techniques of AE testing) can be achieved through good understanding and skillful utilization of the Lamb wave body of knowledge.

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Ultrasonic and acoustic emission testing contrasted An arbitrary mechanical excitation applied to a plate will generate a multiplicity of Lamb waves carrying energy across a range of frequencies.

Such is the case for the acoustic emission wave.

In acoustic emission testing, the challenge is to recognize the multiple Lamb wave components in the received waveform and to interpret them in terms of source motion.

This contrasts with the situation in ultrasonic testing, where the first challenge is to generate a single, well-controlled Lamb wave mode at a single frequency.

But even in ultrasonic testing, mode conversion takes place when the generated Lamb wave interacts with flaws, so the interpretation of reflected signals compounded from multiple modes becomes a means of flaw characterization.

Plate or Lamb waves are similar to surface waves except they can only be generated in materials a few wavelengths thick.

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2.2.5 Rayleigh Characteristics Rayleigh waves are a type of surface wave that travel near the surface of solids. Rayleigh waves include both longitudinal and transverse motions that decrease exponentially in amplitude as distance from the surface increases.

There is a phase difference between these component motions. In isotropic solids these waves cause the surface particles to move in ellipses in planes normal to the surface and parallel to the direction of propagation – the major axis of the ellipse is vertical. At the surface and at shallow depths this motion is retrograde 逆行, that is the in-plane motion of a particle is counterclockwise when the wave travels from left to right. http://en.wikipedia.org/wiki/Rayleigh_wave

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Rayleigh waves are a type of surface acoustic wave that travel on solids.

They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in nondestructive testing for detecting defects. They are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

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Q29: The longitudinal wave incident angle which results in formation of a Rayleigh wave is called: A. B. C. D.

Normal incidence The first critical angle The second critical angle Any angle above the first critical angle

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Surface (or Rayleigh) waves travel the surface of a relatively thick solid material penetrating to a depth of one wavelength.

Surface waves combine both (1) a longitudinal and (2) transverse motion to create an elliptic orbit motion as shown in the image and animation below.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/rayleigh.swf Charlie Chong/ Fion Zhang

The major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an individual atom from the surface increases the width of its elliptical motion decreases. Surface waves are generated when a longitudinal wave intersects a surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves are useful because they are very sensitive to surface defects (and other surface features) and they follow the surface around curves.

Because of this, Rayleigh waves can be used to inspect areas that other waves might have difficulty reaching.

Wave velocity:  Longitudinal wave velocity =1v,  The velocity of shear waves through a material is approximately half that of the longitudinal waves, (≈0.5v)  Surface waves are generated when a longitudinal wave intersects a surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. ≈(0.87~0.95)x0.5v

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The major axis of the ellipse is perpendicular to the surface of the solid.

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Surface wave

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Surface wave or Rayleigh wave are formed when shear waves refract to 90.

The whip-like particle vibration of the shear wave is converted into elliptical motion by the particle changing direction at the interface with the surface. The wave are not often used in industrial NDT although they do have some application in aerospace industry. Their mode of propagation is elliptical along the surface of material, penetrating to a depth of one wavelength. They will follow the contour of the surface and they travel at approximately 90% of the velocity of the shear waves.

Depth of penetration of about one wavelength

Direction of wave propagation

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Surface wave has the ability to follow surface contour, until it meet a sharp change i.e. a surface crack/seam/lap. However the surface waves could be easily completely absorbed by excess couplant of simply touching the part ahead of the waves.

Transducer Wedge

Surface discontinuity

Specimen

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Surface wave - Following Contour

Surface wave

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Surface wave – One wavelength deep

λ

λ

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Rayleigh Wave

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http://web.ics.purdue.edu/~braile/edumod/waves/Rwave_files/image001.gif

Rayleigh Wave

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Love Wave

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http://web.ics.purdue.edu/~braile/edumod/waves/Lwave_files/image001.gif

Love Wave

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Surface (Rayleigh) waves are not as common as the longitudinal and shear waves, but are used to great advantage in a limited number of applications that require an ability of the wave to follow the contours of irregularly shaped surfaces such as jet engine blades and vanes.

Rayleigh waves extend from the surface to a depth of about one wavelength into the material and thus are only sensitive to surface or very near-surface flaws.

They are very sensitive to surface conditions including the presence of residual coupling compounds as well as finger damping.

Rayleigh waves are usually generated by mode conversion using angle beam search units designed to produce shear waves just beyond the second critical angle.

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Other Reading: Rayleigh Waves Surface waves (Rayleigh waves) are another type of ultrasonic wave used in the inspection of materials. These waves travel along the flat or curved surface of relatively thick solid parts. For the propagation of waves of this type, the waves must be traveling along an interface bounded on one side by the strong elastic forces of a solid and on the other side by the practically negligible elastic forces between gas molecules. Surface waves leak energy into liquid couplants and do not exist for any significant distance along the surface of a solid immersed in a liquid, unless the liquid covers the solid surface only as a very thin film. Surface waves are subject to attenuation in a given material, as are longitudinal or transverse waves. They have a velocity approximately 90% of the transverse wave velocity in the same material. The region within which these waves propagate with effective energy is not much thicker than about one wavelength beneath the surface of the metal.

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At this depth, wave energy is about 4% of the wave energy at the surface, and the amplitude of oscillation decreases sharply to a negligible value at greater depths. Surface waves follow contoured surfaces. For example, surface waves traveling on the top surface of a metal block are reflected from a sharp edge, but if the edge is rounded off, the waves continue down the side face and are reflected at the lower edge, returning to the sending point.

Surface waves will travel completely around a cube if all edges of the cube are rounded off. Surface waves can be used to inspect parts that have complex contours.

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Q110: What kind of wave mode travel at a velocity slightly below the shear wave and their modes of propagation are both longitudinal and transverse with respect to the surface? a) b) c) d)

Rayleigh wave Transverse wave L-wave Longitudinal wave

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Q: Which of the following modes of vibration exhibits the shortest wavelength at a given frequency and in a given material?

A. B. C. D.

longitudinal wave compression wave shear wave surface wave

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Q192: Surface waves are reduced to an energy level of approcimately 1/25 of the original power at a depth of ?

A. B. C. D.

25mm 102mm 1 wavelength 4 wavelength

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2.2.6 Lamb Wave: Lamb waves propagate in solid plates. They are elastic waves whose particle motion lies in the plane that contains the direction of wave propagation and the plate normal (the direction perpendicular to the plate). In 1917, the english mathematician horace lamb published his classic analysis and description of acoustic waves of this type. Their properties turned out to be quite complex. An infinite medium supports just two wave modes traveling at unique velocities; but plates support two infinite sets of lamb wave modes, whose velocities depend on the relationship between wavelength and plate thickness.

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Since the 1990s, the understanding and utilization of lamb waves has advanced greatly, thanks to the rapid increase in the availability of computing power. Lamb's theoretical formulations have found substantial practical application, especially in the field of nondestructive testing.

The term rayleigh–lamb waves embraces the rayleigh wave, a type of wave that propagates along a single surface. Both rayleigh and lamb waves are constrained by the elastic properties of the surface(s) that guide them. http://en.wikipedia.org/wiki/Lamb_wave http://pediaview.com/openpedia/Lamb_waves

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Types of Wave

New!  Plate wave- Love  Stoneley wave  Sezawa

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Plate or Lamb waves are the most commonly used plate waves in NDT. Lamb waves are complex vibrational waves that propagate parallel to the test surface throughout the thickness of the material. Propagation of Lamb waves depends on the density and the elastic material properties of a component. They are also influenced a great deal by the test frequency and material thickness. Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material. Lamb waves will travel several meters in steel and so are useful to scan plate, wire, and tubes.

Lamb wave influenced by: (Dispersive Wave) ■ ■ ■ ■

Density Elastic material properties Frequencies Material thickness

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Plate or Lamb waves are similar to surface waves except they can only be generated in materials a few wavelengths thick.

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http://www.ndt.net/ndtaz/files/lamb_a.gif

Plate wave or Lamb wave are formed by the introduction of surface wave into a thin material. They are a combination of (1) compression and surface or (2) shear and surface waves causing the plate material to flex by totally saturating the material. The two types of plate waves:

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With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves. Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate. This is sometimes called the extensional mode because the wave is “stretching and compressing” the plate in the wave motion direction. Wave motion in the symmetrical mode is most efficiently produced when the exciting force is parallel to the plate. The asymmetrical Lamb wave mode is often called the “flexural mode” because a large portion of the motion moves in a normal direction to the plate, and a little motion occurs in the direction parallel to the plate. In this mode, the body of the plate bends as the two surfaces move in the same direction.

The generation of waves using both piezoelectric transducers and electromagnetic acoustic transducers (EMATs) are discussed in later sections.

