Ut testing section 5 measurement techniques

Section 5: Measurement Techniques

Content: Section 5: Measurement Techniques 5.1: Normal Beam Inspection 5.2: Angle Beams 5.3: Reflector Sizing 5.4: Automated Scanning 5.5: Precision Velocity Measurements 5.6: Attenuation Measurements 5.7: Spread Spectrum Ultrasonics 5.8: Signal Processing Techniques 5.9: Scanning Methods 5.10: Scanning Patterns 5.11: Pulse Repetition Rate and Penetration 5.12: Interferences & Non Relevant Indications 5.13: Entry Surface Variables 5.14: The Concept of Effective Distance 5.15: Exercises

Expert at works

5.1: Normal Beam Inspection Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the wellknown relationship:

where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time. d = vt/2 or v = 2d/t

d1 = v½t

d2 = v½t

= d1+d2

2vt

2vt

A-Scan

A Scan

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Precision ultrasonic thickness gages usually operate at frequencies between 500 kHz and 100 MHz, by means of piezoelectric transducers that generate bursts of sound waves when excited by electrical pulses. A wide variety of transducers with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, 1. lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, 2. while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials.

0.5 MHz ~ 100 MHz

In thickness gauging, ultrasonic techniques permit quick and reliable measurement of thickness without requiring access to both sides of a part.

Accuracy's as high as Âą1 micron or Âą0.0001 inch can be achieved in some applications. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological specimens.

On-line or in-process measurement of extruded plastics or rolled metal often is possible, as is measurements of single layers or coatings in multilayer materials. Modern handheld gages are simple to use and very reliable.

5.2: Angle Beams I Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas.

Ɵ = Angle of reflection, T=Material thickness, S= Sound path, Surface distance = SinƟ x S, Depth= CosƟ x S

A-Scan

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Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

Ɵ = Angle of reflection, T=Material thickness, S= Sound path, Skip = 2(T x TanƟ), Leg = T/CosƟ, V Path = 2 x Leg

A-Scan

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Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Dead Zone

Near Surface Detectability with Angle Beam Transducer

Flaw Location

Flaw Location with Angle Beam Transducer

Flaw Location with Angle Beam Transducer

Flaw Location with Angle Beam Transducer

Flaw Location with Angle Beam Transducer

Why angle beam assemblies are used Cracks or other discontinuities perpendicular to the surface of a test piece, or tilted with respect to that surface, are usually invisible with straight beam test techniques because of their orientation with respect to the sound beam.

Perpendicular cracks do not reflect any significant amount of sound energy from a straight beam because the beam is looking at a thin edge that is much smaller than the wavelength, and tilted cracks may not reflect any energy back in the direction of the transducer. This situation can occur in many types of welds, in structural metal parts, and in many other critical components. An angle beam assembly directs sound energy into the test piece at a selected angle. A perpendicular crack will reflect angled sound energy along a path that is commonly referred to as a corner trap, as seen in the illustration below.

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http://www.olympus-ims.com/en/applications/angle-beam-transducers

The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece (first leg test) or, after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test). A variety of specific beam angles and probe positions are used to accommodate different part geometries and flaw types. In the case of angled discontinuities, a properly selected angle beam assembly can direct sound at a favorable angle for reflection back to the transducer.

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http://www.olympus-ims.com/en/applications/angle-beam-transducers

How they work -- Snell's Law A sound beam that hits a surface at perpendicular incidence will reflect straight back. A sound beam that hits a surface at an angle will reflect forward at the same angle.

Sound energy that is transmitted from one material to another bends in accordance with Snell's Law of refraction. Refraction is the bending of a sound beam (or any other wave) when it passes through a boundary between two materials of different velocities. A beam that is traveling straight will continue in a straight direction, but a beam that strikes a boundary at an angle will be bent according to the formula:

Typical angle beam assemblies make use of mode conversion and Snell's Law to generate a shear wave at a selected angle (most commonly 30, 45, 60, or 70 degrees) in the test piece. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material, and if the angle is high enough, all of the energy in the second material will be in the form of shear waves.

There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon: (1) First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials.

(2) Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave, and minimum flaw size resolution increases as the wavelength of a sound beam gets smaller.

Selecting the right angle beam assembly The parameters that affect angle beam performance include not only the (1) beam angle generated by the wedge, but also (2) transducer frequency and (3) element size. The optimum beam angle will generally be governed by the geometry of the test piece and the orientation of the discontinuities that the test is intended to find. Transducer frequency affects penetration and flaw resolution: 1. As frequency increases, the distance the sound wave will travel in a given material decreases, but resolution of small discontinuities improves.

2. As frequency decreases, the distance the sound wave will travel increases but the minimum detectable flaw size will become larger.

3. Similarly, larger element sizes may decrease inspection time by increasing coverage area, but the reflected echo amplitude from small discontinuities will decrease. Smaller element sizes will increase reflection amplitude from small discontinuities, but the inspection may take longer because the smaller beam covers less area.

These conflicting factors must be balanced in any given application, based on specific test requirements.

Contoured wedges

The IIW recommends the use of a contoured wedge whenever the gap between the wedge and the test surface exceeds 0.5 mm (approximately 0.020 in.). Under this guideline, a contoured wedge should be used whenever part radius is less than the square of a wedge dimension (length or width) divided by four:

where R = radius of test surface W = width of wedge if testing in axial orientation, length of wedge if testing in circumferential orientation Of course switching to a small wedge, if possible within the parameters of inspection requirements, will improve coupling on curved surfaces. As a practical matter, contouring should be considered whenever signal strength diminishes or couplant noise increases to a point where the reliability of an inspection is impaired.

Focused dual element angle beams The vast majority of angle beam assemblies use single element, unfocused transducers. However, in some tests involving highly attenuating or scattering materials such as coarse grain cast stainless steel, focused dual element angle beams are useful. Because they have separate transmitting and receiving elements, dual element transducers can typically be driven at higher excitation energies without noise problems associated with ringdown or wedge noise. Focusing permits a higher concentration of sound energy at a selected depth within the test piece, increasing sensitivity to discontinuities in that region.

High temperature wedges Standard angle beam assemblies are designed for use at normal environmental temperatures only. For situations where metal must be inspeced at elevated temperature, special high temperature wedges are available. Some of these wedges will tolerate brief contact with surfaces as hot as 480째 C or 900째 F. However, it is important to note that high temperature wedges require special attention with regard to the sound path they generate. With any high temperature wedge, sound velocity in the wedge material will decrease as it heats up, and thus the refracted angle in metals will increase as the wedge heats up. If this is of concern in a given test, refracted angle should be verified at actual operating temperature. As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult.

Surfaces as hot as 480째C / 900째F

threaded

snap-in

steel with a shear wave velocity of approximately 3,250 M/S or 0.1280 in/uS.