Keywords: Symmetrical = extensional mode Asymmetrical = flexural mode

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When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

Lamb waves – 2 modes

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Symmetrical = extensional mode Asymmetrical = flexural mode

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Symmetrical = extensional mode Asymmetrical = flexural mode

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Symmetrical = extensional mode

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Other Reading: Lamb Wave Lamb waves, also known as plate waves, are another type of ultrasonic wave used in the nondestructive inspection of materials. Lamb waves are propagated in plates (made of composites or metals) only a few wavelengths thick. A Lamb wave consists of a complex vibration that occurs throughout the thickness of the material. The propagation characteristics of Lamb waves depend on the density, elastic properties, and structure of the material as well as the thickness of the test piece and the frequency. Their behavior in general resembles that observed in the transmission of electromagnetic waves through waveguides.

There are two basic forms of Lamb waves: ď Ž Symmetrical, or dilatational ď Ž Asymmetrical, or bending

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The form is determined by whether the particle motion is symmetrical or asymmetrical with respect to the neutral axis of the test piece. Each form is further subdivided into several modes having different velocities, which can be controlled by the angle at which the waves enter the test piece.

Theoretically, there are an infinite number of specific velocities at which Lamb waves can travel in a given material. Within a given plate, the specific velocities for Lamb waves are complex functions of plate thickness and frequency.

In symmetrical (dilatational) Lamb waves, there is a compressional (longitudinal) particle displacement along the neutral axis of the plate and an elliptical particle displacement on each surface (Fig. 4a). In asymmetrical (bending) Lamb waves, there is a shear (transverse) particle displacement along the neutral axis of the plate and an elliptical particle displacement on each surface (Fig. 4b). The ratio of the major to minor axes of the ellipse is a function of the material in which the wave is being propagated.

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Fig. 4 Diagram of the basic patterns of (a) symmetrical (dilatational) and (b) asymmetrical (bending) Lamb waves. The wavelength, , is the distance corresponding to one complete cycle.

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Q1: The wave mode that has multiple or varying wave velocities is: A. B. C. D.

Longitudinal waves Shear waves Transverse waves Lamb waves

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2.2.7 Dispersive Wave: Wave modes such as those found in Lamb wave have a velocity of propagation dependent upon the operating frequency, sample thickness and elastic moduli. They are dispersive (velocity change with frequency) in that pulses transmitted in these mode tend to become stretched or dispersed.

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Dispersion refers to the fact that in a real medium such as water, air, or glass, a wave traveling through that medium will have a velocity that depends upon its frequency. Dispersion occurs for any form of wave, acoustic, electromagnetic, electronic, even quantum mechanical. Dispersion is responsible for a prism being able to resolve light into colors and defines the maximum frequency of broadband pulses one can send down an optical fiber or through a copper wire. Dispersion affects wave and swell forecasts at sea and influences the design of sound equipment. Dispersion is a physical property of the medium and can combine with other properties to yield very strange results. For example, in the propagation of light in an optical fiber, the glass introduces dispersion and separates the wavelengths of light according to frequency, however if the light is intense enough, it can interact with the electrons in the material changing its refractive index. The combination of dispersion and index change can cancel each other leading to a wave that can propagate indefinitely maintaining a constant shape. Such a wave has been termed a soliton.

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http://www.rpi.edu/dept/chem-eng/WWW/faculty/plawsky/Comsol%20Modules/DispersiveWave/DispersiveWave.html

Discussion Subject: Wave Mode and Velocity As mentioned earlier, using AE inspection in conjunction with other NDE techniques can be an effective method in gauging the location and nature of defects. Since source locations are determined by the time required for the wave to travel through the material to a sensor, it is important that the velocity of the propagating waves be accurately calculated. This is not an easy task

For many applications, Lamb waves are of primary concern because they are able to give the best indication of wave propagation from a source whose distance from the sensor is larger than the thickness of the material.

since wave propagation depends on the material in question and the wave mode being detected.

Question: from the additional reading, “Lamb waves, also known as plate waves, are another type of ultrasonic wave used in the nondestructive inspection of materials. Lamb waves are propagated in plates (made of composites or metals) only a few wavelengths thick�. Discuss on this statement.

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4.0 Equipment Acoustic emission testing can be performed in the field with portable instruments or in a stationary laboratory setting. Typically, systems contain a sensor, preamplifier, filter, and amplifier, along with measurement, display, and storage equipment (e.g. oscilloscopes, voltmeters, and personal computers). Acoustic emission sensors respond to dynamic motion that is caused by an AE event. This is achieved through transducers which convert mechanical movement into an electrical voltage signal. The transducer element in an AE sensor is almost always a piezoelectric crystal, which is commonly made from a ceramic such as Lead Zirconate Titanate (PZT).

Transducers are selected based on operating frequency, sensitivity and environmental characteristics, and are grouped into two classes: (1) resonant and (2) broadband.

The majority of AE equipment is responsive to movement in its typical operating frequency range of 30 kHz to 1 MHz. For materials with high attenuation (e.g. plastic composites), lower frequencies may be used to better distinguish AE signals. The opposite holds true as well.

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Key Points: • Two classes: resonant and broadband.

• The majority of AE equipment is responsive to movement in its typical operating frequency range of 30 kHz to 1 MHz. • For materials with high attenuation (e.g. plastic composites), lower frequencies may be used to better distinguish AE signals. The opposite holds true as well.

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The majority of AE equipment is responsive to movement in its typical operating frequency range of

30 kHz to 1 MHz

.

For materials with high attenuation (e.g. plastic composites), lower frequencies may be used to better distinguish AE signals. The opposite holds true as well.

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Q. The most common range of acoustic emission testing is?

A. 100-300KHz B. 10-15KHz C. 500-750KHz D. 1-5mHz What is the standard answer? (more reading) 2015/09/04, best guess “A”

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http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvU XJQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET

Equipment- Probes

Case

Damping materials

Electrode Piezoelectric element

Wear plate

Couplants Specimen

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Equipment- Probe

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Ideally, the AE signal that reaches the mainframe will be free of background noise and electromagnetic interference. Unfortunately, this is not realistic.

However, sensors and preamplifiers are designed to help eliminate unwanted signals. First, the preamplifier boosts the voltage to provide gain and cable drive capability. To minimize interference, a preamplifier is placed close to the transducer; in fact, many transducers today are equipped with integrated preamplifiers. Next, the signal is relayed to a bandpass filter for elimination of low frequencies (common to background noise) and high frequencies.

Following completion of this process, the signal travels to the acoustic system mainframe and eventually to a computer or similar device for analysis and storage. Depending on noise conditions, further filtering or amplification at the mainframe may still be necessary.

Schematic Diagram of a Basic Four-channel Acoustic Emission Testing System

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FIGURE 16.5 The main elements of a modern acoustic emission detection system.

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After passing the AE system mainframe, the signal comes to a detection/measurement circuit as shown in the figure directly above. Note that multiple-measurement circuits can be used in multiple sensor/channel systems for source location purposes (to be described later). At the measurement circuitry, the shape of the conditioned signal is compared with a threshold voltage value that has been programmed by the operator. Signals are either continuous (analogous to Gaussian, random noise with amplitudes varying according to the magnitude of the AE events) or burst-type. Each time the threshold voltage is exceeded, the measurement circuit releases a digital pulse. The first pulse is used to signify the beginning of a hit. (A hit is used to describe the AE event that is detected by a particular sensor. One AE event can cause a system with numerous channels to record multiple hits.) Pulses will continue to be generated while the signal exceeds the threshold voltage.

Once this process has stopped for a predetermined amount of time, the hit is finished (as far as the circuitry is concerned). The data from the hit is then read into a microcomputer and the measurement circuit is reset.

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Hit Driven AE Systems and Measurement of Signal Features Although several AE system designs are available (combining various options, sensitivity, and cost), most AE systems use a hit-driven architecture. The hitdriven design is able to efficiently measure all detected signals and record digital descriptions for each individual feature (detailed later in this section).

During periods of inactivity, the system lies dormant. Once a new signal is detected, the system records the hit or hits, and the data is logged for present and/or future display.

Also common to most AE systems is the ability to perform routine tasks that are valuable for AE inspection. These tasks include quantitative signal measurements with corresponding time and/or load readings, discrimination between real and false signals (noise), and the collection of statistical information about the parameters of each signal.

AET

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AET

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6.0 AE Signal Features With the equipment configured and setup complete, AE testing may begin.

The sensor is coupled to the test surface and held in place with tape or adhesive. An operator then monitors the signals which are excited by the induced stresses in the object. When a useful transient, or burst signal is correctly obtained, parameters like amplitude, counts, measured area under the rectified signal envelope (MARSE), duration, and rise time can be gathered. Each of the AE signal feature shown in the image is described below.

Abbreviation: measured area under the rectified signal envelope (MARSE)

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AET Signals

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Amplitude, A, is the greatest measured voltage in a waveform and is measured in decibels (dB). This is an important parameter in acoustic emission inspection because it determines the detectability of the signal.

Signals with amplitudes below the operator-defined, minimum threshold will not be recorded.

Rise time, R, is the time interval between the first threshold crossing and the signal peak. This parameter is related to the propagation of the wave between the source of the acoustic emission event and the sensor. Therefore, rise time is used for qualification of signals and as a criterion for noise filter.

Duration, D, is the time difference between the first and last threshold crossings. Duration can be used to identify different types of sources and to filter out noise. Like counts (N), this parameter relies upon the magnitude of the signal and the acoustics of the material.

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MARSE, E, sometimes referred to as energy counts, is the measure of the area under the envelope of the rectified linear voltage time signal from the transducer. This can be thought of as the relative signal amplitude and is useful because the energy of the emission can be determined. MARSE is also sensitive to the duration and amplitude of the signal, but does not use counts or user defined thresholds and operating frequencies. MARSE is regularly used in the measurements of acoustic emissions.

Counts, N, refers to the number of pulses emitted by the measurement circuitry if the signal amplitude is greater than the threshold. Depending on the magnitude of the AE event and the characteristics of the material, one hit may produce one or many counts. While this is a relatively simple parameter to collect, it usually needs to be combined with amplitude and/or duration measurements to provide quality information about the shape of a signal

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7.0 Data Display Software-based AE systems are able to generate graphical displays for analysis of the signals recorded during AE inspection. These displays provide valuable information about the detected events and can be classified into four categories: ■ ■ ■ ■

location, activity, intensity, and data quality (crossplots).

Location displays identify the origin of the detected AE events. These can be graphed by X coordinates, X-Y coordinates, or by channel for linear computed-source location, planar computed-source location, and zone location techniques.

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Examples of each graph are shown to the right.

Activity displays show AE activity as a function of time on an X-Y plot (figure below left).

Each bar on the graphs represents a specified amount of time. For example, a one-hour test could be divided into 100 time increments. All activity measured within a given 36 second interval would be displayed in a given histogram bar. Either axis may be displayed logarithmically in the event of high AE activity or long testing periods. In addition to showing measured activity over a single time period, cumulative activity displays (figure below right) can be created to show the total amount of activity detected during a test. This display is valuable for measuring the total emission quantity and the average rate of emission.

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Intensity displays are used to give statistical information concerning the magnitude of the detected signals. As can be seen in the amplitude distribution graph to the near right, the number of hits is plotted at each amplitude increment (expressed in dB’s) beyond the user-defined threshold. These graphs can be used to determine whether a few large signals or many small ones created the detected AE signal energy. In addition, if the Y-axis is plotted logarithmically, the shape of the amplitude distribution can be interpreted to determine the activity of a crack (e.g. a linear distribution indicates growth).

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The fourth category of AE displays, crossplots, is used for evaluating the quality of the data collected. Counts versus amplitude, duration versus amplitude, and counts versus duration are frequently used crossplots. As shown in the final figure, each hit is marked as a single point, indicating the correlation between the two signal features. The recognized signals from AE events typically form a diagonal band since larger signals usually generate higher counts. Because noise signals caused by electromagnetic interference do not have as many threshold-crossing pulses as typical AE source events, the hits are located below the main band. Conversely, signals caused by friction or leaks have more threshold-crossing pulses than typical AE source events and are subsequently located above the main band. In the case of ambiguous data, expertise is necessary in separating desirable Charlie Chong/ Fion Zhang

Amplitude/counts Signal Analysis

The recognized signals from AE events typically form a diagonal band since larger signals usually generate higher counts. Because noise signals caused by electromagnetic interference do not have as many threshold-crossing pulses as typical AE source events,

Conversely, signals caused by friction or leaks have more threshold-crossing pulses than typical AE source events and are subsequently located above the main band.

Because noise signals caused by electromagnetic interference do not have as many threshold-crossing pulses as typical AE source events, the hits are located below the main band Charlie Chong/ Fion Zhang

8.0 AE Source Location Techniques Multi-Channel Source Location Techniques: Locating the source of significant acoustic emissions is often the main goal of an inspection. Although the magnitude of the damage may be unknown after AE analysis, follow up testing at source locations can provide these answers.

As previously mentioned, many AE systems are capable of using multiple sensors/channels during testing, allowing them to record a hit from a single AE event. These AE systems can be used to determine the location of an event source. As hits are recorded by each sensor/channel, the source can be located by knowing the velocity of the wave in the material and the difference in hit arrival times among the sensors, as measured by hardware circuitry or computer software. By properly spacing the sensors in this manner, it is possible to inspect an entire structure with relatively few sensors.

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Source location techniques assume that AE waves travel at a constant velocity in a material. However, various effects may alter the expected velocity of the AE waves (e.g. reflections and multiple wave modes) and can affect the accuracy of this technique. Therefore, the geometric effects of the structure being tested and the operating frequency of the AE system must be considered when determining whether a particular source location technique is feasible for a given test structure.

Keywords: â– Reflections and multiple wave modes â– Geometric effects

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â– Linear Location Technique Several source location techniques have been developed based on this method.

One of the commonly used computedsource location techniques is the linear location principle shown to the right.

Linear location is often used to evaluate struts on truss bridges. When the source is located at the midpoint, the time of arrival difference for the wave at the two sensors is zero. If the source is closer to one of the sensors, a difference in arrival times is measured.

To calculate the distance of the source location from the midpoint, the arrival time is multiplied by the wave velocity. Whether the location lies to the right or left of the midpoint is determined by which sensor first records the hit.

This is a linear relationship and applies to any event sources between the sensors.

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Because the above scenario implicitly assumes that the source is on a line passing through the two sensors, it is only valid for a linear problem. When using AE to identify a source location in a planar material, three or more sensors are used, and the optimal position of the source is between the sensors. Two categories of source location analysis are used for this situation: zonal location and point location.

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â– Zonal Location Technique As the name implies, zonal location aims to trace the waves to a specific zone or region around a sensor.

This method is used in anisotropic materials or in other structures where sensors are spaced relatively far apart or when high material attenuation affects the quality of signals at multiple sensors. Zones can be lengths, areas or volumes depending on the dimensions of the array. A planar sensor array with detection by one sensor is shown in the upper right figure. The source can be assumed to be within the region and less than halfway between sensors.

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When additional sensors are applied, (1) arrival times and (2) amplitudes help pinpoint the source zone. The ordered pair in lower right figure represents the two sensors detecting the signal in the zone and the order of signal arrival at each sensor. When relating signal strength to peak amplitude, the largest peak amplitude is assumed to come from the nearest sensor, second largest from the next closest sensor and so forth.

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â– Point Location In order for point location to be justified, signals must be detected in a minimum number of sensors: (1) two for linear, (2) three for planar, (3) four for volumetric. Accurate arrival times must also be available. Arrival times are often found by using (a) peak amplitude or the (b) first threshold crossing. The velocity of wave propagation and exact position of the sensors are necessary criteria as well. Equations can then be derived using sensor array geometry or more complex algebra to locate more specific points of interest.

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9.0 AE Barkhausen Techniques The Barkhausen effect The Barkhausen effect refers to the sudden change in size of ferromagnetic domains that occur during magnetization or demagnetization. During magnetization, favorably oriented domains develop at the cost of less favorably oriented domains.

These two factors result in minute jumps of magnetization when a ferromagnetic sample (e.g. iron) is exposed to an increasing magnetic field (see figure).

Domain wall motion itself is determined by many factors like microstructure, grain boundaries, inclusions, and stress and strain. By the same token, the Barkhausen effect is too a function of stress and strain.

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Barkhausen Noise Barkhausen noise can be heard if a coil of wire is wrapped around the sample undergoing magnetization. Abrupt movements in the magnetic field produce spiking current pulses in the coil. When amplified, the clicks can be compared to Rice Krispies or the crumbling a candy wrapper. The amount of Barkhausen noise is influenced by material imperfections and dislocations and is likewise dependent on the mechanical properties of a material.

Currently, materials exposed to high energy particles (nuclear reactors) or cyclic mechanical stresses (pipelines) are available for nondestructive evaluation using Barkhausen noise, one of the many branches of AE testing.

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Hysterisis Loop- magnetization or demagnetization.

Barkhausen noise generated if the magnetic field was induced on the areas with discontinuiies (throughout the whole loop)

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10. Applications Acoustic emission is a very versatile, non-invasive way to gather information about a material or structure. Acoustic Emission testing (AET) is be applied to inspect and monitor pipelines, pressure vessels, storage tanks, bridges, aircraft, and bucket trucks, and a variety of composite and ceramic components. It is also used in process control applications such as monitoring welding processes. A few examples of AET applications follow. â– Weld Monitoring During the welding process, temperature changes induce stresses between the weld and the base metal. These stresses are often relieved by heat treating the weld. However, in some cases tempering the weld is not possible and minor cracking occurs. Amazingly, cracking can continue for up to 10 days after the weld has been completed. Using stainless steel welds with known inclusions and accelerometers for detection purposes and background noise monitoring, it was found by W. D. Jolly (1969) that low level signals and more sizeable bursts were related to the growth of microfissures and larger cracks respectively. ASTM E 749-96 is a standard practice of AE monitoring of continuous welding.