5.3: Reflector Sizing There are many sizing methods, these include: 5.3.1

Crack Tip Diffraction

When the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length (a) of a crack can be determined by a technique known as tip diffraction. One common application of the tip diffraction technique is to determine the length of a crack originating from on the backside of a flat plate as shown below. In this case, when an angle beam transducer is scanned over the area of the flaw, the principle echo comes from the base of the crack to locate the position of the flaw (Image 1). A second, much weaker echo comes from the tip of the crack and since the distance traveled by the ultrasound is less, the second signal appears earlier in time on the scope (Image 2).

Crack Tip Diffraction Methods

No animation.

Crack height (a) is a function of the ultrasound velocity (v) in the material, the incident angle (Q2) and the difference in arrival times between the two signal (dt). Since the incident angle and the thickness of the material is the same in both measurements, two similar right triangle are formed such that one can be overlayed on the other. A third similar right triangle is made, which is comprised on the crack, the length dt and the angle Q2. The variable dt is really the difference in time but can easily be converted to a distance by dividing the time in half (to get the one-way travel time) and multiplying this value by the velocity of the sound in the material. Using trigonometry an equation for estimating crack height from these variables can be derived as shown below.

Crack Tip Diffraction Method

The equation is complete once distance dt is calculated by dividing the difference in time between the two signals (dt) by two and multiplying this value by the sound velocity.

5.3.2 6 dB Drop SizingFor Large Reflector (greater than beam width), i.e. there is no BWE.

6 dB Drop Method

6 dB Drop Method

6 dB Drop Method

www.youtube.com/embed/hsR17WA3nHg

6 dB Drop Method

5.3.3

The 20 dB drop sizing method

We can use a beam plot to find the edge of a defect by using the edge of the sound beam.

If we know the width of a beam at a certain distance from the crystal, we can mark the distance across a defect from where the extreme edges of the beam touch each end of the defect and then subtract the beam width to get the defect size.

When the signal from the defect drops by 20dB from its peak, we judge that the edge of the beam is just touching the end of the defect. We can find the width of the sound beam at that range by consulting the beam plot that we have made Note: The peak of the defect is normally taken as being the last peak on the screen before the probe goes off the end of the defect, not necessarily the maximum signal from a defect.

20 dB Drop Method

20 dB Drop Sizing- For Small Reflector (smaller than beam width).

To use this method the transducer beam width need to be first determined.

Construction of a beam edge plot -20dB – Normal Beam Find the hole at a depth of 13mm on an IOW block with a 0 degree probe and maximise the signal. Move the probe until you get the highest signal you can from the hole, then turn the signal to FSH using gain. Mark the position of the middle of the probe on the side of the block.

Move the probe to one side until the signal drops to 10%FSH (-20dB) and mark the centre of the probe on the side of the block.

Move the probe to the other side of the hole until the signal drops to 10%FSH (-20dB) and mark the centre of the probe on the block.

Use the distances between the marks on the block to plot the beam on a piece of graph paper. Measure 13mm depth on the paper then mark the distances of the probe centre at -20dB from the beam centre at 100%FSH on either side.

Now find the 25mm hole and maximise the signal, turning it to 100%FSH. Move the probe to either side of the hole marking the centre of the probe on the side of the block where the signal drops by 20dB. Measure 25mm on the paper and use the distances on the block to plot the beam dimensions at 25mm.

Repeat using the 32mm hole. Join up the points marking the probe centre at 20dB to obtain a beam plot.

Note that we have only drawn the beam width in one plane, so the probe must be marked accordingly and used to measure defects in this plane.

We use knowledge of the beam spread to size defects, find the edges and hence their width, length and sometimes orientation.

Construction of a beam edge plot -20dB – Angle Beam

5.3.4 Equalization Back Wall Sizing- The probe moving off the edges of the reflector until the amplitude is equal to the rising BWE

5.3.5 Maximum Amplitude Techniques The technique is used for small reflector. The probe moving off the edges of the reflector until the amplitude is maximum and the line joining the boundary is the size of reflector cluster.

5.3.6 The DGS Method Distance Gain Size Method. The technique is used to find the equivalent reflector size by comparing the gain between the flaw and the known size reflector.

5.4: Automated Scanning Ultrasonic scanning systems are used for automated data acquisition and imaging. They typically integrate a ultrasonic instrumentation, a scanning bridge, and computer controls. The signal strength and/or the time-of-flight of the signal is measured for every point in the scan plan. The value of the data is plotted using colors or shades of gray to produce detailed images of the surface or internal features of a component. Systems are usually capable of displaying the data in A-, B- and C-scan modes simultaneously. With any ultrasonic scanning system there are two factors to consider: â– how to generate and receive the ultrasound. â– how to scan the transducer(s) with respect to the part being inspected.

Automatic Scanning

The most common ultrasonic scanning systems involve the use of an immersion tank as shown in the image above. The ultrasonic transducer and the part are placed under water so that consistent coupling is maintained by the water path as the transducer or part is moved within the tank. However, scanning systems come in a large variety of configurations to meet specific inspection needs. In the image to the right, an engineer aligns the heads of a squirter system that uses a through-transmission technique to inspect aircraft composite structures. In this system, the ultrasound travels through columns of forced water which are scanned about the part with a robotic system. A variation of the squirter system is the "Dripless Bubbler" scanning system, which is discussed below.

Dripless Bubbler

It is often desirable to eliminate the need for the water coupling and a number of state-of-the-art UT scanning systems have done this. Laser ultrasonic systems use laser beams to generate the ultrasound and collect the resulting signals in an noncontact mode. Advances in transducer technology has lead to the development of an inspection technique known as air-coupled ultrasonic inspection. These systems are capable of sending ultrasonic energy through air and getting enough energy into the part to have a useable signal. These system typically use a through-transmission technique since reflected energy from discontinuities are too weak to detect.

The second major consideration is how to scan the transducer(s) with respect to the part being inspected. When the sample being inspected has a flat surface, a simple raster-scan can be performed. If the sample is cylindrical, a turntable can be used to turn the sample while the transducer is held stationary or scanned in the axial direction of the cylinder. When the sample is irregular shaped, scanning becomes more difficult. As illustrated in the beam modeling animation, curved surface can steer, focus and defocus the ultrasonic beam. For inspection applications involving parts having complex curvatures, scanning systems capable of performing contour following are usually necessary.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/AppleScan/Apple2.swf

5.5: Precision Velocity Measurements Changes in ultrasonic wave propagation speed, along with energy losses, from interactions with a materials microstructures are often used to nondestructively gain information about a material's properties.

Measurements of sound velocity and ultrasonic wave attenuation can be related to the elastic properties that can be used to characterize the texture of polycrystalline metals. These measurements enable industry to replace destructive microscopic inspections with nondestructive methods.