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â– Bucket Truck (Cherry Pickers) Integrity Evaluation Accidents, overloads and fatigue can all occur when operating bucket trucks or other aerial equipment. If a mechanical or structural defect is ignored, serious injury or fatality can result. In 1976, the Georgia Power Company pioneered the aerial manlift device inspection. Testing by independent labs and electrical utilities followed. Although originally intended to examine only the boom sections, the method is now used for inspecting the pedestal, pins, and various other components. Normally, the AE tests are second in a chain of inspections which start with visual checks. If necessary, follow-up tests take the form of magnetic particle, dye penetrant, or ultrasonic inspections.

Experienced personnel can perform five to ten tests per day, saving valuable time and money along the way. ASTM F914 governs the procedures for examining insulated aerial personnel devices.

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AET Application

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â– Gas Trailer Tubes Acoustic emission testing on pressurized jumbo tube trailers was authorized by the Department of Transportation in 1983. Instead of using hydrostatic retesting, where tubes must be removed from service and disassembled, AET allows for in situ testing. A 10% over-pressurization is performed at a normal filling station with AE sensors attached to the tubes at each end. A multichannel acoustic system is used to detection and mapped source locations. Suspect locations are further evaluated using ultrasonic inspection, and when defects are confirmed the tube is removed from use. AET can detect subcritical flaws whereas hydrostatic testing cannot detect cracks until they cause rupture of the tube. Because of the high stresses in the circumferential direction of the tubes, tests are geared toward finding longitudinal fatigue cracks.

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â– Bridges Bridges contain many welds, joints and connections, and a combination of load and environmental factors heavily influence damage mechanisms such as fatigue cracking and metal thinning due to corrosion. Bridges receive a visual inspection about every two years and when damage is detected, the bridge is either shut down, its weight capacity is lowered, or it is singled out for more frequent monitoring. Acoustic Emission is increasingly being used for bridge monitoring applications because it can continuously gather data and detect changes that may be due to damage without requiring lane closures or bridge shutdown. In fact, traffic flow is commonly used to load or stress the bridge for the AE testing.

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â– Aerospace Structures Most aerospace structures consist of complex assemblies of components that have been design to carry significant loads while being as light as possible. This combination of requirements leads to many parts that can tolerate only a minor amount of damage before failing. This fact makes detection of damage extremely important but components are often packed tightly together making access for inspections difficult. AET has found applications in monitoring the health of aerospace structures because sensors can be attached in easily accessed areas that are remotely located from damage prone sites. AET has been used in laboratory structural tests, as well as in flight test applications. NASA's Wing Leading Edge Impact Detection System is partially based on AE technology. The image to the right (above) shows a technician applying AE transducers on the inside of the Space Shuttle Discovery wing structure. The impact detection system was developed to alert NASA officials to events such as the sprayed-on-foam insulation impact that damaged the Space Shuttle Columbia's wing leading edge during launch and lead to its breakup on reentry to the Earth's atmosphere.

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Others  Fiber-reinforced polymer-matrix composites, in particular glass-fiber reinforced parts or structures (e.g. fan blades)  Material research (e.g. investigation of material properties, breakdown mechanisms, and damage behavior)  Inspection and quality assurance, (e.g. wood drying processes, scratch tests)  Real-time leakage test and location within various components (small valves, steam lines, tank bottoms)  Detection and location of high-voltage partial discharges in transformers  Railroad tank car and rocket motor testing There are a number of standards and guidelines that describe AE testing and application procedures as supplied by the American Society for Testing and Materials (ASTM). Examples are ASTM E 1932 for the AE examination of small parts and ASTM E1419-00 for the method of examining seamless, gas-filled, pressure vessels.

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End of Reading 3

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Study Note 4: ASTM E1316 Term & Definitions

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http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Section B: Acoustic Emission (E750, E1067, and E1118) The boldface designations in parentheses indicate the standards from which the terms in that section were derived.

The terms defined in Section B are the direct responsibility of Subcommittee E07.04 on Acoustic Emission Method.

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E1316-05

• acoustic emission (AE)- the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient waves so generated. Acoustic emission is the recommended term for general use. Other terms that have been used in AE literature include (1) stress wave emission, (2) microseismic activity, and (3) emission or acoustic emission with other qualifying modifiers.

• Acoustic emission channel- see channel, acoustic emission.

• acoustic emission count (emission count) (N)- see count, acoustic emission.

• Acoustic emission count rate- see count rate, acoustic emission (emission rate or count rate) (N ).

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E1316-05

• acoustic emission event- see event, acoustic emission.

• acoustic emission event energy- see energy, acoustic event.

• Acoustic emission sensor- see sensor, acoustic emission.

• acoustic emission signal amplitude- see signal amplitude, acoustic emission.

• acoustic emission signal (emission signal)- see signal, acoustic emission.

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E1316-05

• acoustic emission signature (signature)- see signature, acoustic emission.

• • acoustic emission transducer- see sensor, acoustic emission.

• Acoustic emission waveguide- see waveguide, acoustic emission.

• acousto-Ultrasonics (AU)- a nondestructive examination method that uses induced stress waves to detect and assess diffuse defect states, damage conditions, and variations of mechanical properties of a test structure. The AU method combines aspects of acoustic emission (AE) signal analysis with ultrasonic materials characterization techniques.

• adaptive location- source location by iterative 反复的 use of simulated sources in combination with computed location.

• AE activity, n- the presence of acoustic emission during a test.

• AE amplitude- See dBAE.

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• AE rms, n- the rectified, time averaged AE signal, measured on a linear scale and reported in volts.

• AE signal duration- the time between AE signal start and AE signal end.

• AE signal end- the recognized termination of an AE signal, usually defined as the last crossing of the threshold by that signal.

• AE signal generator- a device which can repeatedly induce a specified transient signal into an AE instrument.

• AE signal rise time- the time between AE signal start and the peak amplitude of that AE signal.

• AE signal start- the beginning of an AE signal as recognized by the system processor, usually defined by an amplitude excursion 远足/旅途/前进 exceeding threshold.

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• array, n- a group of two or more AE sensors positioned on a structure for the purposes of detecting and locating sources. The sources would normally be within the array.

• arrival time interval (∆tij)- see interval, arrival time.

• attenuation, n- the decrease in AE amplitude per unit distance, normally expressed in dB per unit length.

• average signal level, n- the rectified, time averaged AE logarithmic signal, measured on the AE amplitude logarithmic scale and reported in dBae units (where 0 dBae refers to 1 μV at the preamplifier input).

• burst emission- see emission, burst.

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• channel, acoustic emission- an assembly of a sensor, preamplifier or impedance matching transformer, filters secondary amplifier or other instrumentation as needed, connecting cables, and detector or processor.

NOTE 2- A channel for examining fiberglass reinforced plastic (FRP) may utilize more than one sensor with associated electronics. Channels may be processed independently or in predetermined groups having similar sensitivity and frequency characteristics.

0 dB= 0 = 20log (I/Io), (I/Io) = 1 (no attenuation)

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• continuous emission- see emission, continuous.

• count, acoustic emission (emission count) (N)- the number of times the acoustic emission signal exceeds (crossing) a preset threshold during any selected portion of a test.

• count, event (Ne)- the number obtained by counting each discerned 分清 acoustic emission event once.

• count rate, acoustic emission (emission rate or count rate) (N)- the time rate at which emission counts occur. (N/s?)

• count, ring-down- see count, acoustic emission, the preferred term.

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• couplant- a material used at the structure-to-sensor interface to improve the transmission of acoustic energy across the interface during acoustic emission monitoring. • cumulative (acoustic emission) amplitude distribution F(V)- see distribution, amplitude, cumulative. • cumulative (acoustic emission) threshold crossing distribution Ft(V)- see distribution, threshold crossing, cumulative.

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• dBAE - a logarithmic measure of acoustic emission signal amplitude, referenced to 1 μV at the sensor, before amplification.

Signal peak amplitude dBAE (dBAE) = (dB1μV at sensor) = 20 log10(A1/Ao)

(1)

where: Ao = 1 μV at the sensor (before amplification), and A1 = peak voltage of the measured acoustic emission signal (also before amplification).

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Acoustic Emission Reference Scale: dBAE Value 0 20 40 60 80 100

Voltage at Sensor 1μV 10 μV 100 μV 1 mV 10 mV 100 mV

DISCUSSION- In the case of sensors with integral preamplifiers, the Ao reference is before internal amplification.

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AE signal amplitude measured as a ratio of 1ÎźV in dBAE

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• dead time- any interval during data acquisition when the instrument or system is unable to accept new data for any reason. (E 750)3 • differential (acoustic emission) amplitude distribution F(V)- see distribution, differential (acoustic emission) amplitude f(V).

• differential (acoustic emission) threshold crossing distribution ft(V)- see distribution, differential (acoustic emission) threshold crossing.

• distribution, amplitude, cumulative (acoustic emission) F(V)- the number of acoustic emission events with signals that exceed an arbitrary amplitude as a function of amplitude V. • distribution, threshold crossing, cumulative (acoustic emission) Ft (V)- the number of times the acoustic emission signal exceeds an arbitrary threshold as a function of the threshold voltage (V).

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• distribution, differential (acoustic emission) amplitude f(V)- the number of acoustic emission events with signal amplitudes between amplitudes of V and V + ∆V as a function of the amplitude V. f(V) is the absolute value of the derivative of the cumulative amplitude distribution F(V).

• distribution, differential (acoustic emission) threshold crossing ft (V)- The number of times the acoustic emission signal waveform has a peak between thresholds V and V + ∆V as a function of the threshold V. ft(V) is the absolute value of the derivative of the cumulative threshold crossing distribution Ft(V).

• distribution, logarithmic (acoustic emission) amplitude g(V)- the number of acoustic emission events with signal amplitudes between V and α V (where α is a constant multiplier) as a function of the amplitude. This is a variant of the differential amplitude distribution, appropriate for logarithmically windowed data.

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• dynamic range- the difference, in decibels, between the overload level and the minimum signal level (usually fixed by one or more of the noise levels, low-level distortion, interference, or resolution level) in a system or sensor.

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effective velocity, n- velocity calculated on the basis of arrival times and propagation distances determined by artificial AE generation; used for computed location. emission, burst- a qualitative description of the discrete signal related to an individual emission event occurring within the material.

NOTE 3- Use of the term burst emission is recommended only for describing the qualitative appearance of emission signals. Fig. 1 shows an oscilloscope trace of burst emission signals on a background of continuous emission. emission, continuous- a qualitative description of the sustained signal level produced by rapidly occurring acoustic emission from structural sources, leaks, or both.

NOTE 4- Use of the term continuous emission is recommended only for describing the qualitative appearance of emission signals. Fig. 2 and Fig. 3 show oscilloscope traces of continuous emission signals at two different sweep rates.

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FIG. 1 Burst Emission on a Continuous Emission Background. (Sweep Rate5 ms/cm.)

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FIG. 1 Burst Emission on a Continuous Emission Background. (Sweep Rate5 ms/cm.)

Burst Emission

Continuous Emission Background

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FIG. 2 Continuous Emission. (Sweep Rate- 5 ms/cm.)

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FIG. 3 Continuous Emission. (Sweep Rate- 0.1 ms/cm.)

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energy, acoustic emission event- the total elastic energy released by an emission event. energy, acoustic emission signal- the energy contained in a detected acoustic emission burst signal, with units usually reported in joules and values which can be expressed in logarithmic form (dB, decibels). evaluation threshold- a threshold value used for analysis of the examination data. Data may be recorded with a system examination threshold lower than the evaluation threshold. For analysis purposes, dependence of measured data on the system examination threshold must be taken into consideration. event, acoustic emission (emission event)- a local material change giving rise to acoustic emission. event count (Ne)- see count, event. event count rate (NË™ e)- see rate, event count.

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examination area- that portion of a structure being monitored with acoustic emission. examination region- that portion of the test article evaluated using acoustic emission technology.

Felicity effect- the presence of acoustic emission, detectable at a fixed predetermined sensitivity level at stress levels below those previously applied.

(E 1067) Felicity ratio- the ratio of the stress at which the Felicity effect occurs to the previously applied maximum stress. (E 1067, E 1118) NOTE 5- The fixed sensitivity level will usually be the same as was used for the previous loading or test. (E 1118)

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instrumentation dead time- see dead time, instrumentation. first hit location- a zone location method defined by which a channel among a group of channels first detects the signal. (the location of the channel?

Probe?) floating threshold- any threshold with amplitude established by a time average measure of the input signal. (E 750) hit- the detection and measurement of an AE signal on a channel. interval, arrival time (∆tij)- the time interval between the detected arrivals of an acoustic emission wave at the ith and jth sensors of a sensor array.

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Kaiser effect- the absence of detectable acoustic emission at a fixed sensitivity level, until previously applied stress levels are exceeded. location accuracy, n- a value determined by comparison of the actual position of an AE source (or simulated AE source) to the computed location. location, cluster, n- a location technique based upon a specified amount of AE activity located within a specified length or area, for example: 5 events within 12 linear inches or 12 square inches.

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location, computed, n- a source location method based on algorithmic analysis of the difference in arrival times among sensors.

NOTE 6- Several approaches to computed location are used, including linear location, planar location, three dimensional location, and adaptive location. a) linear location, n- one dimensional source location requiring two or more channels. b) planar location, n- two dimensional source location requiring three or more channels. c) 3D location, n- three dimensional source location requiring five or more channels. d) adaptive location, n- source location by iterative 反复的/叠代的 use of simulated sources in combination with computed location.

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Linear, Planar, 3D 3D Linear

Planar

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3D

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location, continuous AE signal, n- a method of location based on continuous AE signals, as opposed to hit or difference in arrival time location methods.

NOTE 7- This type of location is commonly used in leak location due to the presence of continuous emission. Some common types of continuous signal location methods include signal attenuation and correlation analysis methods. (a) signal attenuation-based source location, n- a source location method that relies on the attenuation versus distance phenomenon of AE signals. By monitoring the AE signal magnitudes of the continuous signal at various points along the object, the source can be determined based on the highest magnitude or by interpolation or extrapolation of multiple readings. (b) correlation-based source location, n- a source location method that compares the changing AE signal levels (usually waveform based amplitude analysis) at two or more points surrounding the source and determines the time displacement of these signals.

The time displacement data can be used with conventional hit based location techniques to arrive at a solution for the source site.

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NOTE 7- This type of location is commonly used in leak location due to the presence of continuous emission. Some common types of continuous signal location methods include signal attenuation and correlation analysis methods. (a)

(b)

signal attenuation-based source location, n- a source location method that relies on the attenuation versus distance phenomenon of AE signals. By monitoring the AE signal magnitudes of the continuous signal at various points along the object, the source can be determined based on the highest magnitude or by interpolation or extrapolation of multiple readings. correlation-based source location, n- a source location method that compares the changing AE signal levels (usually waveform based amplitude analysis) at two or more points surrounding the source and determines the time displacement of these signals. The time displacement data can be used with conventional hit based location techniques to arrive at a solution for the source site.

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Methods of Location  Hit method  Differential time method  Continuous method - signal attenuation-based source location - correlation-based source location

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location, source, n- any of several methods of evaluating AE data to determine the position on the structure from which the AE originated. Several approaches to source location are used, including zone location, computed location, and continuous location. location, zone, n- any of several techniques for determining the general region of an acoustic emission source (for example, total AE counts, energy, hits, and so forth).

NOTE 8- Several approaches to zone location are used, including independent channel zone location, first hit zone location, and arrival sequence zone location. (a) independent channel zone location, n- a zone location technique that compares the gross amount of activity from each channel. (b) first-hit zone location, n- a zone location technique that compares only activity from the channel first detecting the AE event. (c) arrival sequence zone location, n- a zone location technique that compares the order of arrival among sensors.

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logarithmic (acoustic emission) amplitude distribution g(V)- see distribution, logarithmic (acoustic emission) amplitude. overload recovery time- an interval of nonlinear operation of an instrument caused by a signal with amplitude in excess of the instrument’s linear operating range. performance check, AE system- see verification, AE system. pressure, design- pressure used in design to determine the required minimum thickness and minimum mechanical properties. processing capacity- the number of hits that can be processed at the processing speed before the system must interrupt data collection to clear buffers or otherwise prepare for accepting additional data. processing speed- the sustained rate (hits/s), as a function of the parameter set and number of active channels, at which AE signals can be continuously processed by a system without interruption for data transport.

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˙e)- the time rate of the event count. rate, event count (N rearm delay time- see time, rearm delay. ring-down count- see count, acoustic emission, the preferred term. sensor, acoustic emission- a detection device, generally piezoelectric, that transforms the particle motion produced by an elastic wave into an electrical signal. signal, acoustic emission (emission signal)- an electrical signal obtained by detection of one or more acoustic emission events. signal amplitude, acoustic emission- the peak voltage of the largest excursion attained by the signal waveform from an emission event.

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signal overload level- that level above which operation ceases to be satisfactory as a result of signal distortion, overheating, or damage. signal overload point- the maximum input signal amplitude at which the ratio of output to input is observed to remain within a prescribed linear operating range. signal strength- the measured area of the rectified AE signal with units proportional to volt-sec. (?)

DISCUSSION- The proportionality constant is specified by the AE instrument manufacturer. signature, acoustic emission (signature)- a characteristic set of reproducible attributes of acoustic emission signals associated with a specific test article as observed with a particular instrumentation system under specified test conditions.