Of interest in velocity measurements are longitudinal wave, which propagate in gases, liquids, and solids. In solids, also of interest are transverse (shear) waves. The longitudinal velocity is independent of sample geometry when the dimensions at right angles to the beam are large compared to the beam area and wavelength. The transverse velocity is affected little by the physical dimensions of the sample.

Pulse-Echo and Pulse-Echo-Overlap Methods Rough ultrasonic velocity measurements are as simple as measuring the time it takes for a pulse of ultrasound to travel from one transducer to another (pitch-catch) or return to the same transducer (pulse-echo). Another method is to compare the phase of the detected sound wave with a reference signal: slight changes in the transducer separation are seen as slight phase changes, from which the sound velocity can be calculated. These methods are suitable for estimating acoustic velocity to about 1 part in 100. Standard practice for measuring velocity in materials is detailed in ASTM E494.

ASTM E494 - 10 Measuring Ultrasonic Velocity in Materials Active Standard ASTM E494 | Developed by Subcommittee: E07.06 Book of Standards Volume: 03.03

Precision Velocity Measurements (using EMATs) Electromagnetic-acoustic transducers (EMAT) generate ultrasound in the material being investigated. When a wire or coil is placed near to the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region. If a static magnetic field is also present, these currents will experience Lorentz forces of the form F=JxB where F is a body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.

EMATs

http://www.resonic.com/error%20scan.swf http://www.resonic.com/scan2.swf

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The most important application of EMATs has been in nondestructive evaluation (NDE) applications such as flaw detection or material property characterization. Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarizations that would be difficult to realize with fluid coupled piezoelectric probes. In the inference of material properties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.

Differential velocity is measured using a T1-T2---R fixed array of EMAT transducer at 0, 45째, 90째 or 0째, 90째 relative rotational directions depending on device configuration:

EMAT Driver Frequency: 450-600 KHz (nominal) Sampling Period: 100 ns Time Measurement Accuracy: -- Resolution 0.1 ns -- Accuracy required for less than 2 KSI Stress Measurements: Variance 2.47 ns -- Accuracy required for texture: Variance 10.0 Ns ------ W440 < 3.72E-5 ------ W420 < 1.47E-4 ------ W400 < 2.38E-4

Time Measurement Technique Fourier Transform-Phase-Slope determination of delta time between received RF bursts (T2-R) - (T1-R), where T2 and T1 EMATs are driven in series to eliminate differential phase shift due to probe liftoff.

Slope of the phase is determined by linear regression of weighted data points within the signal bandwidth and a weighted y-intercept. The accuracy obtained with this method can exceed one part in one hundred thousand (1:100,000).

5.6: Attenuation Measurements Ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The velocity of the ultrasonic waves is influenced by the elastic moduli and the density of the material, which in turn are mainly governed by the amount of various phases present and the damage in the material. Ultrasonic attenuation, which is the sum of the (1)absorption and the (2)scattering, is mainly dependent upon the damping capacity and scattering from the grain boundary in the material. However, to fully characterize the attenuation required knowledge of a large number of thermo-physical parameters that in practice are hard to quantify.

Ao Ut A

Relative measurements such as the change of attenuation and simple qualitative tests are easier to make than absolute measure. Relative attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. However, significant variations in microstructural characteristics and mechanical properties often produce only a relatively small change in wave velocity and attenuation. Absolute measurements of attenuation are very difficult to obtain because the echo amplitude depends on factors in addition to amplitude.

The most common method used to get quantitative results is to use an ultrasonic source and detector transducer separated by a known distance.

By varying the separation distance, the attenuation can be measured from the changes in the amplitude. To get accurate results, the influence of coupling conditions must be carefully addressed. To overcome the problems related to conventional ultrasonic attenuation measurements, ultrasonic spectral parameters for frequency-dependent attenuation measurements, which are independent from coupling conditions are also used. For example, the ratio of the amplitudes of higher frequency peak to the lower frequency peak, has been used for microstructural characterization of some materials.

Attenuation:

Ao

Ut A

Attenuation:

5.7: Spread Spectrum Ultrasonics Spread spectrum ultrasonics makes use of the correlation of continuous signals rather than pulse-echo or pitch-catch techniques.

Spread spectrum ultrasonics is a patented new broad band spread-spectrum ultrasonic nondestructive evaluation method. In conventional ultrasonics, a pulse or tone burst is transmitted, then received echoes or throughtransmission signals are received and analyzed.

In spread spectrum ultrasonics, encoded sound is continuously transmitted into the part or structure being tested. Instead of receiving echoes, spread spectrum ultrasonics generates an acoustic correlation signature having a one-to-one correspondence with the acoustic state of the part or structure (in its environment) at the instant of the measurement. In its simplest embodiment, the acoustic correlation signature is generated by cross correlating an encoding sequence, with suitable cross and auto correlation properties, transmitted into a part (structure) with received signals returning from the part (structure).

Section of bi-phase modulated spread spectrum ultrasonic waveform

Multiple probes may be used to ensure that acoustic energy is propagated through all critical volumes of the structure. Triangulation may be incorporated with multiple probes to locate regions of detected distress. Spread spectrum ultrasonics can achieve very high sensitivity to acoustic propagation changes with a low level of energy.

Two significant applications of Spread Spectrum Ultrasonics are: 1. Large Structures that allow ultrasonic transducers to be "permanently" affixed to the structures, eliminating variations in transducer registration and couplant. Comparisons with subsequent acoustic correlation signatures can be used to monitor critical structures such as fracture critical bridge girders. In environments where structures experience a great many variables such as temperature, load, vibration, or environmental coupling, it is necessary to filter out these effects to obtain the correct measurements of defects.

In the example below, simulated defects were created by setting a couple of steel blocks on the top of the bridge girder.

Spread Spectrum UT

2. Piece-part assembly line environments where transducers and couplant may be precisely controlled, eliminating significant variations in transducer registration and couplant. Acoustic correlation signatures may be statistically compared to an ensemble of known "good" parts for sorting or accepting/rejecting criteria in a piece-part assembly line environment.

Impurities in the incoming steel used to forge piece parts may result in sulfite stringer inclusions. In this next example simulated defects were created by placing a magnetized steel wire on the surface of a small steel cylindrical piston used in hydraulic transmissions.

Two discrimination technique are tested here, which are SUF-1 and SUF-2, with the latter giving the best discrimination between defect conditions. The important point being that spread spectrum ultrasonics can be extremely sensitive to the acoustic state of a part or structure being tested, and therefore, is a good ultrasonic candidate for testing and monitoring, especially where scanning is economic unfeasible.

EMATs with Spread Spectrum Ultrasonic

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5.8: Signal Processing Techniques Signal processing involves techniques that improve our understanding of information contained in received ultrasonic data. Normally, when a signal is measured with an oscilloscope, it is viewed in the time domain (vertical axis is amplitude or voltage and the horizontal axis is time). For many signals, this is the most logical and intuitiveç&#x203A;´č§&#x201A;ç&#x161;&#x201E; way to view them. Simple signal processing often involves the use of gates to isolate the signal of interest or frequency filters to smooth or reject unwanted frequencies.

When the frequency content of the signal is of interest, it makes sense to view the signal graph in the frequency domain. In the frequency domain, the vertical axis is still voltage but the horizontal axis is frequency.

Display

Time/Magnitude domain

Frequency /Magnitude domain

The frequency domain display shows how much of the signal's energy is present as a function of frequency. For a simple signal such as a sine wave, the frequency domain representation does not usually show us much additional information. However, with more complex signals, such as the response of a broad bandwidth transducer, the frequency domain gives a more useful view of the signal.

Fourier theory says that any complex periodic waveform can be decomposed into a set of sinusoids with different amplitudes, frequencies and phases. The process of doing this is called Fourier Analysis, and the result is a set of amplitudes, phases, and frequencies for each of the sinusoids that makes up the complex waveform. Adding these sinusoids together again will reproduce exactly the original waveform. A plot of the frequency or phase of a sinusoid against amplitude is called a spectrum.

Fourier Analysis

Fourier Analysis

Fourier Analysis

The following Fourier Java applet, adapted with permission of Stanford University, allows the user to manipulate discrete time domain or frequency domain components and see the relationships between signals in time and frequency domains.

The top row (light blue color) represents the real and imaginary parts of the time domain. Normally the imaginary part of the time domain signal is identically zero.

The middle row (peach color) represents the the real and imaginary parts of the frequency domain.

The bottom row (light green color) represents the magnitude (amplitude) and phase of the frequency domain signal. Magnitude is the square root of the sum of the squares of the real and imaginary components. Phase is the angular relationship of the real and imaginary components. Ultrasonic transducer manufactures often provide plots of both time domain and frequency domain (magnitude) signals characteristic of each transducer. Use this applet to explore the relationship between time and frequency domains.

Fourier Analysis

5.9: Scanning Methods Direct contact,

Direct contact,

single element probe

dual element probe

Through transmission

Fixed delay

Immersion testing

5.9.1

Pulse Echo Method

Pulse Echo Method: Sound pressure on axis (schematic) for the incident wave (top) and the wave reflected from a reflector in form a circular disc (bottom).

Pulse Echo Method

Pulse Echo Method

Pulse Echo Method- Schematic screen pictures obtained by the pulse-echo method. a Small flaw in sound beam; b two small flaws in sound beam; c large flaw in sound beam, smaller second flaw and back wall masked; d large, obliquely orientated flaw, back wall masked; e small flaw but no back wall echo because the axis of the beam is not incident at right angles on back wall; f strong attenuation of sound beam due to scattering, no echo from flaw or back wall, only "grass"

Pulse Echo Method

Pulse Echo Method- Multiple echoes in a plate. a schematic; b actual screen picture without time or thickness scale; steel plate 50 mm thick, frequency 4 MHz

Amplitude loss: Inverse Square Law

Influence of Shadow on axial defects

Influence of reflector orientation on signal

Influence of reflector size on signal

Pulse Echo Method

IP BE

F

plate

delamination

0

2

4

6

8

IP = Initial pulse F = Flaw BE = Backwall echo

10

Pulse Echo Method

s

Probe

Sound travel path

Work piece

Flaw

5.9.2

Pitch-Catch Methods

Advantage: Sensitive to near surface defect Capable of penetrating thicker material due to pitch-catch mode.

Disadvantage: It measures only sound energy loss at the receiver, without giving details information of location.

5.9.2.1 Pitch-Catch Methods- Through Transmission Through transmission testing uses two search units; one unit is used as a transmitter and the other unit is used as a receiver, as shown in Figure below.

With this technique, the ultrasonic beam passes through the test piece or is attenuated by one or more discontinuities. Total or partial attenuation of the signal is possible depending on the severity of the discontinuity. Both transducers must be properly coupled with a liquid coupling agent to obtain reliable results. As with other techniques using two search units, greater efficiency may be obtained by using a ceramic element in the transmitting search unit and a lithium sulfate element in the receiving unit.

Pitch-Catch Methods- Through Transmission

Pitch-Catch Methods- Through Transmission

Through transmission signal

1

T

R

1

2

T

R

2 0

Flaw

2

4

6

Back wall Echo

8

10

5.9.2.2 Pitch-Catch Methods- Tandem The tandem method, the examination is normally carried out using two similar 45째 angle probes, one probe operating as the transmitter and the other probe as receiver. For wall thicknesses greater than approximately 160 mm, probes with different transducer sizes are preferred in order to ensure approximately the same beam diameters in the examination zone.

The use of probe angles other than 45째 may be necessary to comply with particular geometrical conditions. Probe angles that give rise to mode conversions shall be avoided. The probes are located in a line with their acoustic axis in the same direction. In this way the sound beam from the rear probe will, after reflection from the opposite surface, intersect the sound beam from the front probe at the centre of the examination zone.

Extract from: EN 583-4 Non-destructive testing - Ultrasonic examination - Part 4: Examination for discontinuities perpendicular to the surface

Figure 1 shows the relationship between the spacing of the probes (y) and the examination depth of the cross point (tm) and the height of the examination zone (tz). When examining objects with plane parallel surfaces the distance between the probes can be defined using the following equation: y = 2 tan α (d – tm) or 2 tan α (bottom depth)

Distance Between Transmitter / Receiver Probes y Depth tm

α x

α Plate Thickness d

L tan α = L / (d + d - tm) , L = 2d- tm tan α , tan α = x / tm , x = tm tan α y = L - x = (2d - tm tan α) – (tm tan α) y = tan α (2d-tm- tm) = 2 tan α (d - tm)

Video on Through Transmission Methods

www.youtube.com/embed/bRgCLb2cDU4?list=UUSOUDD4-FPV4tzqvUnquwXQ

5.9.3

Immersion Methods

Many of the same techniques used in contact testing can be used in immersion testing. One advantage of immersion testing is that water makes a very effective coupling agent. A wetting agent is often used with the water to reduce surface tension and minimize air bubble formation on probes and test parts.

The main advantages of immersion testing are: ■ ■ ■ ■

speed of inspection, immersion medium provides excellent coupling, ability to direct the sound at any desired angle, and the ease of incorporating automatic scanning techniques.

With immersion testing, the time to send the beam through the water is usually greater than the time to send the beam through the test piece. All immersion search units are basically straight beam units that are directed to produce either longitudinal or shear waves in the test material.