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signature, acoustic emission (signature)- a characteristic set of reproducible attributes of acoustic emission signals associated with a specific test article as observed with a particular instrumentation system under specified test conditions.

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stimulation- the application of a stimulus such as force, pressure, heat, and so forth, to a test article to cause activation of acoustic emission sources. system examination threshold- the electronic instrument threshold (see evaluation threshold) which data will be detected. transducers, acoustic emission- see sensor, acoustic emission. verification, AE system (performance check, AE system)- the process of testing an AE system to assure conformance to a specified level of performance or measurement accuracy. (This is usually carried out prior to, during and/or after an AE examination with the AE system connected to the examination object, using a simulated or artificial acoustic emission source.)

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voltage threshold—a voltage level on an electronic comparator such that signals with amplitudes larger than this level will be recognized. The voltage threshold may be user adjustable, fixed, or automatic floating. (E 750) waveguide, acoustic emission—a device that couples elastic energy from a structure or other test object to a remotely mounted sensor during AE monitoring. An example of an acoustic emission waveguide would be a solid wire of rod that is coupled at one end to a monitored structure, and to a sensor at the other end.

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End of Reading 4

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Study Note 5: Q&A

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Q1. The most common range of acoustic emission testing is?

A. 100-300KHz B. 10-15KHz C. 500-750KHz D. 1-5mHz Q2. Discontinuities that are readily detectable by acoustic emission testing are: A. all of the above.

B. leaks.

C. plastic deformation.

D. growing cracks.

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Q3. The total energy loss of a propagating wave is called: A. attenuation.

B. dispersion.

C. diffraction.

D. scatter.

Q4. The Kaiser effect refers to: A. the behavior where emission from a source will not occur until the previous load is exceeded.

B. velocity changes due to temperature changes.

C. low amplitude emissions from aluminum structures.

D. none of the above.

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Q5. The felicity effect is useful in evaluating: A. fiber-reinforced plastic components.

B. high alloy steel castings.

C. large structural steel members. (?)

D. ceramics.

Q6. The Kaiser effect is useful in distinguishing: A. mechanical noise from growing discontinuities.

B. electrical noise from mechanical noise.

C. electrical noise from growing discontinuities.

D. electrical noise from continuous emissions.

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Q7. The terms ""counts"" refers to: A. the number of times a signal crosses a preset threshold.

B. the number of events from a source.

C. the number of transducers required to perform a test.

D. none of the above.

Q8. The acoustic emission signal amplitude is related to: A. the intensity of the source. (as well as source nearness to the transducer?)

B. the preset threshold.

C. the band pass filters.

D. background noises.

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Q9. Threshold settings are determined by: A. the background noise level.

B. the test duration.

C. the attenuation of the material.

D. the graininess of the material.

Q10. Background noise can be reduced by: A. electronic filtering.

B. using flat response amplifiers.

C. using in-line amplifiers.

D. using heavier gauge coaxial cable.

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Q11. What is the Dunegan Corollary?

A. It states that if acoustic emissions are observed prior to a previous maximum load, some type of new damage must have occurred.

B. When the applied load is high enough to cause significant emissions even though the previous maximum load was not reached. (felicity effect) C. Gauging signal arrival times or differences in the spectral content of true AE signals and background noise.

D. the number of times a signal crosses a preset threshold. (count, n)

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Q12. What is the Felicity Effect?

A. When the applied load is high enough to cause significant emissions even though the previous maximum load was not reached.

B. Gauging signal arrival times or differences in the spectral content of true AE signals and background noise.

C. It states that if acoustic emissions are observed prior to a previous maximum load, some type of new damage must have occurred.

D. the number of times a signal crosses a preset threshold.

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Q13. The Felicity Ratio is: A. The load where considerable AE resumes, divided by the maximum applied load (F/D).

B. Gauging signal arrival times or differences in the spectral content of true AE signals and background noise.

C. It states that if acoustic emissions are observed prior to a previous maximum load, some type of new damage must have occurred.

D. the number of times a signal crosses a preset threshold.

Q14. Examples of electronic filtering: A. Gauging signal arrival times or differences in the spectral content of true AE signals and background noise.

B. using in-line amplifiers.

C. using flat response amplifiers.

D. an electronic filter.

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Q15. A Hsu-Nielson Source: A. measures the effects of attenuation on an AE signal.

B. using in-line amplifiers.

C. using flat response amplifiers.

D. an electronic filter.

Q16. Two types of AE transducers are: A. resonant and broadband.

B. barium and silica C. active and passive.

D. low frequency and high frequency.

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Q17. The most common AE transducer element is made of: A. lead zirconate titanate (PZT).

B. barium titanate C. Quartz D. barium sulfide.

Q18. The term ""MARSE"" refers to: A. the measure of the area under the envelope of the rectified linear voltage time signal from the transducer.

B. the number of events from a source.

C. the number of transducers required to perform a test.

D. the number of times a signal crosses a preset threshold.

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Q19. The term ""rise time"" refers to: A. the time interval between the first threshold crossing and the signal peak.

B. the number of events from a source.

C. the measure of the area under the envelope of the rectified linear voltage time signal from the transducer.

D. the number of times a signal crosses a preset threshold.

Q20. The term ""duration"" refers to: A. is the time difference between the first and last threshold crossings.

B. the number of events from a source.

C. the number of transducers required to perform a test.

D. low frequency and high frequency.

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Q21. The term ""amplitude"" refers to: A. is the greatest measured voltage in a waveform and is measured in decibels (dB).

B. is the time difference between the first and last threshold crossings.

C. the measure of the area under the envelope of the rectified linear voltage time signal from the transducer.

D. the time interval between the first threshold crossing and the signal peak.

Q22. AE displays provide valuable information about the detected events and can be classified into four categories: A. location, activity, intensity, and data quality (crossplots).

B. X,Y,Z, and L.A scan, B scan, C, scan, and Z scan.

C. Class 1, Class 2, Class 3, and Class 4

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Q23. Four types of AE Source Location Techniques: A. Multi-source location, Linear Location, Zonal location, and Point Location.

B. A scan, B scan, C, scan, and Z scan.

C. location, activity, intensity, and data quality (crossplots).

D. Source location, Zonal location, Point Location, and Linear location.

Q24. The term ""Barkhausen Noise"" refers to: A. the sudden change in size of ferromagnetic domains that occur during magnetization or demagnetization.

B. low amplitude emissions from aluminum structures.

C. the behavior where emission from a source will not occur until the previous load is exceeded.

D. velocity changes due to temperature changes

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End of Reading 5

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Study Note 6: High Strength Steel- TWIP Steel (Twinning as source of Acoustic Emission)

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Multi Phase Twinning-Induced Plasticity (TWIP) Steel (Korean Article ) The iron-manganese TWIP steels, which contain 17-20% of manganese, derive their exceptional properties from a specific strengthening mechanism: twinning.

The iron-manganese TWIP steels, which contain 17-20% of manganese, derive their exceptional properties from a specific strengthening mechanism: twinning. The steels are fully austenitic and nonmagnetic, with no phase transformation. The formation of mechanical twins during deformation generates high strain hardening, preventing necking and thus maintaining a very high strain capacity.

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The properties of different steels are determined by their crystal lattice structures, that is the spatial arrangement of their atoms. Adding alloying elements makes certain crystal structures more likely to form which allows the properties of the steel to be fine-tuned. It is concluded from thermodynamic calculations that a combination of manganese, silicon and aluminum would probably be suitable for the development of the new lightweight construction steel. These elements are lighter than iron and they force the crystal lattice into certain structures: iron can switch between different crystal lattices, or iron atoms can switch their positions and form different arrangements in the crystal lattices.

There is, for example, an FCC.: face-centered cubic arrangement, known as "austenite". In this case, the iron atoms sit on the corners of the crystal lattice cube with an atom in the center of each face of the cube. Then there is the BCC.: body-centered cubic layout. Again, the iron atoms are arranged on the corners, but with another one in the cube's center. There is also a type in which the iron atoms are distributed in a hexagonal arrangement. The bodycentered cubic and the hexagonal forms are both traditionally referred to as martensite. The crystal lattice changes, and with it, the character of the steel, depending on the alloy element content (the alien atoms in the crystal lattice).

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Conventional high strength steels were manufactured by adding the alloying elements such as Nb, Ti, V, and/or P in low carbon or IF (interstitial free) steels. These steels can be manufactured under the relatively simple processing conditions and have widely been applied for weight reduction.

However, as the demands for weight reduction are further increased, new families of high strength steel have been developed. These new steels grades include DP (dual phase), TRIP (TRansformation Induced Plasticity), FB (ferrite-bainite), CP (complex phase) and TWIP (TWin Induced Plasticity) steels.

The critical part of the steel manufacturing steels is to control the processing parameters so that the microstructure and, hence, the strength-elongation balance could be optimized. Various high added value products are developed to satisfy increasing customer demands, as shown in Figure 1.