Immersion Methods For immersion testing of steel and aluminum in water, the water path shall be at least 1” for every 4” thickness of the specimen (or ¼ of specimen thickness minimum). If the transducer is too close, the 2nd front reflection will appeared between the 1st front reflection and the 1st backwall echo and this may be wrong interpreted as discontinuity.

Immersion Methods- Since sound waves travel about four times faster in steel and aluminum than they do in water, a general rule of thumb is that the water distance should be 1/4 Ts the part thickness plus 1/4 in (6mm). When immersion testing is used for tapered plates, there should be a uniform water path above the test surface. With immersion testing, false indications from contoured surfaces will result in broad-based noise echoes.

Ts Minimum + [Âźâ&#x20AC;? 6mm (?)]

Immersion Methods- The water path shall be ¼ of specimen thickness minimum. (plus 6mm)

Minimum + [¼ “(?)]

Modified Immersion Methods- Bubbler Chamber

Modified Immersion Methods â&#x20AC;&#x201C; Irrigation Dam

Angle Beam Immersion Methods Note the small front surface reflection. This due to the inclined incident angle reflected away from the transducer.

Straight Beam Immersion Methods

1

2 water delay

surface = sound entry backwall

flaw

IP

1

IE

IP

2

IE

BE

BE F

0

2

4

6

8

10

0

2

4

6

8

10

Angle Beam Immersion Methods- Pipe & Tubing Testing .

Angle Beam Immersion Methods- Weld Testing

Immersion Testing Set-up

Immersion Testing Set-up

Manipulators The manipulator is primarily intended to provide a means of scanning the test specimen with an immersed transducer. ď Ž The manipulator is mounted on a traversing mechanism, which allows movement of the manipulator from side to side. ď Ž The traversing mechanism is an integral component of the bridge assembly.

A search tube is usually held rigid at right angles to the surface of the test specimen. Locking knobs are provided on the manipulator to allow positioning of the search tube in two planes for angle-beam testing.

Manipulators Bridge

Bridge Manipulator

Bridges When the manipulator is automated, electric motors are added to power the bridge carriage, the traversing mechanism, and the up and down movement of the search tube. The pulse-echo unit and the recording unit are also mounted on the bridge, with all power cords secured overhead to allow movement of the bridge along the full length of the tank.

Wands / Support Tubes The support tube for the immersion probe is sometimes called a wand. Its vertical height can be adjusted to vary water path distance and the adjuster which can manipulate probe angle of incidence at the tip of the wand.

Immersion Testing Set-up

Manipulator

Bridge

Wand / Tube

Immersion Testing Set-up

Manipulator

Bridge

Wand / Tube

Immersion Testing Set-up Manipulator

Bridge Wand / Tube

Other Reading (Olympus)- Angle Beam Immersion Methods Immersion transducers offer three major advantages over contact transducers: 1. Uniform coupling reduces sensitivity variations.

2. Reduction in scan time due to automated scanning.

3. Focusing of immersion transducers increases sensitivity to small reflectors.

Focusing Configurations Immersion transducers are available in three different configurations: • unfocused (“flat”), • spherically (“spot”) focused, and • cylindrically (“line”) focused.

Focusing is accomplished by either the addition of a lens or by curving the element itself. The addition of a lens is the most common way to focus a transducer.

An unfocused transducer may be used in general applications or for penetration of thick materials. A spherically focused transducer is commonly used to improve sensitivity to small flaws and a cylindrical focus is typically used in the inspection of tubing or bar stock. Examples of spherical and cylindrical focusing are shown in Figure (17) below.

Cylindrical

Spherical

Unfocused transducer By definition, the focal length of a transducer is the distance from the face of the transducer to the point in the sound field where the signal with the maximum amplitude is located. In an unfocused transducer, this occurs at a distance from the face of the transducer which is approximately equivalent to the transducerâ&#x20AC;&#x2122;s near field length. Because the last signal maximum occurs at a distance equivalent to the near field, a transducer, by definition, can not be acoustically focused at a distance greater than its near field.

Focus may be designated in three ways: FPF (Flat Plate Focus) - For an FPF focus, the lens is designed to produce a maximum pulse/echo response from a flat plate target at the distance indicated by the focal length PTF (Point Target Focus) - For a PTF focus, the lens is designed to produce a maximum pulse/echo response from a small ball target at the distance indicated by the focal length OLF (Optical Limit Focus) - The OLF designation indicates that the lens is designed according to the lens makerâ&#x20AC;&#x2122;s formula from physical optics and without reference to any operational definition of focal length. The OLF designation describes the lens and ignores diffraction effects.

Video on Immersion Testing

www.youtube.com/embed/W07-Z9at=UUSOUDD4-FPV4tzqvUnquwXQ

Q: In immersion testing, to remove the second water reflection (2nd entry surface signal) from between the entry surface signal and the first back reflection, you should: A. B. C. D.

Increase repetition rate Decrease frequency Decrease sweep length Increase water depth

Q110: In addition to other functions, a transducer manipulator in a mechanical immersion-scanning unit permits: A. B. C. D.

Use of the through transmission techniques Use of high scanning speed Detection of obliquely oriented discontinuities Utilization of skill operators

Q1: Which of the following scanning methods could be classified as an immersion type test?

A. Tank in which the transducer and test piece are immersed B. Squirter bubbler method in which the sound is transmitted in a column of flowing water C. Scanning with a wheel-type transducer with the transducer inside a liquid filled tire D. All of the above

Q2: In an immersion test of a piece of steel or aluminum, the water distance appears on the display as a fairly wide space between the initial pulse and the front surface reflection because of: A. Reduced velocity of sound in water as compared to test specimen B. Increased velocity of sound in water as compared to test specimen C. Temperature of the water D. All of the above

Q2: Using the immersion method, a distance amplitude curve (DAC) for a 19 mm diameter, 5 MHz transducer shows the high point of the DAC at the B/51 mm block. One day later, the high point of the DAC for the same transducer is at the J/102 mm block. Assuming the calibration has not change, this would indicate that the transducer: A. B. C. D.

Is improving in resolution Is becoming defective Has the beam of smaller transducer Both A & B

Hint: B leads to C, thus D is the standard answer. http://www.ndt-instrument.com/UltrasonicThicknessGauge.asp?sort=Ultrasonic+Flaw+Detector

Q176: To evaluate and accurately locate discontinuities after scanning a part with a paintbrush transducer, it is generally necessary to use a: A. Transducer with a smaller crystal B. Scrubber C. Grid map D. Crystal collimator

38. The component in a conventional immersion system which spans the width of the immersion tank is called: A. An articulator.

B. A bridge.

C. A manipulator.

D. A search tube.

5.10: Scanning Patterns

Scanning Patterns

5.11: Pulse Repetition Rate and Penetration The energy of the generated sound depend on the pulse repetition rate, the higher the repetition rate the higher the energy and the sound able to penetrate thicker material. However if the PRR is excessive, ghost signal may formed, this is due to the fact that the next sequence of pulse is generated before the expected returning signal reaching the receiver.

1. The pulse repetition frequency or pulse repetition rate PRR: is the number of pulse of ultrasonic energy that leave the probe in a given time (per second). Each pulse of energy that leave the probe must return before the next pulse leave, otherwise they will collide causing ghost echoes.

2. Transit time: The time taken for the pulse to travel from the probe and return 3. Clock interval: The time between pulse leaving the probe.

The transit time must be shorter than the Clock interval else, ghost signal may formed. Typically the Clock interval should be 5 time the transit time.

PRR- Pulse Repetitive Frequency/Rate and Maximum Testable Thickness Clock interval = 1/PRR When Transit time = Clock interval For pulse echo method: Maximum testable length = ½ x Velocity x Clock interval Typically the Clock interval should be 5 time the transit time, i.e. the sound path should travel 5 times the maximum testable length. (1st BWE, 2nd BWE, 3rd BWE, 4th BWE to 5th BWE.) Note: The Clock interval has neglected the time occupied by each pulse.

Pulse Repetition Rate and Penetration

Pulse Repetition Rate and Penetration

Pulse-Length and Near Surface Sensitivity

Q186: The maximum scanning speed possible is primarily determined by: A. B. C. D.

The frequency of transducer Viscous drag problem The pulse repetition rate of test instrument The persistency of the ultrasonic instrument display

Q200: When setting up an ultrasonic inspection, the repetition frequency for the ultrasonic instrument should be set: A. B. C. D. E.

So that its period is at least as long as the operating time The same as the transducer resonance frequency As low as possible to avoid over-pulsing and distortion According to the instrument manual None of the above

5.12: Interferences & Non-Relevant Indications Following are signal interferences that may produce non-relevant UT indications: 1.

2.

3.

4.

5.

6.

7.

Electrical interference Transducer interference Test specimen geometric interference Test specimen surface interferences Test material structure interferences Test material internal mode conversion interference UT techniques induced interferences (In correct PRR/ Band width/ Frequency selection/ Excessive Beam Spread/ etc.)

Transducer Interference- Transducer internal reflections & Mode conversion may cause interference

Specimen Surface Interference Excessive surface roughness, air bubbles on the surface (on the transducer front, specimen front and back for immersion techniques.

Surface wave for testing near the edges

Specimen Surface Interference

? ?

Specimen Surface Interference- You can determined whether the signal is from the surface wave or the refracted wave simply by touching the surface ahead of the wave (assuming the velocity of surface wave at 0.9 of the shear wave)

Mode Conversion Interference The mode conversion interference during testing of long cylindrical specimen with longitudinal wave often appeared after the first back wall echo. The signal can be easily distinguished and ignore.

Material Geometric Interference False signals may generated due to the test specimen structural configurations resulting in spurious signals.

Non Relevant Indications Transducer with Excessive Beam Spread may generate signal, usually after the 1st BWE. The example below the convex surface defocused the beam and lead to excessive beam spread, using a proper contoured probe may eliminate the problem. However excessive contour may results in generation of surface wave.

Non Relevant Indication Large grain size especially casting may cause excessive hash or grass signal.

Properly selecting probe with lower frequency may relieve the problem.

However this can only de accomplished with reduction in sensitivity.

Non Relevant Indication Large grain size at heat affected zone HAZ (CGHAZ) may cause localized signal due to large grain size. The signal may be wrongly assessed as a defect.

Non Relevant Indications The geometric abnormalities at root penetration and weld surface (crown) may reflect the sound path, returning to the receiver as signals. To distinguished the non relevant indications, finger touching will damped the signals. Further testing may be necessary to ensure the signals were not from the surface defects like surface crack. Any near surface indication that are unusually consistent could be a non relevant indication.

5.13: Entry Surface Variables Entry surface variables include: 1. surface roughness 2. surface coatings 3. couplant condition.

5.13.1 Surface Roughness Surface roughness will have several possible effects on the inspection of a test piece. In contact testing roughness on a gross scale results from: weld spatter, plate scale, dirt (sand) and rough cast surfaces from sand casting.

These irregularities will cause some points of contact to push away the couplant and force it into the lower areas around the probe. If the couplant is not sufficiently viscous it will drain away quickly and fail to couple the probe to the test piece. See Figure 8-3.

http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm

Entry surface variables: Surface roughness

Air Gap Low Viscosity Couplant

High Points of Rough Surface

In addition to reduced coupling, which will reduce signal amplitudes, the rough surface increases the rate of wear on the probe. On an otherwise smooth surface isolated protrusions such as weld spatter can hinder or stop probe motion or in the case of mechanized systems there may be sufficient force to move the probe past the obstruction but this could result in damaging the probe by either tearing it from its mounting or severely scoring the plastic wedge. When the dirt on the test piece is very fine (similar to a flour texture) coupling can be prevented due to surface tension preventing the liquid couplant penetrating to the metal. Unless a transfer value has been established between test piece and calibration piece, this could go undetected.

In addition to affecting coupling, surface roughness tends to reduce signal amplitude by scattering and focusing the beam. This applies to both contact and immersion testing.

Whether uniform or irregular, a rough surface has the potential to present a scattering effect at an interface where a beam impinges. The degree of scattering is based on the ratio of roughness to wavelength.

When roughness is less than about 1/10 a wavelength, scatter will be negligible.

To reduce signal losses due to scattering an operator can select a lower frequency probe. With a wavelength of 0.37mm in water for a 4MHz probe, signal loss due to scatter can occur for irregularities as small as about 0.04mm.

In addition to signal reduction another effect of surface irregularities is to redirect and mode convert some energy which when returned to the probe can be the source of spurious signals. In contact testing false indications from standing waves resulting from scatter on rough surfaces will normally have short sound paths. They can be eliminated as true flaws by failing to locate any trace of indication from the full skip or from the opposite side.

Unless done properly, removal of surface roughness by mechanical means can result in further scattering problems. Small curved gouges left by a grinding wheel used to remove spatter or machining grooves can form small lenses. The affect of grinding can be unpredictable. Some of the lensing may concentrate the beam thereby increasing signal amplitude, or, the lens effect may be a de-focusing of the beam, again resulting in lower than expected signal amplitudes.

Uniform surface preparation by sand or shot blasting usually provides a good surface for ultrasonic testing. Removal of excess metal by a hand held grinding wheel is commonly used on weld caps and roots. When a pipe weld has had its root ground flush and inspection can only be performed from the outside diameter, quality of grinding can result in unnecessary repair calls if grinding has been along the weld axis. The small grooves made by the grinding wheel run parallel to the root edge and are easily confused with lack of fusion, missed edge or undercut defects.

Keywords on Rough Surface: 1. The degree of scattering is based on the ratio of roughness to wavelength. When roughness is less than about 1/10 a wavelength, scatter will be negligible.

2. Consequences of Surface Roughness: ď Ž Signal reduction ď Ž Redirect and mode convert some energy which when returned to the probe can be the source of spurious signals.

3. The False Indications: In contact testing false indications from standing waves resulting from scatter on rough surfaces will normally have short sound paths. They can be eliminated as true flaws by failing to locate any trace of indication from the full skip or from the opposite side

5.13.2 Surface Coatings Surface coatings are added to protect a surface from corrosion or to enhance its appearance. Thin films, such as oxide layers, anodizing layers or electroplated finishes, and the slightly thicker coatings of paint or lacquer are usually well bonded to the surface. Quality of bond may be compared to the uncoated reference block by a simple transfer value. Even a slight loss due to the coating may be preferable to removing the coating and trying to inspect on the rough surface it hides.

When thickness testing is done on a painted surface the paint thickness can add error to the reading. For example: A nominal 25mm steel plate has a cellulose paint coating of 0.5mm. Vsteel = 5980m/s, V paint = 2600m/s.

If a digital thickness meter is calibrated on a 25mm thick piece of the steel plate without the paint coating and then placed on the painted surface an error will occur.

The coating is sufficiently thin that its interface with the metal will occur in the dead zone but the duration of time spent in the paint is added to the travel time to the opposite wall of the plate. If the true plate thickness at the point of measurement is 25.16mm and the paint coating is 0.5mm thick, the time in the paint is 0.5/ 2.6 x 106 = 0.19Âľs. 0.19 microseconds is equivalent to 1.15mm in steel. The reading on the digital meter would combine the two thickness as though all travel was in steel. This results in 25.16 + 1.15 = 26.31mm as the indicated thickness.

This problem can be overcome by using an A-scan display and measuring the interval between the first and second echo instead of the main bang and first echo. This is shown in Figure 8-4.

Thickness Measurement with Surface Coating Figure 8-4.

5.13.3 Couplant Condition Both contact and immersion methods utilize intervening media to transfer sound from the probe into the test piece and back to the receiver. With immersion methods it is accomplished by a single fluid medium. In contact testing there are nearly always at least two intervening media; the delayline or protective face and the thin film of coupling fluid or grease. Attenuation and acoustic velocity are the two main properties that dictate the performance of a couplant. Attenuation affects amplitude of the signal and velocity will determine both transit time and refracted angles. But attenuation and velocity of couplants are not independent properties. Each is a function of other parameters. Unless these parameters are controlled or in some way compensated for, gross variations from the reference value or calibration conditions can result.

Attenuation of couplants varies with material composition as would be expected. Published attenuation values are available for many materials as indicated in the table below. Attenuation coefficients are often quoted in Neper which allow for frequency dependence. 1 Np = 8.686 dB. Attenuation per unit length= Attenuation Coefficient x f 2 x 8.686 dB/cm.

Table 8-1 indicates Attenuation Coefficient of some common liquids.

-15

In more practical terms, for water with longitudinal wave of 500KHz this would mean an attenuation of about 5 dB per meter.

Example: Attenuation factor for water = 25.3x 10-15 Neper Frequency= 0.5MHz The attenuation = Attenuation Coefficient x f 2 The attenuation = 25.3 x 10-15 x (0.5x106)2 Neper/ cm The attenuation = 25.3 x 10-15 x (0.5x106)2 x 8.686 = 0.055 dB/cm or 5.5 dB/m Since such long water path lengths are not normally used the 0.005 dB/mm attenuation is considered negligible. But for the heavier oils attenuations 200 to 500 times greater can have significant effects on signal amplitude and frequency content.

For the fixed delay-lines or wedge materials used in contact testing attenuation variations can be far more pronounced and variation between manufacturers can cause considerable response differences.

Table 8.2

For example the plastics listed in table 8-2 are typical wedge materials selected by manufacturers and based on velocity for refraction purposes, but attenuation differences would result in noticeable amplitude response variation and frequency content of transmitted waveforms. Since the operator rarely knows what wedge material a manufacturer has used, little can be done to correct for potential variations in periodic inspections where results of tests taken with one or more years separation are compared.

For example the plastics listed in table 8-2 are typical wedge materials selected by manufacturers and based on velocity for refraction purposes, but attenuation differences would result in noticeable amplitude response variation and frequency content of transmitted waveforms. Since the operator rarely knows what wedge material a manufacturer has used, little can be done to correct for potential variations in periodic inspections where results of tests taken with one or more years separation are compared.

5.13.4 More Reading: What is Neper ?

The Neper (unit symbol Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the decibel and Bel, the Neper is unit of the International System of Quantities (ISQ), but not part of the International System of Units (SI), but it is accepted for use alongside the SI. The Neper is a natural linear unit of relative difference, meaning in Neper (logarithmic units), relative differences add, rather than multiply. This property is shared with logarithmic units in other bases, such as the Bel.

http://en.wikipedia.org/wiki/Neper

Like the decibel, the Neper is a unit in a logarithmic scale. While the Bel uses the decadic (base-10) logarithm to compute ratios, the Neper uses the natural logarithm, based on Euler's number (e â&#x2030;&#x2C6; 2.71828).

The value of a ratio in Neper is given by

where x1 and x2 are the values of interest, and ln is the natural logarithm.

In the ISQ, the Neper is defined as 1 Np = 1.

1 Np = ln (2.718) when the ration of

= 2.718

The neper is defined in terms of ratios of field quantities (for example, voltage or current amplitudes in electrical circuits, or pressure in acoustics), whereas the decibel was originally defined in terms of power ratios. A power ratio 10 log r dB is equivalent to a field-quantity ratio 20 log r dB, since power is proportional to the square (Joule's laws) of the amplitude.

Hence the Neper and dB are related via: The decibel and the Neper have a fixed ratio to each other. Example: For a ratio of (x1/x2) The (voltage) level ratio is:

Hence:

Q31: Rough surfaces can cause undesirable effects which are noticeable when parts are tested ultrasonically, include: A. Annular maximum rings B. An increase in width of front face echo and consequent loss of resolving power C. Acoustic mismatch D. Asymmetrical modes Q32: Rough surfaces cause echo amplitude from discontinuities within the part to: A. B. C. D.

Increase Decrease Not change Change frequency

5.14

The Concept of Effective Distance

Effective distance in ultrasonic testing is a distance which take into account of the required sound beam overlap and hits. The effective distance always smaller in the calculation.

Keywords: Loop or full path = Thickness x 2 Effective Distance Deff = Beam Diameter / overlap, or Effective Distance Deff = Beam Diameter / No. of hits Effective Distance Deff = Loop x hits T = Time (of interest) T = Deff / Velocity PRR = 1/T RPM = Revolution per minute Maximum linear traversing speed V for effective inspection = RPM x Deff

Scanning Speed: Scanner speed = (PRR × Effective diameter of probe / Number of hits) Speed of test part = (PRR × Effective diameter of probe / Number of hits) Where: Effective dia. of probe = Dia. of probe – 2 [ (Dia. of probe) × (Percent of overlap between scan / 100) ] Linear speed of disc or pipe in mm/ s = (2πr x RPM / 60)

Q&A on The Concept of Effective Distance

e c n a st i D

Q1: A tubular product is tested by AUT (or UT). The tube is rotated at 500 RPM If beam diameter is 10 mm and overlap between scan is 50%.

Calculate maximum length of the tube that can be tested.

Answer: The effective distance covered by each revolution (Deff) is 5mm.

For 500 RPM the total distance covered: 500 x 5 mm = 2500 mm The inspection rate = 2500/ minute#

Effective coverage 50% overlap = 5mm

Beam diameter (coverage) =10mm

Q2: A steel bar with 200 mm thick is scanned by UT. Minimum number of hits required is 10. What is the maximum PRR to avoid the ghost echo. (VL, steel = 0.59 cm/Îźs ) Answer: The total distance to travel Deff = Thickness x [2 x 10 hits] = 400cm = 4m The time taken to complete the 10 loops (T) = [effective distance / Velocity] T= 4/ 5900 =1/1475 The maximum pulse repetition rate PRR = 1/T = 1475

Distance travelled by 10hits= 10x 200x2=4m

10 hits

200mm

Q3: Minimum number of hits required is 2. What is the maximum allowable axial speed for a probe with effective diameter of 102 mm and PRR of 800 Hz. Answer: The diameter of the beam = 102mm The effective distance by one pulse, Deff = Beam Diameter / No of hits Deff = 102/2 = 51mm The total distance covered 800 pulse = 51x800 =40.8m The axial speed is 40.8m/s

Q4: Assuming that Minimum 3 pulses are required to trigger the alarm in AUT (or UT) . What would be (maximum) scanning speed to detect 0.5 mm size defect while using 4 MHz probe, 10 mm beam diameter with PRR of 0.5 kHz. Answer: The distance Covered by scan in 1 second = PRR x Diameter = 500 x 10 = 5m/s The effective distance covered = D/ hits = 5/3 = 1.667m/s Hint: the size of the defect and mode frequency has no implication of the calculation

Q5: What is the maximum PRR is needed for contact test of steel material with 100 mm thick using longitudinal wave. Minimum number of hits required is 10. (VL, steel = 0.59 cm/Îźs ) Answer: Distance = 100 x 2 =200mm for single loop.

Effective distance Deff = 200 x10 =2000 mm Time = Deff / Velocity = 1/2950 PRR = 1/T = 2950

Q6: What is the maximum PRR is needed for immersion testing of aluminum with 80 mm thick using longitudinal wave. The water path is 10 mm.

Minimum number of hits required is 20. (VL, aluminum = 6.32 × 103 m/s, VL, water = = 1.48 × 103 m/s) Answer: Single Water path =10mm, Single Al path= 80mm LoopH2O = 0.02 m, loopAl = 0.16 m Time for traversing single hit = [0.02/1480] + [0.16/6320] = 3.883 x 10-5 s Time for traversing 20 hits T’ = 20 x T = 3.883s PRR = 1/T’ = 1287

Q7: A steel plate size 6.2 m Ă&#x2014;1.8 m Ă&#x2014;0.1m is scanned using 25 mm diameter normal probe and overlap between scan is 20%. Minimum number of hits required is 15. Calculate the inspection time if scanning speed is 500 mm/s.

Answer: (Standard Answer) Effective area of probe = 25 x0.8 x 500 = 10000 mm2 Area of plate = 6200 x 1800 mm2 T = (6200 x 1800)/ 10000 = 1116 s

20% overlap

25mm Effective area = 25 x 500 x 0.8 = mm2

Scanning speed 500 mm/s

Q7: A steel plate size 6.2 m Ă&#x2014;1.8 m Ă&#x2014;0.1m is scanned using 25 mm diameter normal probe and overlap between scan is 20%. Minimum number of hits required is 15. Calculate the inspection time if scanning speed is 500 mm/s.

Answer: In-correct answer.

Area covered by probe in 1 second = 25 x 500 = 12500 mm2 Effective plate area = 6200 x 1800 x 1.2 mm2 Time to scan the plate = [6200 x1800 x 1.2 / 12500] = 1071.36s (maintaining the probe area, increase the surface area by the overlap) Hint: the plate thickness is of no concern.

Q8: Assume that the minimum PRR is needed for contact test of a given steel plate is 240 Hz. How much volume would be covered under test if the PRR is set at 120 Hz. Answer: Volume inspected by ultrasound per unit time = PRR x Pulse length x Cross sectional area of beam at point of interest. = PRR x constant (k) Where k = Pulse length x Cross sectional area of beam at point of interest The ration of volume covered = 120k / 250k x 100 = 50%

5.15: Questions & Answers Exercises

The 6 dB Method For Large Reflector (greater than beam width), i.e. there is no BWE.

Compared 6 dB Drop Sizing with Equalization Technique The Equalization Back Wall Sizing- The probe moving off the edges of the reflector until the amplitude is equal to the rising BWE

Q1 What is the correct water path between the transducer and the steel front surface to focused a transducer for a area of interest at ½ below a steel surface?

Given that: Focal length of transducer in water = 6” Velocity of sound in water= 1484 m/s Velocity of sound in steel = 5920 m/s Equivalent depth in water for ½ steel depth = 4x ½ = 2” The water path= 6”- 2” = 4”

Q5: Ultrasonic inspection is being for a circumferential weld of a pressure vessel. Equipment is calibrated at the beginning of the examination and scanning started at 9.00 AM. In between calibration is also done and at 12.00 noon the equipment is not functioning properly. Still 30 % of weld is to be examined. As per procedure and as level II one can: A. go ahead with the scanning by doing recalibration B. perform the inspection fully from the beginning after recalibration C. bring another equipment and proceed scanning from the left out place D. check and recalibrate the equipment and continue scanning from the portion where scanning started after calibration at 11.00 AM

Q5: When dissimilar metal welds is to be tested ultrasonically and scanning is to be performed from both sides of the weld the calibration blocks shall be made from: A. B. C. D.

material having higher tensile strength both the materials material subjected to heat treatment calibration block material is aluminum

Break Time

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