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Keywords:  DP (dual phase),  TRIP (TRansformation Induced Plasticity),  FB (ferrite-bainite),  CP (complex phase) and  TWIP (TWin Induced Plasticity) steels.

The critical part of the steel manufacturing steels is to control the processing parameters so that the microstructure and, hence, the strength-elongation balance could be optimized.

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Figure 1: Ductility-strength relationship of mild and high strength steels

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Figure 1: Ductility-strength relationship of mild and high strength steels (M)

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Recently, new group of austenitic steels with 15-25 percent of manganese contents and 3 percent of aluminum and silicon has been developed for automotive use. This group is divided into transformation induced plasticity steels (HMS-TRIP) and twinning induced plasticity steels (HMS-TWIP) due to the characteristic phenomena occurring during plastic deformation inside the grains.

At 700 MPa, the TRIP steels are also exceptionally strong. However, their ductility is moderate, at approximately 35 percent. This characteristic – ductile yet strong – is the result of changes in the crystal lattice. When forces act on the steel, it changes from the face-centered cubic form, austenite to the body centered cubic form, martensite. It is the collective shear of the crystal lattice planes (the transformation) that makes traditional TRIP steel ductile.

Keywords: (improved formability?)

When forces act on the steel, it changes from the face-centered cubic form, austenite to the body centered cubic form, martensite.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

However, with conventional TRIP steel, a certain amount of the austenite portion is transformed to martensite – a rigid crystal structure that allows hardly any stretching. In crash tests, this steel offers only about 5 percent additional ductility.

With the increased share of manganese, silicon and aluminum atoms in the iron crystal, the TRIP effect is twice as profound, thus providing double additional ductility. The reason for twinning is that the alloy elements make two martensitic transformations possible – first a change from austenite to hexagonal martensite, and then from the hexagonal structure to the bodycentered cubic martensite.

Keypoints: TWIP Hardening Mechanism?

The reason for twinning is that the alloy elements make 2 (two) Martensitic transformations possible – (1) first a change from austenite to hexagonal martensite, and then from the hexagonal structure to the (2) tetragonal body-centered cubic martensite.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

with conventional TRIP steel, a certain amount of the austenite portion is transformed to martensite – a rigid crystal structure that allows hardly any stretching. In crash tests, this steel offers only about 5 percent additional ductility.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Back to Basic: Martensite Martensite is a body-centered tetragonal form of iron in which some carbon is dissolved. Martensite forms during quenching, when the face centered cubic lattice of austenite is distorted into the tetragonal body centered tetragonal structure without the loss of its contained carbon atoms into cementite and ferrite. Instead, the carbon is retained in the iron crystal structure, which is stretched slightly so that it is no longer cubic. Martensite is more or less ferrite supersaturated with carbon. Compare the grain size in the micrograph with tempered martensite.

Charlie Chong/ Fion Zhang

http://www.threeplanes.net/martensite.html

Martensitic Transformation: Mysterious Properties Explained The difference between austenite and martensite is, in some ways, quite small: while the unit cell of austenite is a perfect cube, in the transformation to martensite this cube is distorted so that it's slightly longer than before in one dimension and shorter in the other two. The mathematical description of the two structures is quite different, for reasons of symmetry, but the chemical bonding remains very similar. Unlike cementite, which has bonding reminiscent of ceramic materials, the hardness of martensite is difficult to explain in chemical terms. The explanation hinges on the crystal's subtle change in dimension, and the speed of the martensitic transformation.

Austenite is transformed to martensite on quenching at approximately the speed of sound - too fast for the carbon atoms to come out of solution in the crystal lattice. The resulting distortion of the unit cell results in countless lattice dislocations in each crystal, which consists of millions of unit cells.

These dislocations make the crystal structure extremely resistant to shear stress - which means, simply that it can't be easily dented and scratched.

Picture the difference between shearing a deck of cards (no dislocations, perfect layers of atoms) and shearing a brick wall (even without the mortar).

Charlie Chong/ Fion Zhang

http://www.threeplanes.net/martensite.html

Keywords: Hexagonal & BCC Martensite The reason for twinning is that the alloy elements make two martensitic transformations possible – first a change from austenite to hexagonal martensite, and then from the hexagonal structure to the body-centered cubic martensite.

Charlie Chong/ Fion Zhang

http://www.threeplanes.net/martensite.html

The twinning causes a high value of the instantaneous hardening rate (n value) as the microstructure becomes finer and finer. The resultant twin boundaries act like grain boundaries and strengthen the steel. TWIP steels combine extremely high strength with extremely high formability. The n value increases to a value of 0.4 at an approximate engineering strain of 30% and then remains constant until a total elongation around 50%. At the same time, it hardens without breaking and it resists tensile pressures up to 1100 MPa and it could be stretched to approximately 90 percent of its length without breaking (Figure 2).

It is means in practice that when forces act on the steel, as in the deep draw process, some of the austenite first transforms to the first martensite stage, the hexagonal crystal form. When the steel is put under increasing stress, the hexagonal lattice switches to the final, body-centered cubic form, similar to conventional TRIP steel. This means that the steel retains a good part of its ductility even after deep draw processing.

Austenite → ε martensite → γ martensite

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http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Figure 2: The stress-strain diagram clearly shows the differing characters of TRIP and TWIP steel. TRIP steel can resist high stresses without deforming.

TWIP steel deforms with low stresses, but does not break until strain reaches around 90 percent.

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http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Dotted red line: more representing the higher tensile strength of TWIP Steel?

TWIP Steel ?

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http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Also, the TRIP steel is particularly useful for side impact protection. The material deforms and absorbs the energy of the impact. It also becomes very strong as it hardens, which prevents the side sections from collapsing too much and protects vehicle occupants from injury.

However, the double TRIP effect does not explain why an alloy with 15-25 manganese content is particularly ductile. This is caused by small faults in the crystal structure called "stacking faults". Stacking faults can be visualized as a shift in the grid of atomic planes neatly arranged side by side and one on top of the other. If an extra stack of two atomic planes is introduced into the lattice from above, the regular stacking sequences are disturbed and therefore form a stacking fault. This folding mechanism takes place on a mirror plane, creating regularly mirrored sections of crystal. Experts refer to this as twinning, which is what manifests itself externally as extreme ductility.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Typical mechanical property ranges of these different steels are indicated in Figure 3. It is obvious that High Manganese Steels show extraordinary strength-ductility relationships with a resist tensile stress up to 1100 MPa. Conventional high-strength bodywork steels rupture at around 700 MPa or even less.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Figure 3: The diagram shows the very high stresses that TRIP/TWIP steels (red) can resist, compared to conventional deep drawing steels (blue).

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

High manganese steels composed of single austenite phase or multi phase with high fraction of austenite phase can be alloyed with a large amount of alloying elements. Effect of alloying element on properties of high manganese steels is shown in Table 1.

■C As discussed above, carbon improves the stability of austenite and strengthens the steels. It inhibits the formation of ε-martensite by increasing the stacking fault energy.

■ Mn

Manganese stabilizes austenite. However if its content is less than 15%, α'martensite is formed, which aggravates the formability.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

Table 1: Effect of alloying elements on properties of high manganese steels

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

■ Mn

The γ => ε transformation temperatures decrease with increasing Mn content.

■ Si Silicon improves strength by solid solution strengthening. ilicon addition is effective for refining ε martensite plates and increasing fracture strength, although it does not improve ductility.

■ Al The high aluminum content in high manganese steels increases the stacking fault energy of austenite. The formation of ε-martensite is suppressed by aluminum addition. An aluminum addition is also very effective for improving of low temperature toughness. Aluminum can segregate on the grain boundaries during solidification, and produce a low melting point intermetalic compound such as Fe2Al5 having a melting point about 1170°C on the grain boundaries, which cause a weakness in the casting structure.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

■ B, Ti, Zr Adding small amounts of boron, titanium and zirconium into the high manganese steels HMS alloyed with aluminum can improve the hot ductility of the steels.

■N Nitrogen is an effective strengthening element in austenite e.g. adding nitrogen to the Fe16.5Mn alloy decrease the martensite start temperature and also reduces the volume fraction of ε-martensite.

TWIP steels have very good mechanical advantages for the improvement of the automotive design, a very good crash resistance and they also reduce the vehicle weight. This new class of steels is a good example of the development of new materials for the benefit of the human being.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207

End of Reading 6

Charlie Chong/ Fion Zhang

Acoustic Emission Technique the optimum solution for leakage detection and location on water pipelines Marco Fantozzi ASM Brescia S.p.A., Via Lamarmora 230, 25124 Brescia, Italy.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

ABSTRACT Leaks in water pipelines cause unnecessary waste of limited resources, thus the necessity of leakage prevention and detection.

The experience of water distribution companies shows that the reduction of leakage and the preservation of a low leakage level can be achieved with a strategy that requires a loss analysis followed by leak detection and location survey.

Effective techniques of leak detection by acoustic emission have been developed and tested and this paper describes the experience and results obtained with the application of these techniques in the last fifteen years in several water systems including but not limited to those managed by ASM BRESCIA S.p.A. in Italy.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

ASM introduction Since 1908, ASM is the Municipal Services Board of Brescia, which is a town of 200,000 inhabitants situated in the North of Italy. ASM, which is largely owned by the Municipality of Brescia, is in charge of several services, the main of which being: production and distribution of electricity, district heating, street lighting and traffic lights, distribution of natural gas, collection, treatment and distribution of drinking water, sewage treatment, urban transport, parking management, telematic services, collection and disposal of urban solid waste (including separate waste collection, landfill management and incineration of the rest with combined production of energy and heat).

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

Brescia, Italy

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

Active approach to leak detection The water systems managed by ASM, whose extension is around 2,200 km are constructed mainly of ductile iron and cast iron pipes. Over 120 boreholes and 30 spring sources supply the networks delivering to users a total of 47 million mc a year. Since 1988, ASM BRESCIA S.p.A. has been engaged in an active program of leakage reduction.

Various methods of leakage monitoring and detection have been employed

by ASM. They include:  District metering technique and step testing (using quadrina insertion flow meters and data loggers)  Leak detection and location using leak noise correlators  Area surveys using acoustic loggers (Aqualogs)  Analysis of the results by the Company's Maintenance Database ASM's commitment to leakage reduction is demonstrated by the reduced level of leakage achieved in many of the managed water networks.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

District metering technique ASM decided to divide the network into a number of small zones called districts that has proved by experience in different parts of the world, to be the most efficient method of controlling leakage. Then, permanently closing the boundary valves and installing flow meters on the few supplying mains can continuously monitor the level of leakage. If an increase is registered in the night consumption, a team is sent in to locate the leaks. In this way, leakage is under permanent control, but intervention occurs only at the optimum moment.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

Leak detection and location The modern leak noise correlator is now the most effective and widely used system for leak detection and location. For this reason the leak inspection on water pipelines using the cross-correlation method were standardised in 1991 by a work group of the CNR (Italian National Research Council).

The code of practice highlights those elements necessary for carrying out the leak detection survey in order to improve the quality and standardize the activity. This document can be used by the Water Distribution Companies as well as by Service Companies as a useful reference.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

â– Object and target The method of testing requires the use of sensing devices placed on existing pipelines fittings as well as conditioning, acquisition and signal analysis instrumentation in order to detect and locate the leaks.

The method described applies to the control of underground supply and distribution water pipelines of steel, ductile iron, cast iron, asbestos cement, polyethylene and PVC. Cast iron, steel or asbestos cement pipe sections of a maximum length of 250 meters can be controlled by using non-intrusive sensing devices (accelerometers) and up to 600 meters by intrusive sensing devices (hydrophones).

The maximum controllable length of plastic pipes such as PVC or (high and low density) polyethylene is 50 metres only, when accelerometers are used, and 120 meters when hydrophones are used.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

■ Method of testing (Linear) The method of testing requires the use of non intrusive sensing devices (accelerometers) or intrusive devices (hydrophones) placed on existing pipeline fittings as well as conditioning, acquisition and signal analysis instrumentation in order to detect and locate leaks.

The location of the leaking point in the pipe is obtained knowing: the distance between the sensors that span the leak, the propagation velocity of the leak sound in the pipeline and the time delay, measured by the cross-correlation function (see figure 2), that the leak sound takes to reach the two sensors.

D = 2x + v∙∆T, x = ½•(D- v•∆T) T2v X= T1v

X= T1v ∆d = v(T1-T2) =v∙∆T

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

X = distance of the point of leak from the reference sensing device; D = distance between the two sensing devices; V = propagation wave speed; ∆t = time delay obtained from the peak position of the cross-correlation function.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

Fig 2: Cross-correlation function plot.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

Fig 3: Coherence function plot.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

The figure 1 shows how the position of the leaking point may be obtained and the figure 2 shows an example of the cross-correlation function.

The diagram in figure 2 shows that the position of the leak, in relation to the two sensing devices, is determined by detecting the maximum of the crosscorrelation function related to the time delay of the signals. (?)

The coherence function shown in figure 3 allows establishing the reliability rating of the measure carried out. It expresses the dependence of the signals, detected at the two measurement points A and B, from a common leak noise source. The Coherence is normally represented between zero and one, therefore, the nearer the coherence is to one the closer is the link between the two detected signals. (?)

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

On site inspection results The results obtained over a sample of 4820 km of water distribution network in different Italian cities that have been surveyed using the cross-correlation technique in the last ten years are now outlined.

During the systematic survey concerning the above mentioned networks about half consisting of cast and ductile iron pipes and the other half of steel pipes and asbestos cement pipes (only 33 km of plastic pipes have been inspected) - a total of 3450 water leakages have been detected.

Out of the detected leaks, 3312 (96%) have been located exactly and have undergone repair. Some of the remaining 174 leaks have been located during the repair excavation at distances greater than 3-4 meters. The location errors are essentially due to the uncertainty of the used distance between sensors.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

Area surveys using acoustic loggers In the last few years other acoustic techniques have been developed to optimise water leakage management in identifying leakage areas prior to directing leak detection operators to pinpoint the leak.

Thus have been developed systems for acoustic noise monitoring and recording that can be permanently or time limited installed at hydrants, valves or house connections. These "noise loggers" record typical noises in the network during low consumption hours at night and identify areas of potential leakage for further investigation. The ultimate advance consists in transmission of leak presence from the noise loggers to a receiver module, which may be hand carried or vehicle-mounted.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

â– Noise Logger This logger is installed at fittings via a simple magnetic coupling, and is battery powered with no maintenance requirement, and no problems for being immersed in water.

The separation distance between loggers depend mainly on the pipe material, with plastic pipes requiring closer spacing than metallic.

Each unit is intelligent and adapts itself to the environment. If no leak is present, a radio signal is transmitted to indicate normal background conditions. However, as soon as a leak is detected, the unit enters an alarm state and transmits a radio signal to indicate a "leak condition". Signals are received by a module that can be mounted in a patrolling vehicle, or can be easily hand-held. This receiving module analyses and "homes in" on signals to identify the location of units indicating a "leak condition", and thus the approximate position of a likely leak.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

The reading of an area meter could easily include the monitoring of the loggers within it, so that new leaks are localised at exactly the same time as increases in the night flow are noticed.

This should mean a prescribed leakage level can be easily maintained, because the detection time is greatly reduced.

This innovative technology offers the possibility of continuous, permanent monitoring for leakage for the entire distribution system or just for those parts that are known problem areas.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

THE FUTURE The next step will probably be the automatic cross-correlation analysis between permanently installed loggers.

The noise logger will be enabled to correlate a leak position with an adjacent logger and transmit the exact position by interfacing through SCADA with a GIS system. This process would enhance the leakage control process significantly.

Comments: SCADA (supervisory control and data acquisition) is a system operating with coded signals over communication channels so as to provide control of remote equipment .

A geographic information system (GIS) is a system designed to capture, store, manipulate, analyze, manage, and present all types of spatial or geographical data.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

CONCLUSIONS In the last fifteen years the use of acoustic emission techniques has shown that leaks can be accurately identified and localised much faster than with any conventional method.

These experiences in leakage detection and location have proved that the application of acoustic techniques gives Water industry the most effective tools of conserving precious water resources.

In particular, the use of cross-correlation to detect and locate the leaks on underground pipelines has gained larger and larger approval within the water industry, because it offers a more accurate location of the leak, less dependence from operator interpretation and it can be used in very noisy conditions.

The obtainable benefits due to the application of the considered technique are dependent on the care and manner in which it is applied and the results are as good as the operators strictly observe the guideline.

With the application of the "noise loggers" which record typical noises in the network during low consumption hours at night is now possible the permanent acoustic monitoring of the distribution network. This new technology will help to achieve further leakage reduction without increasing the costs for water leak detection.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm

End of Reading 7

Charlie Chong/ Fion Zhang

■ ωσμ∙Ωπ∆ ∇ º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λ α ρτ√ ≠≥ѵФε ≠≥ѵФdsssa

Charlie Chong/ Fion Zhang

More Reading

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Others Reading  http://www.globalspec.com/reference/63985/203279/Chapter-10-AcousticEmission-Testing  http://www.corrosionsource.com/(S(vf34kqncr0uklwzu0ioy5dz2))/FreeCont ent/3/Combatting+Liquid+Metal+Attack+by+Mercury+in+Ethylene+and+Cr yogenic+Gas+PlantsTask+1+-+Non-Destructive+Testing  http://www.ndt.net/ndtaz/index.php?id=2  https://www.ndeed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE _Index.htm

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Peach – 我爱桃子

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Good Luck

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Good Luck

Charlie Chong/ Fion Zhang

https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang