Volume 10 Issue 2 September 2011 by ram jnde

Volume 10 issue 2 September 2011

from the Chief Editor This issue of the Journal of Nondestructive Testing and Evaluation has technical papers on 4 separate topics of interest to the NDT community. The paper on detection of disbonds in adhesively bonded structures using thermal imaging techniques brings forth a non-contact technique for measurement of defects in components that are now using adhesive for joining metals. This technique has excellent applications for thin plate like structures. The authors use 2 thermography methods and confirm their results using ultrasonic pulse echo measurements. The paper on measuring vibration inside the FBR core is an interesting and novel approach since the conventional pulse echo method is employed in a position sensing mode for the measurement of vibration. This method certainly finds application in solving problems where other forms of vibration measurement are not feasible. The demonstration of this technique under sodium pool is a commendable achievement. The use of frequency spectrum for extracting the vibration values is also interesting. The paper on the inversion of conductivity vs. frequency measurement to predict the depth profiling of conductivity values (as a function of depth) provides a powerful tool for processes such as peening and machining for measuring the material state such as residual stress, hardness, etc., with depth. The fourth paper provides an industry perspective on the use of ultrasonic methods for detecting entrapped foreign objects inside the pressurized tanks with access only from the outside. The authors extend the time of flight diffraction technique (TOFD) commonly used in the detection of cracks in welded plates to a new application for pressure vessels. The BASICS article introduces the Impact Echo technique that is commonly used in the NDT of civil structures including piers, columns, beams, etc. The Compton Backscattering technique introduced in the HORIZONS holds great promise to solve the limitation of the current industrial radiography procedures, by providing a single side access method for the NDT of objects. This technology is already becoming popular in the security screening devices, as may be found in airports, and the feasibility of applying this technique for NDT is showing excellent results. The NDT PUZZLE challenges the reader once again. The IQFORUM has been left out of this issue since the previous articles has failed to invoke any response from the readers. The NEWS and the PATENTS sections will keep the audience up-to-date on the recent activities in the field of NDT. The NDE2011 is scheduled for 6th -10th of December 2011 at the CHENNAI TRADE CENTRE. This promises to be a MEGA event with expected registrations exceeding 1500 with extensive participation of international delegates and speakers. The industrial exhibition with about 80+ booths will have the latest NDT tools and instruments on display. There are 4 preconference WORKSHOPS on Aerospace, Automobile, Concrete and Boilers. I encourage all of you to register at the earliest and plan to be a part of this EVENT. For more information, please visit www.nde2011.com. Dr. Krishnan B alasubramaniam Balasubramaniam Professor Centre for Non Destructive Evaluation IITMadras, Chennai balas@iitm.ac.in jndte.isnt@gmail.com URL: http://www.cnde-iitm.net/balas

Journal of Non Destructive Testing & Evaluation

vol 10 issue 2 September 2011

I S N T - National Governing Council Chapter - Chairman & Secretary President Shri K. Thambithurai President-Elect Shri P. Kalyanasundaram Vice-Presidents Shri V. Pari Swapan Chakraborty Shri D.J.Varde Hon.General Secretary Shri R.J.Pardikar Hon. Treasurer Shri T.V.K.Kidao

Ahmedabad

Kota

Shri D.S. Kushwah, Chairman, NDT Services, 1st Floor, Motilal Estate, Bhairavnath Road, Maninagar, Ahmedabad 380 028. dskushwah@icenet.net Shri Rajeev Vaghmare, Hon. Secretary C/o Modsonic Instruments Mfg. Co. Pvt. Ltd. Plot No.33, Phase-III, GIDC Industrial Estate Naroda, Ahmedabad-382 330 modsonic@modsonic.com

Shri R.C. Sharma, Chairman Associate Director (QA), Rawatbhata 323 307 rcsharma@npcil.co.in Shri S.V. Lele, Hon. Secretary, T/IV – 5/F, Anu Kiran Colony, PO Bhabha Nagar, Rawatbhata 323 307. svlele@npcil.co.in

Mumbai Bangalore Prof.C.R.L.Murthy, Chairman Dept. of Aerospace Engg, Indian Institute of Science, Bangalore 560012 Email : crlmurty@aero.iisc.ernet.in crlmurty@aero.iisc.ernet.in

Hon. Joint Secretaries Shri Rajul R. Parikh

Chennai Immediate Past President Shri Dilip P. Takbhate Past President Shri S.I.Sanklecha Members Shri Anil V. Jain Shri Dara E. Rupa Shri D.K.Gautam Shri Diwakar D. Joshi Dr. Krishnan Balasubramaniam Shri Mandar A. Vinze Shri B.B.Mate Shri G.V. Prabhugaunkar Shri B.K.Pangare Shri M.V. Rajamani Shri P.V. Sai Suryanarayana Shri Samir K. Choksi Shri B.K. Shah Shri S.V. Subba Rao Shri Sudipta Dasgupta Shri N.V. Wagle Shri R.K. Singh Shri A.K.Singh (Kota) Shri S. Subramanian Shri C. Awasthi Brig. P. Ganesham Shri Prabhat Kumar Shri P. Mohan Shri R. Sampath Shri V. Sathyan Shri B. Prahlad Ex-officio Members Managing Editor, JNDT&E Shri V. Pari Chairman, NCB & Secretary, QUNEST Dr. Baldev Raj Controller of Examination, NCB Dr. B. Venkatraman President, QUNEST Prof. Arcot Ramachandran All Chapter Chairmen/Secretaries Permanent Invitees Shri V.A.Chandramouli Prof. S. Rajagopal Shri G. Ramachandran & All Past Presidents of ISNT

vol 10 issue 2 September 2011

Shri R.S. Vaghasiya, Chairman, B 4/7, Sri Punit Nagar, Plot 3, SV Road, Borivile West, Mumbai 400 092. ravji.vaghasiya@gmail.com Shri Samir K. Choksi, Hon. Secretary, Director, Choksi Brothers Pvt. Ltd., 4 & 5, Western India House, Sir P.M.Road, Fort, Mumbai 400 001. Choksiindia@yahoo.co.in

Nagpur

Shri R. Sundar, Chairman Director of Boilers, Tamil Nadu Shri R. Balakrishnan, Hon. Secretary, No.13, 4th Cross Street, Indira Nagar, Adyar, Chennai 600 020. rbalkrishin@yahoo.co.in

Shri Pradeep Choudhari, Chairman Parikshak & Nirikshak, Plot M-9, Laxminagar Nagpur - 440 022 Mr. Jeevan Ghime, Hon. Secretary, Applies NDT & Tech Services, 33, Ingole Nagar, B/s Hotel Pride, Wardha Road, Nagpur 440 005. antstg_ngp@sancharnet.in

Delhi

Pune

Shri Ashok Singhi, Chairman, MD, IRC Engg Services India Pvt. Ltd 612, Chiranjiv Tower 43, New Delhi irc_engg@hotmail.com Shri Dinesh Gupta, Hon.Secretary, Director, Satya Kiran Engg. Pvt. Ltd BU 3 SFS Pitampura, New Delhi 110034 sathyakiran@bal.net.in

Shri PV Dhole, Chairman NDT House, 45 Dr Ambedkar Road, Sangam Bridge, Pune- 411 001 info@technofour.com Shri VB Kavishwar, Hon Secretary, NDT House, 45 Dr Ambedkar Road, Sangam Bridge, Pune- 411 001 eddysonic@gmail.com

Sriharikota Hyderabad Shri M. Narayan Rao, Chairman, Chairman & Managing Director, MIDHANI, Kanchanbagh, Hyderabad 500 058. cmd.midhani@ap.nic.in Shri Jaiteerth R. Doshi, Hon.Secretary, Scientist, Project LRSAM DRDL, Hyderabad 500 058. joshidrdl@gmail.com

Jamshedpur Dr N Parida, Chairman, Senior Deputy Director Head, MSTD, NML, Jamshedpur - 831 007 nparida@nmlindia.org Mr. GVS Murthy, Hon. Secretary, MSTD, NML, Jamshedpur gvs@mnlindia.org / gvsmurthy_mnl@yahoo.com

Kalpakkam Shri YC Manjunatha, Chairman Director ESG, IGCAR, Kalpakkam – 603 102 ycm@igcar.gov.in Shri BK Nashine, Hon.Secretary Head, ED &SS, C&IDD, FRTG IGCAR, Kalpakkam – 603 102 bknash@igcar.gov.in

Kochi Shri CK Soman, Chairman, Dy. General Manager (P & U), Bharat Petroleum Corporation Ltd. (Kochi Refinery), PO Ambalamugal 682 302. Kochi somanck@bharatpetroleum.in Shri V. Sathyan, Hon. Secretary, SM (Project), Bharat Petroleum Corporation Ltd. (Kochi Refinery), PO Ambalamugal-682 302. Kochi sathyanv@bharatpetroleum.in

Kolkata Shri Swapan Chakraborty, Chairman Perfect Metal Testing & Inspection Agency, 46, Incinerator Road, Dum Dum Cantonment, Kolkata 700 028. permeta@hotmail.com Shri Dipankar Gautam, Hon. Secretary, 4D, Eddis Place, Kolkata-700 019. eib1956@gmail.com

Shri S.V. Subba Rao, Chairman, General Manager, Range Operations SDSL, SHAR Centre Sriharikota 524124. svsrao@shar.gov.in Shri G. Suryanarayana, Hon. Secretary, Dy. Manager, VAB, VAST, Satish Dhawan Space Centre, Sriharikota-524 124. isnt@shar.gov.in

Tarapur Shri PG Behere, Vice Chairman, AFFF, BARC, Tarapur-401 502. pgbehere1@rediffmail.com Shri Jamal Akftar, Hon.Secretary, TAPS 1 & 2, NPCIL, Tarapur. jakftar@npcil.com

Tiruchirapalli R.J. Pardikar AGM, (NDTL) BHEL Tiruchirapalli 620 014. rjp@bheltry.co.in Shri A.K.Janardhanan, Hon. Secretary, C/o NDTL Building 1, H.P.B.P., BHEL, Tiruchirapalli 620 014. akjn@bheltry.co.in

Vadodara Shri P M Shah, Chairman, Head-(QA) Nuclear Power Corporation Ltd. npcil.bar@gmail.com M S Hemal Thacker, Hon.Secretary, NBCC Plaza, Opp.Utkarsh petrol pump, Kareli Baug, Vadodara-390018. pmetco@gmail.com

Thiruvananthapuram Dr. S. Annamala Pillai, Chairman Group Director, Structural Design & Engg Group, VSSC, Thiruvananthapuram 695022 sannamala@vssc.gov.in Shri. Imtiaz Ali Khan Hon.Secretary, Engineer, Rocket Propellant Plant, VSSC, Thiruvananthapuram 695 022 imtiaz_ali@vssc.gov.in

Visakhapatnam Shri Om Prakash, Chairman, MD, Bharat Heavy Plate & Vessels Ltd. Visakhapatnam 530 012. Shri Appa Rao, Hon. Secretary, DGM (Quality), BHPV Ltd., Visakhapatnam 530 012

Journal of Non Destructive Testing & Evaluation

Journal of Non Destructive Testing & Evaluation About the cover page:

Volume 10 issue 2 September 2011

Contents Basics - Impact Echo Technique

Micro IR Imaging on MEMS Devices The cover page for this issue shows a thermography image of a Micro-ElectronicMechanical-Sensor (MEMS) as-fabricated wafer. The MEMS technology is now becoming very common method for fabrication of mechanical devices such as micro-motors, micro-pumps, etc. that are extremely small in size. MEMS based sensors are also found to be useful in measuring pressure, acceleration, temperature, etc. These devices are so small that they can be embedded inside human bodies, inside aircraft structures, and provide real-time data logging. They are fabricated using micro-lithogragphy methods that are similar to the way IC circuits in computers are fabricated, only not quite so small. The image is obtained using a micro-lens attached to a FLIR IR Camera at 6 micron resolution. This was done in PASSIVE mode. The fractures in the cantilever sensors can be observed using this imaging technique. (Courtesy: Centre for NDE at IIT Madras)

Horizon - NDE by Backscatter Imaging

Chapter News

NDE Events

NDE Patents

NDT Puzzle

Technical Papers Non destructive detection of debonding in adhesively bonded metal/ceramic composite plates Sony Punnose, Amretendu Mukhopadhyay, B. Nagaraja Kowmudi, P. Rama Subba Reddy, V. Madhu and Vikas Kumar

NDE Technique for Reactor Core Vibration Measurement in FBRs R. Ramakrishna, P. Anup Kumar, M. Anandaraj, M. Thirumalai, V. Prakash, C. Anandbabu and P. Kalyanasundaram

On the conversion of multi-frequency â&#x20AC;&#x153;apparentâ&#x20AC;? conductivity data to actual conductivity gradients on peened samples Veeraraghavan Sundararaghavan, Krishnan Balasubramaniam

Ultrasonic Non-Destructive Evaluation (NDE) based internal inspection of pressure vessels for better maintenance practice S.K.Nath and B.H.Narayana

Probe

Chief Editor Prof. Krishnan Balasubramaniam e-mail: balas@iitm.ac.in The Journal is for private circulation to members only. All rights reserved throughout the world. Reproduction in any manner is prohibited. Views expressed in the Journal are those of the authors' alone. Published by Shri RJ Pardikar, General Secretary on behalf of Indian Society for Non Destructive Testing (ISNT) Modules 60 & 61, Readymade Garment Complex, Guindy, Chennai 600032 Phone: (044) 2250 0412 Email: isntheadoffice@gmail.com and Printed at VRK Printing House 3, Potters Street, Saidapet, Chennai 600 015 vrkonline@gmail.com Ph: 09381004771

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Dr T Jayakumar, Ultrasonic & Acoustic Emission Methods

Sri P Kalyanasundaram Advanced NDE Methods

Sri K Viswanathan Radiation Methods

Editorial Board Dr N N Kishore, Sri Ramesh B Parikh, Dr M V M S Rao, Dr J Lahri, Dr K R Y Simha, Sri K Sreenivasa Rao, Sri S Vaidyanathan, Dr K Rajagopal, Sri G Ramachandran, Sri B Ram Prakash

Advisory Panel Prof P Rama Rao, Dr Baldev Raj, Dr K N Raju, Sri K Balaramamoorthy, Sri V R Deenadayalu, Prof S Ramaseshan, Sri A Sreenivasulu, Lt Gen Dr V J Sundaram, Prof N Venkatraman

Objectives The Journal of Non-Destructive Testing & Evaluation is published quarterly by the Indian Society for Non-Destructive Testing for promoting NDT Science and Technology. The objective of the Journal is to provide a forum for dissemination of knowledge in NDE and related fields. Papers will be accepted on the basis of their contribution to the growth of NDE Science and Technology.

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Journal of Non Destructive Testing & Evaluation

Basics Impact echo technique Dr. T. Jayakumar Metallurgy and Materials Group Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India

Co-ordinated by Prof. O. Prabhakar

1. INTRODUCTION Concrete is a composite material consisting of a binding medium with aggregates like gravel, sand etc. embedded in the medium. Ultrasonic and impact-echo test methods are two important methods that are widely used for the nondestructive examination of concrete structures [1]. Testing of thick concrete structures using ultrasonic technique is often difficult due to large scattering and attenuation of the sound energy in the medium that results in poor signal-to-noise ratio of the reflected signal amplitudes. Also, in thick structures, ultrasonic through transmission technique is suggested. This needs accessibility of both surfaces as well as proper alignment of both the transducers, which is quite difficult if not impossible. To overcome these limitations and to reliably examine the concrete structures, impact-echo test method was developed in the mid 1980’s as a nondestructive test method [2, 3]. Since its development, impact echo technique has found its wide applicability in NDE of concrete structures [2, 3]. 1.1 PRINCIPLE OF IMPACT ECHO TECHNIQUE

Tapping an object with a hammer is one of the oldest forms of nondestructive testing techniques based on stress wave propagation. Depending on whether the result is a high-pitched “ringing” sound or a low frequency “rattling” sound, the

integrity of the member can be assessed. The method is subjective, as it depends on the experience of the operator, and it is limited to detecting near surface defects. Despite these inherent limitations, sounding is a useful method for detecting near-surface delaminations, and it has been standardized by ASTM [4]. Impact echo technique may be considered as a sophisticated and instrumented advancement of the sounding technique. It involves introducing a transient stress pulse of predefined frequency band-width into a test object by selecting the size of the spring loaded steel ball impactor and monitoring the surface displacements using a dry-coupled piezoelectric receiver with an integrated amplifier. The obtained impact pulse is analyzed online with the help of a data acquisition system and a computer with necessary signal analysis software. The impact pulse consists of compression (P) and shear (S) waves which propagate into the object along spherical wave fronts, and a Rayleigh (R) wave that propagates along the surface. These waves are reflected by internal defects and the boundaries and the reflected waves propagate back to the surface. At the top surface, the waves are reflected again and they once again propagate into the test object. Thus, a transient resonance condition is setup by multiple reflections of waves between the top surface and internal flaws or external boundaries. A

Journal of Non Destructive Testing & Evaluation

displacement transducer located close to the impact point is used to monitor the surface displacements caused by the arrival of these reflected waves. P-waves are of primary importance in the impact echo testing of concrete structures, because the displacements caused by P-waves are much larger than those caused by S-waves at points located close to the impact point [3, 5, 6]. The amplitude of the reflected Pwave, Areflected is given by (1)

Where Z1 and Z2 are the acoustic impedances of the regions in which the wave is approaching the interface and of the region beyond the interface respectively, and A1 is the amplitude of the particle motion in incident wave. The phase of the reflected wave depends upon the ratio of Z2/Z1. If Z2/Z1 is less than one (at concrete/air interface; Z2/Z1 = 5 x 10-5), then the phase reversion takes place at the interface. Because of the phase reversion at both the interfaces, waves reflected between two concrete/air interfaces produce successive arrival of tension wave (inward displacement) at the impact surface. The frequency of P-wave arrivals at the transducer is determined by transforming the time domain signal into the frequency domain using the fast Fourier transform technique. The frequencies associated with the peaks in the resulting amplitude spectrum represent the dominant frequencies in the waveform. 2. EQUIPMENT The impact echo equipment consists of (a) a hand held unit containing an impacting device (steel ball) for producing low frequency stress waves (sound waves) and a pair of piezoelectric transducers that detect surface displacements caused by reflected waves, (b) a high speed, vol 10 issue 2 September 2011

Basics

analog to digital data acquisition system that receives and digitizes the analog voltage signal from the transducers and transfers it to the computer, (c) a portable computer, and (d) a software program that monitors each test, and guides the data processing to produce output displays viz., time signal, frequency spectrum, and frequency amplitude as a function of % depth, that provides information about the structure being tested. Figure 1 shows various components of a typical impact echo system. 2.1 IMPACTOR

The impactor is usually spherical or spherically tipped. The spring loaded hardened steel balls of various diameters in the range of 1 mm to 16 mm can be used for generating the stress waves of different frequency ranges in the specimen. The highest frequency of the generated stress waves (fmax) can be estimated by the following equation [7]: (2)

Where, fmax is in kHZ and D is the diameter of the hardened steel ball in meters. The impact delivers sufficient energy into the concrete structure so that a well defined amplitude spectrum is obtained with predominant peaks. The impact duration, tc must be less than the round trip travel time for a P-wave during the impact echo testing, i.e.

Where, T is the thickness of the specimen and C p is the P-wave velocity in the concrete specimen. The smaller diameter ball generates higher frequency wave with low energy and is more useful for the near surface defect detection, whereas, the larger diameter ball vol 10 issue 2 September 2011

generates lower frequency wave with more energy and hence is more useful for inspection up to larger depths. A 3 mm diameter impactor (the smallest practical size is about 1.5 mm in diameter) produces stress waves with useful frequencies up to almost 90 kHz, and wavelengths as small as 0.04 m. At the high end of this frequency range, the stress waves begin to get scattered and reflected by the natural inhomogeneous regions in concrete, such as small air inclusions and mortar/aggregate interfaces, with the result that there is more “noise” in the waveform and spectrum. Another consideration in selecting an impactor is the relationship between the wavelength and size of a flaw or discontinuity that can be detected. A flaw of lateral dimension l is “invisible” to stress waves of wavelength greater than l. Combining the fundamental relationship between wave speed, frequency and wavelength, Cp = fl (where l is the wavelength), with Eq. 2 and by using a wave speed of 4000 m/s in concrete, the minimum detectable lateral dimension of the flaw (Lmin) is given by Lmin = 14D

(3)

Thus, the minimum lateral size of a flaw that can be detected, is about 14 times the diameter of the impactor. 2.2 RECEIVER TRANSDUCER

Two broadband, piezoelectric transducers which respond to normal surface displacement are used. These transducers are capable of detecting the small displacements that correspond to the impact generated P-wave traveling along the surface during the velocity measurement and the reflected P-wave from the interface during the impact echo testing. The transducer with a piezoelectric element of a small contact area (a tip diameter of 1.5 mm) and the larger end attached to

a brass backing block has been found suitable. A suitable material is required to be used to couple the transducer to the concrete. A lead sheet approximately 0.25 mm thick is a suitable coupling material for such a transducer. 2.3 SPACER DEVICE

A spacer device is used to hold the transducer at a fixed distance apart during measurement of the wave velocity, as shown in Fig. 1. The transducer tips shall be placed about 300 mm apart and the actual distance between the tips of the transducers be measured and recorded to the nearest 1 mm. 2.4 DATA ACQUISITION SYSTEM AND ANALYSIS SOFTWARE

This consists of hardware and software for acquisition, recording and processing of the signal output of the two transducers. This system can be a portable computer with a two channel data acquisition card or a portable two channel waveform analyzer. The sampling rate for each channel is at least 500 kHz. The system is capable of triggering on the signal from one of the recording channels and acquisition of upto 2048 data points. Higher sampling rate increases the accuracy in the time of flight measurement and larger data length increases the frequency resolution. Figure 2 shows the front panel of the software developed at the authors’ laboratory for impact echo testing. It consists of acquisition of impact pulse for a duration of about 2 – 5 μs depending upon the thickness of the structure, transforming the signal to frequency domain and then converting the frequency spectrum to a depth vs. amplitude graph. The software also calculates the expected peaks in the frequency spectrum based on the thickness and the shape of the structure. The software consists of various additional features such as

Journal of Non Destructive Testing & Evaluation

Basics

Fig. 1

: Components of a typical impact echo system.

Fig. 2

: Front panel of the software for impact-echo testing showing the thickness response of a concrete block of 400 mm thickness.

selecting suitable time and frequency windows for analysis. One of the important additional features in the developed software as compared to the commercial software is the display of depth vs. amplitude spectrum in a linear depth scale. This allows clear visualization of the location of a defect in the structure, if any found. 3. MEASUREMENT OF P-WAVE VELOCITY The P-wave velocity needs to be determined in the structure before

Fig. 3 : Experimental setup for P-wave velocity measurement using procedure A.

Journal of Non Destructive Testing & Evaluation

vol 10 issue 2 September 2011

Basics

(a) Fig. 4

(b)

: Signal acquired and procedure for determination of P-wave velocity by a) directly measuring the travel time of P-wave and b) by measuring the travel time of R-wave.

the start of the impact echo testing. It can be measured in two different ways: by using a two reeiver system for the measurement of velocity on the surface (procedure A) and, by using a single transducer to measure the thickness averaged velocity (procedure B). The details of these measurements are given below: 3.1 PROCEDURE A

This procedure measures the time taken by a wave to travel between two transducers positioned at a known distance apart (300 mm) along the surface of a structure. The schematic of the experimental set up for this procedure is shown in Fig. 3. The wave velocity is calculated by dividing the distance between the two transducers by the travel time. The P-wave velocity can either be calculated directly by measuring the travel time of P-wave or by measuring the travel time of R-wave and then using the relation between R-wave and P-wave velocities, knowing the Poisson’s ratio. 3.1.1 MEASUREMENT OF TRAVEL TIME OF P-WAVE

Figure 4a shows the signal acquired for measurement of P-wave velocity directly. The procedure for determination of P-wave velocity by directly measuring the travel time of P-wave is as follows: As the P-wave generated upon the impact is vol 10 issue 2 September 2011

compression in nature, its arrival can be identified by the first rise in voltage (T 1 in Fig, 4a). The difference in the arrival time of the P-waves at the two transducers (T2T1) can be measured by moving the cursors (Fig. 4a) to locate the rising points in the two voltage plots and this can be used to calculate the Pwave velocity (Cp) as follows: Cp= d/(T2-T1)

(4)

Where d is the distance between the two receivers. 3.1.2 MEASUREMENT OF TRAVEL TIME OF R- WAVE

Figure 4b shows the signal acquired for measurement of R-wave velocity. The procedure for determination of P-wave velocity by measuring the travel time of R-wave is as follows: The arrival of the R-wave is identified by a sharp drop in the voltage due to the tensile nature and high amplitude of the R-wave (T3 and T4 in Fig. 4b). The difference in the arrival time of R-waves at the two transducers (T 4 -T 3) can be measured by bringing the cursors to the first sharp dropping points in each voltage plot (Fig. 4b). The Rwave velocity can be calculated from this as follows: CR= d/(T4-T3)

(5)

and P-wave velocity can be

determined using the following equation [8]:

(6)

where, n is the Poisson’s ratio. The average value of Poisson’s ratio for hardened concretes is 0.2. 3.2 PROCEDURE B

The principle of this procedure is similar to that of impact-echo testing on a concrete structure of known thickness. Impact on the surface of the concrete generates stress waves of which P-wave is of primary importance. The P-wave propagates into the concrete and is reflected from the opposite surface. Multiple reflections of the P-wave between the two surfaces give rise to a transient thickness resonance with a frequency related to the thickness of the concrete and wave velocity in the concrete. A receiving transducer, located adjacent to the impact position, receives the reflected wave and the output of the transducer is captured as a time domain waveform. The frequency of the P-wave arrivals at the transducer is determined by transforming the time domain signal into the frequency domain using the fast Fourier transform technique. The frequencies associated with the peaks in the resulting amplitude spectrum

Journal of Non Destructive Testing & Evaluation

Basics represent the dominant frequencies in the waveform. P-wave velocity can be determined by using the following equation [2]:

(7)

Where, f is the thickness frequency (the frequency of the thickness mode of vibration), T is the thickness and â is the shape factor for the thickness mode of vibration. The values of â for various structures are given in Table 1. 4. MEASUREMENT OF THICKNESS OF CONCRETE STRUCTURE The principle of impact echo testing for measurement of thickness of concrete structure is similar to that explained in section 3.2 (Procedure B for the measurement of the P-wave velocity). Before carrying out the thickness measurement, the wave velocity in the specimen is determined using either the procedure A or B, as described in the previous section. Even though the procedure B is more accurate, it can be used only if the thickness is known in the same structure at any other given location. Then the impact echo test will be carried out as explained in section 3.2. The thickness of the structure is determined by using the following

Fig. 5

: A crack at a depth d gives the same response as a void at that depth

equation obtained by re-arranging Eq. 7.

(8)

5. TESTING OF CONCRETE STRUCTURES FOR DETECTION OF FLAWS/ DEFECTS The experimental setup for detection of flaws in concrete structures is

similar to that discussed in sections 3.2 (procedure B of velocity measurement) and 4 (thickness measurement). The presence of a flaw changes the patterns of stress wave propagation and reflection. These changes shall get reflected both in the waveforms and spectra obtained from impact-echo tests, and they provide both qualitative and quantitative information about the flaws. This section focuses on signal behavior associated with cracks and

Table 1 Various shapes as defined during impact echo testing of concrete structures and corresponding shape factor (ß). Structure shape

Definition

Plate

A structure with two parallel faces, for which the ratio of lateral dimension to thickness is sufficiently large, so that multiple wave reflections from the side boundaries do not reach the transducer within the few milli-seconds in which multiple reflections between the two faces are being recorded. When 1024 data points are recorded at an interval of 2 μs, this ratio is five to six.

0.96

Bar of circular cross section

A structure in which the length is at least three times the diameter

0.92

Bar of square cross section

A structure in which the length is at least three times the thickness

0.87

Bar of rectangular cross section

A structure in which the length is at least three times the largest dimension in the cross section

Function of depth/ breadth (D/B)

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Shape Factor (â)

vol 10 issue 2 September 2011

Fig. 6

Basics

: Comparison of the solid response (left) with the response from a region with a crack of wide lateral extent.

voids in plates, including the special case of a shallow crack or delamination, and the response of plates containing unconsolidated concrete (honeycombing). Although the discussion is focused on plate structures, the interaction of stress waves with flaws and the resulting changes in the waveform and

Fig. 7

spectrum can be generalized to any geometry. A crack or void within a concrete structure forms a concrete/air interface. Cracks with a minimum width (crack opening) of about 0.08 mm cause almost total reflection of a P-wave. The responses from cracks

and voids are similar, because stress waves are reflected from the first concrete/air interface encountered. Thus, a crack at a depth (d) will give the same response as a void whose upper surface (nearest to the impact surface) is at the same depth (Fig. 5).

: Comparison of solid response (left) with response in the vicinity of a flaw of limited lateral extent.

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Fig. 8

: The principle components of the response produced by impact on the surface of a concrete slab containing a shallow delamination: a) flexural mode, and b) thickness mode. The contributions to the spectrum are shown in c) and d).

When tests are carried out to locate flaws in a plate structure, the first step is to determine the response of the solid structure. This is accomplished by performing tests in a region where the structure is known to be solid. If the thickness and wave speed are known, the thickness frequency can be calculated, and tests can be performed until a solid response is obtained. The difference between the solid response and the response when a crack of wide lateral extent (large surface area parallel to the top surface) is present is illustrated in Fig. 6. The depth of the flaw can be determined using Eq. 9, as given below: (9)

The impact echo testing is also influenced by the depth at which a flaw is located. When the lateral dimensions of a crack are comparable to its depth, stress waves are both reflected from the crack and diffracted around it. As a result, Pwave reflections occur both within the layer above the crack and across

the full thickness. However, the full thickness frequency is lower than that of the solid plate because of the reduced stiffness in the vicinity of the crack and because the P-waves must travel a longer path around the crack to reach the bottom surface. The difference between the solid response and the response when a crack of small lateral extent is present, is illustrated in Fig. 7. When the depth of the flaw is greater than about 10 cm, the response from multiple P-wave reflections within the layer above the flaw is relatively strong. If the depth of the flaw is less than 10 cm, flexural vibrations in the thin layer are often excited, and the response is dramatically different. This situation, for example, is frequently encountered on concrete bridge decks, where wide spread cracking 窶田alled delamination- occurs at shallow depths due to corrosion in the reinforcing steel. The resulting signal includes a large-amplitude, low frequency component due to the flexural vibration. The flexural vibrations are similar to the vibration in a drum, and because the resulting

Journal of Non Destructive Testing & Evaluation

surface displacements are far larger than those caused by P-wave arrivals they dominate the signal. The higherfrequency component due to multiple P-wave reflections across the thin layer is weak by comparison, and thus making it sometimes difficult to detect. A schematic representation of the effects of flexural vibration in thin layer is shown in Fig. 8. Flexural vibration, shown in (a), has low frequency (typically 2- 6 kHz) and very large amplitude as compared to surface displacements caused by the arrival of reflected P-waves, shown in Fig. 8(b). Figures 8 (c) and (d) show the corresponding contributions to the spectrum: flexural vibrations produce a highamplitude, low frequency signals that dominate the waveform and spectrum, while the peak resulting from P-wave reflections has a higher frequency and lower amplitude, and is sometimes too small to be seen. There are two methods for amplifying this high frequency peak: (1) using a smaller impactor and (2) digital high pass filtering. In Fig. 8d, vol 10 issue 2 September 2011

Fig. 9

Basics

: Response of honetcombing at about 130 mm (15 kHz) in a concrete block of 250 mm thickness (solid thicknes frequency = 8 kHz).

the high frequency peak is amplified by applying a digital high pass filter to the same signal for which amplitude spectrum is shown in Fig. 8c. A region of unconsolidated concrete typically consists of large number of small, interconnected voids, commonly referred to as “honeycombing”. Such areas include many small concrete/air interfaces over a range of depths, and often they do not have a well-defined external boundary. The usual response of a honeycombed region to an impact-echo test includes a “displaced thickness frequency” – that is, a strong peak at a frequency smaller than that of the solid plate – and one or more additional peaks corresponding to P-wave reflections from a range of depths within the unconsolidated region. The response of honeycombing at about 130 mm in a 250 mm thick concrete plate is shown in Fig. 9. Multiple peaks centered at about 15 kHz (130 mm thickness) and a displaced thickness frequency peak at 5 kHz can be seen in Fig. 9. The expected location of the solid thickness frequency is shown as a vertical cursor at 8 kHz. vol 10 issue 2 September 2011

6. TESTING OF CONCRETE STRUCTURES FOR DELAMINATION OF REINFORCED BARS The experimental setup for the testing of concrete structures for detection of delamination of reinforced rods is similar to that of impact echo testing as described earlier. The waves reflected between two concrete/air interfaces produce

successive arrival of tension wave (inward displacement) at the impact surface due to the phase reversion at both the interfaces. Whereas, waves reflected between a concrete/air interface and a concrete/steel interface produce arrival of alternate compression wave (outward displacement) and tension wave (inward displacement) at the impact surface, which makes the frequency of the arrival of P-wave of similar nature (compression/tension) half to that when reflected between two concrete/air interfaces. It is this difference in the frequency of arrivals of similar nature of P-wave that is used for the identification of delamination of reinforced bars, as the delamination changes the type of interface from concrete/steel to concrete/air. Figure 10 (a) shows the typical impact echo signal of a reinforced rod intact with the concrete, at a depth of 50 mm. The frequency spectrum consists of a cluster of peaks centered at 17.1 kHz (116 mm). These clustered peaks are the characteristic response of steel reinforcement rods without delamination [9]. As discussed

Fig. 10 : Response of reinforced rod (a) without and (b) with delamination Journal of Non Destructive Testing & Evaluation

Basics earlier; the frequency of the arrival of the wave of similar nature is half, and hence Eq. 9 leads to the depth of the reinforced rod as double of that of the actual depth. Further, the exact depth of the reinforcement rod can be determined by applying the correction factor as per the equation given below [9]: Depth calculated from frequency spectrum = t (-0.6 D/t + 1.5)

(10)

Where D and t are the diameter and the actual depth of the reinforcement rod, respectively. The above correction factor is needed to take into account the interference of the wave reflecting from the concrete-steel interface, steelconcrete interface and the waves

going around the rod. This has more importance when the D/t ratio is much less than 1. Figure 10(b) shows the typical response of a delaminated rod at a depth of 35 mm. The frequency spectrum shows the dominant peaks at 6.3 kHz and 59.6 kHz (33 mm). The peak at 59.6 kHz is due to the reflection from concrete/air interface at the depth of 35 mm (delamination) and the peak of 6.3 kHz corresponds to the flexural vibration of the concrete cover of 35 mm thickness. This is a typical response, if the section above the delamination is thin (less than 100 mm), as discussed earlier [9]. Hence, it is clear from the response of the reinforcement rod (with and

without delamination) that, if the location of the reinforcement rod is known apriori, then it can be easily checked by impact echo testing that whether any delamination has occurred. 7. DETERMINING THE DEPTH OF SURFACE OPENING CRACKS The experimental setup for the depth measurement of surface opening crack, as shown in Fig. 11a, is similar to that for the velocity measurement. The only difference is that the crack is kept in between the two transducers and the impact is given in between the crack and the transducer. When an impact is performed, the waves generated travel on the surface and picked up by the transducer 1 on the same side of the crack. Figure 11b shows the signals received by the two transducers for the crack depth of 25 mm. The waves cannot reach the transducer 2 directly on the surface and the first wave to reach transducer 2 is the P-wave diffracted at the crack tip (Fig. 11a). The time lag between the P-wave to reach directly to transducer 1 and after diffraction to transducer 2 can be calculated from Fig. 11 directly. Using the Pwave velocity and the time lag, the depth of the surface opening crack can be determined by the following equation [10]: Depth = ((CP x Dt)2/4 â&#x20AC;&#x201C; H2)1/2

Fig. 11 : (a) Experimental setup and (b) waveform for depth measurement of the surface opening crack Journal of Non Destructive Testing & Evaluation

(11)

Where CP, Dt and H are the P-wave velocity, time lag and the constant distance between the crack and the transducer 2 (= the distance between the crack and the impact point = the distance between the impact point and the transducer 1), respectively. Using this equation, the depth of the crack is found to be 28 mm, which is very close to the actual depth of the crack [11]. Similarly the depth of the surface opening cracks of depth 50 mm and 75 mm could also vol 10 issue 2 September 2011

Basics

be determined with an accuracy of about Âą10 % [11]. 8. CASE STUDIES 8.1 STRUCTURAL INTEGRITY ASSESSMENT OF RING BEAM OF A NUCLEAR REACTOR CONTAINMENT STRUCTURE USING IMPACT ECHO TECHNIQUE

Containment structures of some of the pressurised heavy water reactors (PHWRs) are made of prestressed concrete with pretensioned cables. One of the important components of this containment structure is the ring beam. The inner containment dome of one of the PHWRs got delaminated during construction. Due to sudden release of force in prestressing cables during collapse of the dome and during subsequent detensioning/slackening of the

cables, delaminations in ring beam were suspected. Impact echo testing has been carried out for assessment of the structural integrity of the ring beam [11].

45 mm each at a depth of 50 mm were kept and shaken before the settling of the concrete to produce disbond at the steel-concrete interface.

In order to develop the test procedure for carrying out the impact echo testing, two mock up calibration blocks were made. The block-I (Fig. 12), with a size of 4000 x 4335 mm and representing a circumferential length equivalent to about 4 degrees sector of the ring beam contained simulated flaws, viz. voids of sizes 50, 100 and 200 mm at a depth of 500 mm, surface opening cracks of 25, 50 and 75 mm depth, reinforced bars of diameter 20, 32 and 45 mm at a depth of 50 mm etc. In order to study the response of the delaminations of reinforced rods, various rods of diameter 20, 32 and

The impact echo testing of this block indicated that the 100 mm and 200 mm voids at 500 mm depth could be detected, however, the 50 mm void at 500 mm depth could not be detected. Further, It was also demonstrated that if the location of the reinforcement rod is known apriori, then it can be easily checked by impact echo testing that whether any delamination has occurred (Fig. 9). In order to establish the sensitivity of the technique i.e. the depth at which the 50 mm diameter void could be detected, another test block (block-II) was made. In this block, 50 mm diameter voids were

Fig. 12 :

Photograph of (a) Mock up block-1, (b) voids and (c) reinforcement rods in it.

Fig. 13 :

mpact echo response of (a) reinforcement rod and (b) duct sheath in ring beam of inner containment wall.

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Basics kept at various depths such as 100 mm, 200 mm and 300 mm. The voids at 100 mm and 200 mm depths could be detected. However, the 50 mm diameter void at 300 mm depth could not be detected. Based on these results, the delectability of the Impact-echo system in terms of the depth and the lateral dimension of the defect was also established. The study revealed that a void can be detected, if the ratio of the depth of location to the lateral dimension of the void is less than 5. Further, the impact echo testing on reinforced rods of various diameter at different depths indicated that a reinforced rod can be detected if the ratio of the depth of its location to the diameter of the reinforced rod is less than 3 [11]. Based on the optimized test parameters identified with the help of studies carried out on the mock up blocks, impact echo testing was successfully carried out on the ring beam of the reactor containment structure for assessing its structural integrity. To the best of our knowledge, this was internationally the first time that impact echo technique has been employed for integrity evaluation of a critical component of the containment structure of a nuclear power plant. Figure 13a shows the response of a reinforced rod in the ring beam. The impact echo response of the rod is similar to that of an intact reinforced rod (Fig. 9). Further, a duct sheath at a depth of about 400 mm could also be detected (Fig. 13b) and no other defect indication were observed in the containment structure. The results of velocity measurement and impact echo testing revealed that the ring beam was free from any defects/ anomalies and the reinforced rods near the surface were also found to be properly intact with the concrete. Based on the study, the ring beam was affirmed to be free from any damage. Hence, the dome

was reconstructed using the same ring beam and the reactor is under operation. 8.2 IMPACT ECHO TESTING ON DELHI IRON PILLAR

As discussed earlier, impact echo technique has been developed and successfully utilized for testing of concrete structures. For the first time, the authors have used impact echo technique for testing of a metallic structure [12]. Impact echo technique has been used for the nondestructive evaluation of the Delhi Iron Pillar towards the characterization of its internal structure and to understand the methodology adapted for making the Iron Pillar. The Delhi Iron Pillar has long evoked the admiration of antiquarians and the curiosity of metallurgists, principally because of its excellent state of corrosion resistance and the method of fabrication of such a huge iron object

1600 years ago. Because of the higher attenuation of ultrasonic waves in the pillar, it was not possible to inspect the pillar with conventional ultrasonic technique, even at lower wave frequencies. Hence, impact echo technique has been applied on the iron pillar. As this is the first time to the best of our knowledge, that impact echo technique was being applied for testing of metallic materials, the testing was carried out on mock up cylindrical steel block of similar diameter as that of the Iron Pillar, before testing the iron pillar. The proportionality constant for the relation between the first peak frequency, the p-wave velocity and the dimensions of the structures are determined for the steel structures. A sketch of the Delhi iron pillar is shown in Fig. 14. Impact echo testing was carried out on the cylindrical portion of the Delhi Iron Pillar,

Fig. 14 : Sketch of the Delhi iron pillar showing various portions of the pillar. The schematic of a defect is also shown for which impact echo response is shown in Fig. 14.

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Basics

except the bottom rough portion. The testing was also carried out at the decorated portion on top of the pillar. The decorated portion exhibited higher velocity as compared to that in the cylindrical portion. Impact echo signals acquired at a few locations consisted of extra peaks corresponding to different depths, which were attributed to the presence of random voids. A typical impact echo signal indicating the presence of a defect in the iron pillar is shown in Fig. 15. Further, the aspect ratio of the voids (larger dimensions in axial and circumferential directions as compared to the radial direction) indicated that the pillar would have been forged in the radial direction rather than in axial direction [12]. 9. ADDITIONAL GUIDELINES FOR IMPACT ECHO TESTING The proper selection of digitizing frequency, data length and diameter of the steel balls for giving impact, are of significant importance. For example, when the main concern is to get the full thickness of the structure, lower digitizing frequency, longer data length and larger diameter ball are suitable. Whereas, when detection of flaws near the

surface is the main concern, higher digitizing frequency, shorter data length (to avoid low frequency response) and smaller diameter ball are the best suited. In routine testing, it is usually recommended to start with a large impactor (10 mm diameter or larger) and proceed to use a smaller impactor, if it is necessary to amplify or “bring up” features that are associated with frequencies of about 20 kHz and higher.

interval in time domain (equal to inverse of the digitization frequency) and Δt is the travel time of the wave between two transducers. For Cp= 4000m/s, δt= 2 μs (digitization frequency = 500 kHz) and with a distance between two transducers as 300 mm; et = ±(2x10-6/75x10-6) = ± 0.0267, i.e ± 2.67% or ± 107 m/s. Similarly, the maximum error involved in the thickness (/depth of a defect) measurement due to frequency

10. ERRORS ASSOCIATED WITH VARIOUS MEASUREMENTS

interval is given by

Δf is the frequency interval (=

Impact echo technique is based on the use of digital signal analysis methods. As a result, the time domain waveforms and frequency spectra are composed of discrete points with fixed intervals that depend upon the data acquisition parameters such as digitization rate and total length of the time domain waveform. This results in systematic errors in the velocity and thickness measurements. The maximum errors involved in the velocity measurements using the procedure as discussed in section 3.1 is given

where n is the number of samples recorded) and Δf is the frequency observed corresponding to the thickness (/depth of a defect). For δt= 2 μs, n=2048, CP=4000 m/s, and thickness of concrete = 300 mm; ef = (± 244.14/13333.33) = ± 0.0183 i.e ± 1.83 % or ± 5.5 mm. The % error associated in the measurement of thickness due to the error involved with frequency measurement is the function of the frequency and in turn the thickness also. The % error decreases with decrease in the thickness.

The upper limit of concrete thickness that can be measured by the impact-

; where ät is the sampling

; where ,

Fig. 15 : Impact echo signal obtained at 70 mm below the top of the cylindrical portion of Delhi Iron Pillar showing a defect (shown schematically in Fig, 13) at about 60 mm depth (tested from the surface near the defect, where the waves get reflected from both the defect and the diametrically opposite surface). vol 10 issue 2 September 2011

Journal of Non Destructive Testing & Evaluation

Basics echo technique is usually limited by the low frequency resonant peaks of the receiver transducer appearing at ~1.1-1.4 kHz which correspond to ~1.4 – 1.8 m thickness. Impact-echo technique has been widely used for testing of concrete structures upto ~800 mm thickness and thickness as high as 1.2 m concrete structure is also reported, when a steel ball of 12.5 mm diameter was used as an impactor. For thick concrete structures (thickness = 1.2 m – 2 m), thickness peak obtained in an impact echo test may be indecisive due to the possible presence of the transducer resonant peak. However, it can still be tested for defect detection by performing tests from both the sides of the structure covering half of the thickness from each side.

ACKNOWLEDGEMENTS

The authors are thankful to Shri S.C. Chetal, Director, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam and Dr. Baldev Raj, the previous director of IGCAR, Kalpakkam for their constant encouragement and support.

M. Sansalone and N.J. Carino, “Detecting Delaminations in Concrete Slabs with and without Overlays using the Impact-Echo Method,” Journal of the American Concrete Institute, 1989; 86(2): 175-184.

C. Colla and R. Lausch, “Influence of source frequency on impact-echo data quality for testing concrete structures”, NDT&E International, 2003: 36; 203–213.

J.M. Lin and M. Sansalone, “A procedure for determining Pwave speed in concrete for use in impact-echo testing using a Rayleigh speed wave speed measurement technique”, Innovations in Nondetrctive Testing, SP-168, American Concrete Institute, 1997.

C. Cheng and M. Sansalone, “Effects on Impact-Echo Signals Caused by Steel Reinforcing Bars and Voids Around Bars,” ACI Materials Journal, 1993; 90 (5): 421-434.

REFERENCES 1.

11. CONCLUSIONS The paper presents the basics of the impact echo technique including its physical principle, details of the hardware and software, response of various types of defects, guidelines and associated errors in the impact echo testing. With the help of typical examples and schematics, it has been demonstrated that impact echo testing can be used for the measurement of ultrasonic velocity and thickness of concrete structure as well as for the detection of voids/ delaminations/defects. The depth of the surface opening cracks could be measured with an accuracy of about 10%. Further, if the location of a reinforcement rod is known apriori, then it can be easily checked by impact echo testing that whether any delamination has occurred.

Journal of Non Destructive Testing & Evaluation

Anish Kumar, T. Jayakumar, C. V. Subramanian and M. Thavasimuthu, “Testing of Concrete Structures for Determination of Strength and Detection of Flaws using Low Frequency Ultrasonic and Impact-echo techniques”, J. Nondestructive Testing and Evaluation, 1999; 19(2): 43-46. Mary Sansalone, “Impact-Echo: The Complete Story”, ACI Structural Journal, 1997; 94 (6): 777-786. M. Sansalone and W.B. Streett, Impact-Echo: Nondestructive Testing of Concrete and Masonry, 1997; Bullbrier Press, Jersey Shore, PA. ASTM C 1383-04, “Test Method for Measuring the PWave Speed and the Thickness of Concrete Plates using the Impact-Echo Method,” 2010 Annual Book of ASTM Standards Vol. 04.02, ASTM,West Conshohocken, PA. C. Cheng and M. Sansalone, “The Impact-Echo Response of Concrete Plates Containing Delaminations: Numerical, Experimental, and Field Studies,” Materials and Structures, 1993; 26(159): 274285.

10. Y. Lin and W.C. Su, The Use of Stress Waves for Determining the Depth of Surface-Opening Cracks in Concrete Structures, Materials Journal of the American Concrete Institute, 1996; 93(5): 494-505. 11. Anish Kumar, Baldev Raj, P. Kalyanasundaram, T. Jayakumar, and M. Thavasimuthu NDT&E International, 35 (2002), 213220. 12. Baldev Raj, P. Kalyanasundaram, T. Jayakumar et al., Current Science, 88 (12) (2005) 1948-1956.

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Horizon NDE by backscatter imaging Dr. CV Krishnamurthy Centre for NDE and Department of Physics, IIT Madras

scattered photon, and mec2 is the rest mass energy of the electron (0.511 MeV). The energy of the scattered photon can be calculated from this equation using the angle of deflection and the energy of the incident photon. Figure 1 depicts the process and indicates the angular distribution of scattered energy. Compton scattering can also be expressed in terms of the incident and scattering waves as

NDE by backscatter imaging A visual image is formed by the reflection or more generally by the scattering of light from the viewed object to the eye, i.e., the radiation source (light) and the detector (eyes) are on the same side of the object. Given that x-rays or gamma rays just pass through any material without any deflection, can imaging be possible with scattered radiation at such high energy X-rays or gamma rays? Compton Scattering – Basics Compton scattering, so named after the American physicist, Arthur H. Compton, who was awarded the Nobel Prize in 1927 for his interpretation of the X-ray scatter effect that bears his name is significant as it demonstrates that light cannot be explained purely as a wave phenomenon. Compton’s work convinced the scientific community that light can behave as a stream of particles (photons) whose energy is proportional to the frequency. Compton scattering requires that light is viewed as a particle and not just a wave, because it is the “collision” of the photon with the electron, and the exchange of energy, which accounts for the shift in energy. Compton scattering is the interaction of a high energy photon, as from a gamma ray or high energy X-ray, with an electron, and the resulting “scattered” photon which has a reduced frequency, and therefore reduced energy and is thus termed incoherent scattering. The compton scattering formula is

(1a) where, Ei is the energy of the incident photon, E f is the energy of the vol 10 issue 2 September 2011

Figure 1: (top) Schematic of the Compton scattering from a free electron (from http:// missionscience.nasa.gov/ems/12_gammarays.html) (bottom) Polar diagram of the scatter cross-section (from http://whs.wsd.wednet.edu/) Journal of Non Destructive Testing & Evaluation

HORIZON

(1b)

where

λi

is the initial wavelength of

the photon, λf is the final wavelength after scattering, θ is the scattering angle, and λ c , the compton wavelength, is the wavelength of the resting electron which is 2.426 10-12m. The formula is derived by considering the interaction of a resting electron with an incoming photon under the constraints of conservation of momentum and energy. The formula assumes that the photon is scattering off a free electron. However within the detector the electrons may be bound valence electrons. So the energy received by the electron will not only go into

ionizing the atom, but also kinetic energy of the now free electron. Therefore the kinetic energy will be less by about 10 eV, or about 1 part in 1000 since the final kinetic energy of the electron is 11.27 keV if θ = 180° corresponding to backscatter. The shift in wavelength is inversely proportional to the mass of the particle it is scattering off. So, photons which scatter off of the nucleus would appear unshifted because their shift would be about 1000 times smaller than the shift due to an electron (the maximum shift when the photon is backscattered by nuclear scattering is about 0.067 keV, while for electron scattering it is 11.27 keV). In the energy range from several KeVs to several MeVs, the incident x-ray beam becomes attenuated in three principle ways. They are photoelectric effect, Compton scattering, and pair production. The relative coefficient of three attenuating processes is

illustrated in Figure 2. In this figure, we can find that under the conditions in which we are interested (50KeV < E < 10MeV, Z<30), Compton scattering is the dominant mode of interaction of photons. The probability of the Compton scattering per atom of the absorber depends on the number of electrons available as scattering targets, and therefore increases linearly with the atomic number, Z. The number of photons scattered is then a measure of the electron density of the material which deflected it. This can in turn help identify the presence of an inclusion or a defect as highlighted in Table 1 and Table 2. Compton Scatter Tomography Over the last two decades, Compton scattering based studies have been carried out for density measurements and thickness measurements and on

Table 1: Features of Compton Scattering Most likely to occur As x-ray energy increases

As atomic number of absorber increases As mass density of absorber increases

a) with outer-shell electrons b) with loosely bound electrons a) Increased penetration through specimen without interaction b) Increased Compton scattering relative to photoelectric effect No effect on Compton scattering Proportional increase in Compton scattering

Table 2: Comparison between Compton scatter and transmission densitometric and imaging techniques Scattering 3-D information Imaging achieved directly by scanning Densitometry achieved on anabsolute basis Fractional contrast of defect structure is high especially at high photon energies or low target densities

Figure 2 : (top) Compton scattering from an atom indicating ionization events (from http://whs.wsd.wednet.edu/). (bottom) The relative intensity of photoelectric effect, Compton scattering, and pair production. Journal of Non Destructive Testing & Evaluation

Transmission 2-D information Imaging achieved – Indirectly reconstruction from projections Densitometry requires a ‘caliper” measurement to convert attenuationinto a density value Fractional contrast of defect structure is low due to superposition effects

Energy analysis is possible - can monitor both the Compton and elastic scattering intensities Variable scattering geometry Backscattering geometry available requires access from one side only

Energy analysis of limited value

Aspect ratio unimportant

Aspect ratio important for adequate contrast and avoidance of shape artefacts in CT- images Scanning times are fast

Scanning rates are generally slow for high Z materials unless multibeam/ multidetector arrays can be used

No degrees of freedom Requires access from both sides

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20 associated NDE of voids, delaminations and material losses. Metals, weldments, concrete structures, and composites have been investigated with increasing success due to improvements in detectors and reconstruction algorithms. Figure 3 shows the schematics of two configurations that are routinely employed to carry out compton scattering based NDE to extract the information about density and thickness variation. The incoherent scattering leads to a “soft-collimation” process, by virtue of the fact that each detected energy window corresponds to a particular direction of scattering. Thus, many “points” along the incident beam can be simultaneously inspected as indicated on the right in Fig. 3. In order to avoid ambiguity when relating the energy to angle of scattering using Eq. (1), the detector is confined to almost a point, using a wedge- shaped detector collimator.

HORIZON Figure 4 shows an example of the results of experiments on a pipe illustrating the potential of Compton scattering for NDE applications. According to ASTM E1931 - 09 Standard Guide for X-Ray Compton Scatter Tomography, it is best applied to thinner sections of lower Z materials. Table No: 3 provides a general idea of the range of applicability when using a 160 keV constant potential X-ray source: Compton Back-Scatter Imaging Compton imaging is a visualization technique that uses the kinematics of Compton scattering for the reconstruction of a gamma radiation source image. Compton imaging systems, also known as Compton cameras, are used in the nuclear power industry for site and environmental surveys, in gamma-ray astronomy and in several prototypes of nuclear medical imaging systems.

Figure 3: (left) geometry exploiting the scattering angle for voxel-based inspection, (right) a wide-angle Compton-scatter inspection system (from Ref [5]).

Basic design of Compton imaging systems consists of two planes of position sensitive and energy dispersive gamma-ray detectors as shown in Figure 5. Compton imaging requires that a gamma ray must interact with electrons at least twice—once to induce Compton scattering and once to allow photoelectric absorption— although more than one scatter can occur. When an x- or gamma-ray photon is scattered or absorbed, highenergy electrons are ejected. The subsequent deposition of electron energy produces a large number of ionized atoms. The ionization from Compton scattering or photoelectric absorption is then recorded by the detectors. The gamma-ray interactions must be separated in space sufficiently so that they can be easily distinguished from each other and their positions can be accurately measured to obtain high angular resolution. Electronic collimation gives the possibility to perform gamma-ray imaging without use of mechanical collimators. The potential advantages of the Compton cameras over conventional imaging techniques include a large field of view, increased efficiency, good background suppression and a more compact and lightweight imaging system. Compton cameras are effective over a large energy range (140 keV to 10 MeV) and can be used in an energy selective mode for the separate imaging of the mixed gamma-ray sources. At close ranges (less than few meters) they give possibility for three-dimensional (3D) imaging of objects from a fixed position without detector motion. One of the most promising applications has been that XBT is able to detect metal-free landmines buried in a variety of soil conditions including various types of vegetation as indicated by Figure 6 and Figure 7.

Figure 4: Scattered intensity originating from interactions of 662 keV photons with (a) an iron pipe of wall thickness 2.5 mm filled with different density liquids (petrol, diesel, multipurpose engine oil API CF, water and glycerine) and (b) an iron pipe (length 140 mm and opening 22 mm) having a cut of 1 mm width up to the middle of pipe, covered with insulating material in the form of powder. Detection is by a NaI(Tl) detector with collimators of hole-size of 4 mm in radius for source and slit size of 4 mm in width for detector to define the volume of interest. Analysis was carried out using an inverse matrix approach rather than the usual Monte Carlo method (from Ref [11]). vol 10 issue 2 September 2011

Strikingly similar to light reflection, backscatter signals are particularly strong whenever the incident X-rays interact with explosives, plastics, and other biological items, which typically contain low Z materials. An example of the photo-like images produced from the backscattered signals by commercial systems is shown in Figure 8. Deciphering information by visually observing raw scatter radiographs, or

Journal of Non Destructive Testing & Evaluation

HORIZON

Figure 5: (a) The energies and positions of the first two interactions define a cone of incident angles. (b) The cones can be projected onto a plane or sphere (one circle per gamma-ray event) to produce a 2D image of the source. The source image is reconstructed by the backprojection and intersection of number of conic surfaces in the image space. (c) Image of point gamma-ray source reconstructed by backprojection of cones. This type of information about the possible source distribution requires specialized reconstructions techniques (from Ref [8]). a set of measurements, is not directly possible, due the convoluted nature of scatter indications. The problem is caused by the non-localized diffused nature of scattering; a coded aperture technique (see Figure 9) to detect localized anomalies with backscatter imaging is often employed.

Figure 6: XBT images at two depths on the left of an anti-personnel mine (type PPM-2) shown on the right (from Ref [6]).

Figure 7: XBT images at two depths on the left of an anti-tank mine (type TM-62) shown on the right (from Ref [6]).

Figure 8: Image of a cargo truck carrying drugs (from Ref [9]).

Table 3: ASTM E1931 â&#x20AC;&#x201C; 09 Guidelines (http://www.astm.org/Standards/E1931.htm) Material

Practical Thickness Range

Steel

Up to about 3 mm (1/8 in.)

Aluminum

Up to about 25 mm (1 in.)

Aerospace composites

Up to about 50 mm (2 in.)

Polyurethane Foam

Up to about 300 mm (12 in.)

Journal of Non Destructive Testing & Evaluation

The advantage of merging visual images with the images from a Compton imager can be striking as is shown in Figure 11. In one experiment, the team at Livermore combined a camera and a Compton imager to demonstrate that the system can visually identify a gammaray source containing an isotope of sodium. Compton imaging systems have a number of advantages for nuclear waste characterization, such as identifying hot spots in mixed waste in order to reduce the volume of highlevel waste requiring extensive treatment or long-term storage, imaging large contaminated areas and objects etc. Compton imaging also has potential applications for monitoring of production, transport and storage of nuclear materials and components. Mixed radionuclide sources can be successfully separated by selectively imaging of gamma rays of interest. Other advantages of scatter imaging, compared to transmission imaging, are the flexibility in locating the source and the detector, which do not have to be on opposite sides, the ability to use more than one detector for the same radiation source beam, and the 3-D nature of the scattering process which makes it amenable to multiplanar imaging. Compton imaging is completely passive and gives a 3D image of an vol 10 issue 2 September 2011

22 object from a single viewpoint without needs for scanning or access to both sides of the object. It covers a large energy range, has a wide field of view and does not require mechanical collimation. It also provides good background suppression through the kinematics of the scattering process

HORIZON and through energy filtering. The Xray backscatter technology has the potential for low false alarm rates and a high detection probability. Looking at the high resolution images, a trained operator is able to identify the buried object immediately.

Figure 9: Combining a mask with an antimask whose hole pattern is the inverse of the mask’s pattern effectively removes background signals outside the surveyed area (from Ref [8]).

Figure 10: The Livermore-designed gamma-ray imager is portable and can be placed inside a small truck. Two coded-aperture masks (gray bars shown in the top photo) allow the instrument to image both sides of a road. In the bottom photo, one can see the assembly of the detectors (cylinders shown in the middle). Compton imagers for 3D information are shown in the disk configuration (right top) with orthogonal strips on each side connected to a preamplifier and a digital data-acquisition system to determine the three-dimensional (3D) position for each gamma-ray interaction, and in the cylindrical configuration (right bottom) with the outside contact divided into pixels that, when analyzed with a digital signal processor, provide the necessary 3D information (from Ref [8]).

For further reading: 1.

R.S. Holt and M.J. Cooper, Gamma-ray scattering NDE, NDT International 20 (1987) 161-165

D. Babot, G. Berodias and G. Peix, Detection and sizing by X-ray Compton scattering of nearsurface cracks under weld deposited cladding, NDT& E International 24 (1991) 247-251

Stephen J. Norton, Compton scattering tomography, J. Appl. Phys. 76 (1994) 2007-2015

W.Niemann and S.Zahorodny, Status and future aspects of X-ray backscatter imaging, Review of progress in Quantitative Nondestructive Evaluation 17A (1997) 379-385

A.C. Ho, E.M.A. Hussein, Quantification of gamma-ray Compton-scatter nondestructive testing, Applied Radiation and Isotopes 53 (2000) 541-546

W. Niemann, S. Olesinski, T. Thiele, Detection of buried landmines with x-ray backscatter technology, NDT.net – 7 (2002) No.10

Joseph Callerame, X-Ray Backscatter Imaging:Photography Through Barriers, JCPDSInternational Centre for Diffraction Data (2006) ISSN 1097-0002

Gabriele Rennie, Imagers to provide eyes to see Gamma Rays, S & TR on Gamma-Ray ImagingLawrence Livermore National Laboratory (May 2006)

http://www.as-e.com/ products_solutions/ z_backscatter.asp

10. G. Harding, E.Harding, Compton scatter imaging: A tool for historical exploration, Applied Radiation and Isotopes 68 (2010) 993–1005 11. A. Sharma, B.S.Sandhu, Bhajan Singh, Incoherent scattering of gamma photons for nondestructive tomographic inspection of pipeline, Applied Radiation and Isotopes 68 (2010) 2181–2188 12. h t t p : / / w w w . n d t - e d . o r g / EducationResources

Figure 11: A Compton imager combined with a camera produced this image, which pinpoints the location of the radioactive source - an isotope of sodium (22Na) (from Ref [8]). vol 10 issue 2 September 2011

13. http://en.wikipedia.org/wiki/ Backscatter_X-ray

Journal of Non Destructive Testing & Evaluation

CHAPTER NEWS BANGALORE Executive Committee Meeting & the New EC was elected for 2011-2012 CHENNAI The following courses were conducted *UT Level-II (ASNT) course from 22.04.11to 30.04.11. *MT – Level - II(ASNT),for M.M.Forgings Ltd from 24th Apr, 08th, 15th, 22nd May 2011 *Surface NDT ( MT & PT) Level - II (ASNT) from 20.05.2011 to 26.05.2011. No. of participants for course 20. No. of participants for examination 18. *UT L - II (ASNT) course from 30.05.2011 to 05.06.2011. No. of participants for course 14. No. of participants for examination-18 *RT L - II (ASNT) course from 10.06.2011 to 16.06.2011. No. of participants for course 17. No. of participants for examination-18 Other Activities: ISNT DAY celebrated on 21.04.2011. No of participants : 225 inclusive of members and their family. Chief guest Mr RG Ganesan,Technical Director *The Thambithrai Award for Best Technical Paper presented to Dr. Vaidehi Ganesan Scientist, IGCAR. *Best member of ISNT-CHENNAI CHAPTER and PARI award, Received by Mr RG Ganesan, Joint secretary – ISNT Chennai Chapter. EC Meetings: *01.05.2011*11.06.2011 JAMSHEDPUR Annual General Body Meeting conducted on Date: 29.04.2011 and the new executive committee was elected under the chairmanship of Dr. N. Parida, Scientist from NML Awards : Dr. Amitava Mitra received the prestigious Materials Research Society of India Medal (MRSI Medal) KALPAKKAM Conducted a Technical Talk on X-ray endoscopic inspection of T/TS welds in heat exchangers by eminent Prof. Dr Uwe Ewert on Friday, 22 nd July, 2011 KOLKATA The following courses were conducred Radiography Testing (RT- II) Training & Certification Course from 23rd May to 29th May, 2011. No. of participants for course- 16 No. of participants for examination-121st Magnetic Particle Testing (MPT- II) Training & Certification Course from 15.07.11 to 17.07.11. Four (4) candidates participated & results are awaited.- 1st Penetrant Testing (PT- II) Training & Certification Course from 22.07.11 to 24.07.11. Three (3) candidates

23 participated & results are awaited.-8 t h Radiography Testing (RT- II) Training & Certification Course from 01.08.11 to 08.08.11. Six (6) candidates are likely to participate.-5 th Ultrasonic Testing (UT- II) Training & Certification Course from 05.09.11 to 11.09.11. EC Meetings:1. April’11 2.June,’11.Interaction with ISNT HOThe Chapter had interacted with the ISNT HO, NCB and other Chapters on various occasions In connection with its activities, National Seminars etc Kolkata Chapter paid back Rs. 1.0 Lakh to ISNTHQ in June, 2011 towards adhoc payment against excess income over expenditure during National seminar NDE-2010 held at Kolkata in December, 2010. The A/cs. Statement is in advance stage of preparation & shall be sent to HQ by end August, 2011. The dues against loan taken from HQ for Purchase of Office Premises in 2005 stand at Rs. 1.86 Lakh which we hope to clear by Sept end, 2011 after preparartion of Balance Sheet and income/expenditure statement of Chapter. KOTA Organized: Leak Testing Level -II Course during June, 2011. MUMBAI -APCNDT 2013 committee Meeting was held on 8th April 2011, 19th April 2011, 25th April 2011. -conducted Welding Inspector examination at ITT, on 1st May 2011, -APCNDT 2013 committee Meeting was held on 13th May 2011 and 7th June 2011. -EC Meeting was held on 22rd June 2011 APCNDT 2013 committee Meeting was held on 29th July 2011. -Conducted LT Level II Examination for Kota Chapter on 26- 06- 2011 at Kota.. PUNE Technical Talk: TOFD Technique introduction & Practical Demonstration on Weld”on 14.05.2011 by Shri. Ashok Trivedi EC Meeting held on 29.04.2011 TRICHY Newly added Members during this period a. Associate member : 16 b. Life Members : 01 c. Members : 01 d. Student Members: 82 EC meeting was conducted on june 2011 Following Courses were conducted: a) Radiographers level I-(In association with BARC-Mumbai) from 11.04.2011 to 29.04.2011 b) MPT-Level–II- From 16.06.2011 to 19.06.2011

Journal of Non Destructive Testing vol 10 issue 2 September 2011 & Evaluation

LPT – Level – II – from 13.06.2011 to 15.06.2011 d) Radiography Level – II from 11.07.2011 to 21.07.2011 Invited Lecture : “Introduction to SafeRad Radiography System - an unique & innovative NDT technique” BY Mr. Malcolm Wass U.K. on 11th July 2011. TRIVANDRUM 1. Annual General Body Meeting 2010-11: was held on 28th May 2011. 2. Election of office bearers and EC members for the period 2011-2013 was conducted 3. MR Kurup Memorial Lecture 2011: for the year 2010-11 was delivered by Shri V Srinivasan, Deputy Director, PRSO Entity, VSSC, at Hotel Maurya Rajadhani, Trivandrum on 28th May 2011. VADODARA 1) Executive Committee Meeting held on 25th March, 2011 2) Annual contract for website designing and maintenance for one year at the cost of Rs.5,000/ - (Rupees Five Thousand only) was finalized.

Malaysian International NDT Confer ence and E xhibition Conference Exhibition 2011 (MINDT CE 11) (MINDTCE November 21-22, 2011, Thistle Port Dickson Resort www.msnt.org.my The Malaysian Society for NDT (MSNT) cordially invites you and your staffs to participate and present your paper in the 2011 Malaysian International NDT Conference and Exhibition 2011 (MINDTCE 11). We expect there will be a lot of participation from NDT professionals and with your participation we look forward to sharing knowledge and experience in the field of NDT. MINDTCE 11 is jointly organized with the Malaysian Welding and Joining Society (MWJS) and strongly supported by PETRONAS, International Committee for NDT (ICNDT), Malaysian Nuclera Agency and SIRIM. The organizers invite scientists, engineers, educators, researchers and managers to submit paper to this wonderful conference.

10 issue 2 September 2011 Journal of Nonvol Destructive Testing & Evaluation

National Certification Board - Indian Society for Non Destructive Testing

Announcement ASNT NDT Lev el III E xamination Level Examination Mumbai 28, 29 & 30 November 2011 ASNT NDT Level III Examination will be conducted in the following methods: 1. Basic 2. Radiographic Testing 3. Magnetic Particle Testing 4. Ultrasonic Testing5. Liquid Penetrant Testing 6. Eddy Current Testing 7. Neutron Radiographic Testing 8. Leak Testing 9. Visual Testing 10. Acoustic Emission Testing 11. Thermal / Infrared Testing It may please be noted that the basic examination by itself is not considered as a method. Basic and method examination(s) must be taken to become eligible to receive a certificate for that method(s). The maximum number of examinations that can be taken is six during the three days of the Examination. Dr. B. Venkatraman ASNT Level III Examination Coordinator, Modules 60 & 61, Readymade Garment Complex, SIDCO Industrial Estate, Guindy, Chennai 600 032, India Ph: 91 44 22500412 & 91 44 42038175 / 91 44 27480500 Ext.22306 E Mail: isntheadoffice@gmail.com Alternate E Mail: ncbisnt@gmail.com

Revised Final date for full paper submission 5th September 2011 Notification of acceptance : 15th July 2011 Final date for full paper submission : 5th September 2011

National NDT Awards

No.

Award Name

Sponsored by

ISNT - EEC National NDT Award (R&D)

M/s. Electronic & Engineering Co., Mumbai

ISNT - Modsonic National NDT Award (Industry)

M/s. Modsonic Instruments Mfg. Co. (P) Ltd., Ahmedabad

ISNT - Sievert National NDT Award (NDT Systems)

M/s. Sievert India Pvt. Ltd., Navi Mumbai

ISNT - IXAR Best Paper Award in JNDE (R & D)

M/s. Industrial X-Ray & Allied Radiographers Mumbai

ISNT - Eastwest Best Paper Award in JNDE (Industry)

M/s. Eastwest Engineering & Electronics Co., Mumbai

ISNT - Pulsecho Best Chapter Award for the Best Chapter of ISNT

M/s. Pulsecho Systems (Bombay) Pvt. Ltd. Mumbai

ISNT - Ferroflux National NDT Award

M/s. Ferroflux Products Pune

(International recognition)

ISNT - TECHNOFOUR National NDT Award for Young NDT Scientist / Engineer

ISNT - Lifetime Achievement Award

M/s. Technofour Pune

Note-1: The above National awards by ISNT are as a part of its efforts to recognise and motivate excellence in NDT professional enterpreneurs. Nomination form for the above awards can be obtained from ISNT head office at Chennai, or from the chapters. The filled application are to be sent to Chairman, Awards Committee, Indian Society for Non-destructive Testing, Module No. 60 & 61, Readymade Garment Complex, SIDCO Ind. Estate, Guindy, Chennai-600 032. Telefax : 044-2250 0412 Email: isntheadoffice@gmail.com

vol 10 issue 2 September 2011

Journal of Non Destructive Testing & Evaluation

NDE events We hope that this new feature added to the journal since the last two issues has been useful for the readers in planning their activities in terms of paper submissions, registering for seminars, etc. Please send your feedback, comments and suggestions on this section to mandayam.shyamsunder@gmail.com September 2011 International Congress on Ultrasonics (ICU 2011) September 5 to 8, 2011 ; Gdansk, Poland http://icu2011.ug.edu.pl/ocs233-1/index.php/icu/ icu2011 6th International Conference on Mechanical Stress Evaluation by Neutrons and Synchrotron Radiation (MECA SENS VI) September 7 to 9, 2011 ; Hamburg, Germany http://www.mecasens2011.de/ 8th International Workshop on Structural Health Monitoring (IWSHM 2011) September 13 to 15, 2011; Stanford, CA, USA http://structure.stanford.edu/workshop/ Materials Testing 2011 September 13 to 15, 2011 ; Telford, UK http://www.bindt.org/Events/Exhibitions/MT_2011 5th Conference in Emerging Technologies in NDT (ETNDT) September 19 to 21, 2011; Ioannina, Greece http://www.etech-ndt5.uoi.gr/ 2011 ATA NDT Forum September 26 to 29, 2011; Charlotte, NC, USA http://www.airlines.org/SafetyOps/EM/Pages/ 2011NDTForum.aspx October 2011 V Pan American Conference on NDT October 2 to 6, 2011 ; Cancun, Mexico http://www.copaend5.com/en/index.php VIth International Workshop NDT in Progress October 10 to 12, 2011 : Prague, Czech Republic http://cndt.cz/ndt_in_progress2011/ 2011 IEEE International Ultrasonics Symposium (IUS) October 18 â&#x20AC;&#x201C; October 21 ; Orlando, FL, USA http://ewh.ieee.org/conf/ius_2011/ International Conference & Expo on NDT 2011 (ICENDT 2011) October 18 to 21, 2011; Jakarta, Indonesia http://www.autri.org/ 2011 ASNT Fall Conference & Quality Testing SHow October 24 to 28, 2011 ; Palm Springs, CA, USA http://www.asnt.org/events/conferences/fc11/fc11.htm National Conference of the Italian Society for NonDestructive Testing October 26 to 28, 2011; Florence, Italy http://www.aipnd.it Journal of Non Destructive Testing vol 10 issue 2 September 2011 & Evaluation

November 2011 International Workshop on Smart Materials & Structures and NDT in Aerospace November 2 to 4, 2011 ; Montreal, Quebec, Canada http://www.cansmart.com/ MATEST 2011 International NDT Conference November 2 to 5, 2011; Split, Croatia http://www.hdkbr.hr Singapore International NDT Conference & Exhibition (SINCE 2011) November 3 to 4, 2011 ; Singapore http://www.ndtss.org.sg/ 41st International Conference and NDT Exhibition; NDE for Safety 2011 / Defektoskopie 2011 November 9 to 11, 2011, Ostrava, Czech Republic http://cndt.cz/nde_for_safety2011/ NDE-Tokyo November 16 to 18, 2011; Tokyo, Japan http://www.jma.or.jp/next/en/nde/outline/index.html Malaysia International NDT Conference & Exhibition 2011 (MINDTCE â&#x20AC;&#x2DC;11) November 21 to 22, 2011 ; Malaysia http://www.aindt.com.au/images/stories/page_images/ conferences/international/ mindtce_11_brochure_revision_1.pdf 2011 Aircraft Structural Integrity program Conference November 29 to December 1, 2011 ; San Antonio, Texas http://www.asipcon.com/ 5th IET international conference on railway condition monitoring and non-destructive testing November 29 to 30, 2011; Derby, UK http://www.theiet.org/events/2011/rcm.cfm December 2011 International Conference on NDE in the Steel and Allied industries (NDESAI2011) December 2 to 3, 2011 ; Jamshedpur, India http://www.ndesai2011.com/ National Seminar on NDE (NDE-2011) December 6 to 10, 2011 ; Chennai, India http://www.nde2011.com/ 59th Defense Working Group on Nondestructive Testing December 6 to 8, 2011; Williamsburg, VA, USA http://www.dwgndt.org/

10 issue 2 September 2011 Journal of Nonvol Destructive Testing & Evaluation

Answ ers for P d SSear ear ch 2 Answers Prrevious issue - NDT Wor ord earch

NDT WORDSEARCH - 2 3 ' $ ( 7 % 5 8 6 + , 1 * 9 ' 1 7 ( ( % 2

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vol 10 issue 2 September 2011

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Journal of Non Destructive Testing & Evaluation

NDE patents

Compiled by Dr. M.T.Shyamsunder, GE Global Research, Bangalore, India

We hope that the section on NDE Patents, which featured in the March 2011 issue of this journal, has continued to trigger your curiosity on this very important topic of Intellectual property. We continue this section with a few more facts on patents and a listing of a few selected NDE patents. Please send your feedback, comments and suggestions on this section to mandayam.shyamsunder@gmail.com

Here are some interesting “Firsts” from the world of patents ! FIRST PATENT The first U.S. patent was granted in 1790 to Samuel Hopkins of Philadelphia for “making pot and pearl ashes”-a cleaning formula used in soapmaking. This patent is referred to as Patent X1. Patent numbers were not assigned to patents until 1836. FIRST FEMALE PATENT HOLDER In 1809, Mary Dixon Kies, a native of Killingly, Conn., is reported to have received the first U.S. patent awarded to a woman for a process of weaving straw with silk or thread. Unfortunately, all records of this patent were destroyed in the Patent Office fire of 1836. On the morning of December 15, 1836, the Patent Office, then located at the Blodgett’s Hotel in Washington, D.C., was consumed by fire. Among the lost patent-related materials were an estimated 7,000 models and 9,000 drawings of pending and patented inventions.

FIRST DESIGN PATENT The first design patent was granted to George Bruce of New York City for a typeface. FIRST PLANT PATENT Plant Patent #1 was issued in 1931 to Henry Bosenberg of New Brunswick, NJ for a climbing or trailing rose. Said Mr. Bosenberg of his invention, “My invention now gives the true everblooming character to climbing roses.” YOUNGEST PATENT HOLDER The youngest person to be granted a patent is a four-year-old girl from Houston, Texas, for an aid for grasping round knobs. “This invention relates to a device for grasping drawer or cabinet knobs which is particularly useful to physically impaired persons who would otherwise, due to such impairment, have great difficulty in opening and closing drawers or

cabinets utilizing such knobs.”

(SourceCourtesy: http://www.lib.utexas.edu/ engin/patentlite/firsts/ ) Given below are some interesting statistics of patents in India (Source : IP India Annual reports ; http://www.ipindia.nic.in/) : Continuing our endeavor to provide you updates on NDE and Inspection relate patents, listed below are a few patents from areas related to Radiographic inspection which were issued by USPTO in the last few years. If any of the patents are of interest to you, a complete copy of the patent including claims and drawings may be accessed at http://ep.espacenet.com/ United States Patent 7,885,381 Method for inspecting pipes, and radiographic non-destructive inspection apparatus

FIRST PATENT GRANTED AFTER NUMBERING STARTS Patent numbering started on July 13, 1836. Patent No. 1 was issued to Senator John Ruggles of Thompson, Maine, for a locomotive steam engine for rail and other roads. “A new and useful improvement or improvements on locomotive-engines used on railroads and common roads by which inclined planes and hills may be ascended and heavy loads drawn up the same with more facility and economy than heretofore…”

Journal of Non Destructive Testing & Evaluation

vol 10 issue 2 September 2011

28 fixture in order to illuminate the outer surface of a pipeline or other object with radiation. A first positioning means is provided for coarsely positioning the scanning apparatus relative to the pipeline. A second positioning means is provided for finely positioning the scanning apparatus relative to the pipeline. The second positioning means is operable from a remote location when the radiation source is illuminating the pipeline with radiation. The first and second positioning means provide a plurality of degrees of freedom for positioning the scanning apparatus. United States Patent 7,218,706 Energy discrimination radiography systems and methods for inspecting industrial components Inventors: Hopkins; Forrest Frank, Dixon; Walter Vincent, Bueno; Clifford, Du; Yanfeng, Mohr; Gregory Alan, Fitzgerald; Paul Francis, Birdwell; Thomas William Assignee: General Electric Company (Niskayuna, NY) Inventors: Nagumo; Yasushi, Nukaga; Jun, Kamimura; Hiroshi, Sadaoka ; Noriyuki, Takemori; Satoshi, Kodaira; Kojirou Assignee: Hitachi-GE Nuclear Energy, Ltd. (Ibaraki, JP) The pipe inspection method and apparatus can be used to implement rapid, tomographic inspection of a pipe set up at a narrow location. The pipe inspection method includes: a first step for scanning the pipe by translating a radiation source and radiation detector arranged opposedly to the pipe; a second step for the radiation detector to detect radiation that the radiation source has emitted, at given scanning distance intervals; a third step for creating a transmission image of the pipe, based on a radiation dose that the radiation detector has detected; and a fourth step for constructing a tomogram or stereoscopic image of the pipe, based on the transmission image. Thus, it is possible to provide the pipe vol 10 issue 2 September 2011

inspection method and apparatus that can be used to implement rapid, tomographic inspection of the pipe set up at a narrow location. United States Patent 7,319,738 Delivering X-ray systems to pipe installations Inventors: Lasiuk; Brian W, Griffin; Weston B, Allison; Peter S Assignee: General Electric Company (Schenectady, NY) A mobile radiographic device for use in inspecting pipelines and the like, comprising an articulating aerial boom coupled to a mobile carriage vehicle. A pivot mount is rotatably coupled to the distal end of the aerial boom. A platform having a sliding rail is operatively coupled to the pivot mount. A mounting fixture is rotatably mounted to a cradle, which in turn is coupled to the sliding rail of the platform. A radiation source and a radiation detector are mounted on diametrically opposing sides of the

An energy discrimination radiography system includes at least one radiation source configured to alternately irradiate a component with radiation characterized by at least two energy spectra, where the component has a radiation detector is configured to receive radiation passing through the component and a computer is operationally coupled to the detector. The computer is configured to receive data corresponding to each of the energy spectra for a scan of the component, process the data to generate a multi-energy data set, and decompose the multi-energy data set to generate material characterization images in substantially real time. A method for inspecting the component includes irradiating the component, receiving a data stream of energy discriminated data, processing the energy discriminated data, to generate a multi-energy data set, and decomposing the multi-energy data set, to generate material characterization images in substantially real time.

Journal of Non Destructive Testing & Evaluation

29 United States Patent 7,236,564 Linear array detector system and inspection method Inventors: Hopkins; Forrest Frank, Galish; Andrew Joseph, Ross; William Robert Assignee: General Electric Company (Niskayuna, NY) A linear array detector (LAD) for scanning an object is provided. The detector includes a scintillator layer configured for generating a number of optical signals representative of a fraction of an incident X-ray beam passing through the object. The plane of the scintillator is parallel to the Xray beam. The LAD further includes a two dimensional array of photoconversion elements configured to receive several X-rays of the X-ray beams and configured to generate corresponding electrical signals. An arrangement of the photo-conversion elements is independent of the X-ray paths. United States Patent 7,689,003 Combined 2D and 3D nondestructive examination Inventors: Shannon; Robert E, Hatcher; Clifford, Laloni; Claudio, Forster; Frank, Davis; Fredrick M Assignee: Siemens Energy, Inc. (Orlando, FL) An inspection apparatus applying two dimensional nondestructive examination images onto a three dimensional solid model of a component to display a virtual component that may be manipulated to perform a nondestructive inspection. The two dimensional nondestructive examination images may be acquired from a plurality of views of the component in order to provide full coverage of the surface to be inspected, with appropriate stitching of images in regions of overlap between adjacent views. The two dimensional images may be color or black and white photographs or ultraviolet or infrared images, for example. Multiple types of nondestructive examination images, results of inspection data evaluations, and design, operational and/or

maintenance information may be displayed separately or jointly on the three dimensional solid model. Surface features of interest that are mapped as defined areas on the three dimensional solid model may be displayed simultaneously in different views on 2D and 3D images of the virtual component. United States Patent 7,099,432 X-ray inspection apparatus and X-ray inspection method Inventors: Ichihara; Masaru, Yoshino; Shinji, Inoue; Hiroyuki, Kinoshita; Toshio, Ohuchi; Kazuo

the signal from the sensor (32) in a signal processor (34) and displaying the image on a display (36) for determining defects in the object (12). United States Patent 6,637,266 Non-destructive inspection, testing and evaluation systems for intact aircraft and components and method therefore Inventors: Froom; Douglas Allen

Inventors: Jones; James Wayne

A non-destructive inspection, testing and evaluation system and process is provided for the review of aircraft components. The system provides for a structure configured to contain an inspection and testing apparatus and the aircraft components under inspection. The structure is lined with shielding to attenuate the emission of radiation to the outside of the structure and has corbels therein to support the components that constitute the inspection and testing apparatus. The inspection and testing apparatus is coupled to the structure, resulting in the formation of a gantry for supporting a carriage and a mast is mounted on the carriage. The inspection and testing equipment is mounted on the mast which forms, in part, at least one radiographic inspection robot capable of precise positioning over large ranges of motion. The carriage is coupled to the mast for supporting and allowing translation of the equipment mounted on the mast. The mast is configured to provide yaw movement to the equipment.

Assignee: Siemens Westinghouse Power Corporation (Orlando, FL)

United States Patent 6,466,643

Assignee: Matsushita Electric Industrial Co., Ltd. (Osaka, JP) The X-ray inspection device and the X-ray inspection method according to the present invention are configured to hold an object to be inspected irradiated with an X-ray from an Xray irradiation device, uses a swinging device for performing swinging motion of tilting the object to be inspected at an arbitrary angle and in an arbitrary direction, images the X-ray that passes through the object to be inspected in an X-ray detection device and extracts data of a desired cross section from the X-ray image of the X-ray detection device in a control device. United States Patent 6,873,680 Method and apparatus for detecting defects using digital radiography

A digital radiography apparatus (10) and process for providing images of an object, for example, an exhaust transition duct (12) comprising a core material and an overlying thermal barrier layer, to detect surface and interior defects within the duct (12). Incident energy is provided by an energy source (30), transmitted through the object (12), and sensed by a sensor (32). An image of the object (12) is formed by processing

Journal of Non Destructive Testing & Evaluation

High speed digital radiographic inspection of aircraft fuselages Inventors: Bueno; Clifford, Herd; Kenneth Gordon, Mohr; Gregory Alan, Batzinger; Thomas James, Walsh; Dennis Michael Assignee: General Electric Company (Schenectady, NY) A system and method for radiographic inspection of aircraft fuselages vol 10 issue 2 September 2011

30 includes a radiation source preferably located inside of the fuselage and a radiation detector preferably located outside of the fuselage. A source positioning system is provided for moving the radiation source longitudinally with respect to the fuselage, and a detector positioning system is provided for positioning the radiation detector in longitudinal alignment with the radiation source. The detector positioning system also moves the radiation detector circumferentially with respect to the fuselage. In operation, the radiation detector is moved over the fuselage in a circumferential direction while the radiation source illuminates an adjacent region of the fuselage with radiation. United States Patent 7,244,955 Computed radiography systems and methods of use Inventors: Bueno; Clifford, Corby Jr.; Nelson Raymond, Herd; Kenneth Gordon Assignee: General Electric Company (Niskayuna, NY) A computed radiography (CR) system for imaging an object is provided. The system includes a radiation source, a storage phosphor screen, an illumination source and a two dimensional imager. The radiation source is configured to irradiate the storage phosphor screen, and the storage phosphor screen is configured to store the radiation energy. The illumination source is configured to illuminate at least a sub-area of the storage phosphor screen to stimulate emission of photons from the storage phosphor screen. The two dimensional (2D) imager is configured to capture a two dimensional image from the storage phosphor screen using the stimulated emission photons. A method of reading a storage phosphor screen is also provided. The method includes illuminating at least a sub-area of the storage phosphor screen using an illumination source to stimulate

vol 10 issue 2 September 2011

emission of photons from the storage phosphor screen. The method further includes capturing at least one 2D image using a 2D imager, from at least a sub-array of the storage phosphor screen using the stimulated photons. United States Patent 7,239,435 High-speed, high resolution, wide-format Cartesian scanning systems Inventors: Shahar; Arie A scanning system for writing, printing, direct imaging, plotting, computed radiography, and scanning includes an optical system contains at lest one modulatable radiation source for emitting radiation, a movable collimating lens, a reflector, and a focusing lens and a mechanical system containing a first mechanical carrier spinning about a first axis. A second mechanical carrier spins about a second axis. The second axis is mounted on the first mechanical carrier and is arranged to rotate about the first axis. A third mechanical carrier spins about a third axis. The third axis is mounted on the second mechanical carrier and is arranged to rotate about the second axis. The system also has a movable surface. The collimating lens is arranged to receive the radiation from the one radiation source and to convert it into at least one collimated beam which propagates along an optical path from the collimating lens to the surface via the reflector and the focusing lens to form at least one focused radiation spot on the surface. The mechanical system is arranged to cause the third mechanical carrier of the mechanical system to carry the reflector and the focusing lens of the optical system to move the one focused radiation spot on the surface along a straight line. United States Patent 6,409,383 Automated and quantitative method for quality assurance of digital radiography imaging systems

Inventors: Wang; Xiaohui, Vanmetter; Richard L, Foos; David L, Steklenski; David J Assignee: Eastman Kodak Company (Rochester, NY) A phantom for use in measuring parameters in a digital radiography image system comprising a substantially rectangular member of an x-ray attenuating material; a first rectangular array of landmarks associated with the member for use in geometry measurements; a set of regions associated with the central position of the member for exposure linearity and accuracy measurement and a set of sharp angular edges part of one of the regions for modulation transfer function measurements. United States Patent 6,466,689 Method and system for digital radiography Inventors: MacMahon; Heber Assignee: Arch Development Corp. (Chicago, IL) A system and method for digital imaging. A digital radiological image of a subject is obtained having at least one low density region. The image is processed using first weighting factors in the at least one low density region of the digital image and second weighting factors smaller than the first weighting factors in regions of the digital image other than the at least one low density region. A processed digital image is obtained and a representation of the processed digital image is produced. In the processing of the image, unsharp mask filtering is employed using a processing curve having maximum unsharp mask filtering in the at least one low density region of the digital image and a constant amount of unsharp mask filtering less than the maximum unsharp mask filtering in the regions of the image other than the low density regions.

Journal of Non Destructive Testing & Evaluation

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Journal of Non Destructive Testing & Evaluation

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Journal of Non Destructive Testing & Evaluation

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Journal of Non Destructive Testing & Evaluation

Technical Paper

Non destructive detection of debonding in adhesively bonded metal/ceramic composite plates Sony Punnose, Amretendu Mukhopadhyay, B. Nagaraja Kowmudi, P. Rama Subba Reddy, V. Madhu and Vikas Kumar Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad – 500 058 Email: snbyrec@yahoo.co.in, sony@dmrl.drdo.in

ABSTRACT Studies have been carried out for detecting the debonding in ceramic-metal composite laminates of Zirconia Toughened Alumina bonded to Titanium alloy (ZTA/Ti) plate. Composite panels of ZTA/Ti with simulated debonds have been tested to assess the feasibility of thermography as a non destructive testing method for detection of debonds. The method has been studied vis-àvis ultrasonic technique. Pulse phase and lockin thermography techniques have been used for detection of debonds. The thermogram clearly reveals the debonding as well as the non-uniformity in bonding. Ultrasonic attenuation drop has been measured to detect the debonding in the specimens. The study shows that both thermography and ultrasonic techniques can be adopted for detecting the debonding in the composite plates. Key Words: Ultrasonic, lock-in thermography, pulse phase thermography

INTRODUCTION

Adhesively bonded structures and adhesive joining technology are increasingly being used as alternatives to traditional methods of fastening materials. This has led to an increasing demand on nondestructively evaluating and characterizing this material for quality control. The most common kind of damage in the adhesively bonded structure includes localized lack of or excess of resin and debonding, inhomogenities due to the presence of spurious materials and porosity. The quality (strength and durability) of a bond depends on the interaction of the adhesive with the adherent (surface to bond). There are several factors, e.g. ambient temperature and humidity, pressure applied, that can affect the quality of bonding. Owing to improper fabrication parameters, different types of defects are likely to occur, such as lack of adhesive (bubbles, air layers, foreign materials), cohesion defects (breaking within the adhesive) and bonding defects (breaking at the surfacebond interface). Various Non Destructive Examination (NDE) methods including ultrasound, radiography, acoustic emission, infrared thermography (IRT) have been used for the detection and characterization of defects in composites. Ultrasonic method, being one of the oldest methods, is widely used in the industry for defect characterization. Among other techniques, infrared thermography is a fast growing technique for the inspection of composite sandwich structures. As the defects in bonded structures act as barrier to thermal diffusion, these defects can in-principle be detected by IRT technique. Thermal imaging techniques are being widely used for detection and evaluation of defects and delaminations in composite materials [1-8]. Lock-in thermography is proved to be an effective technique for quantifying the depth of the defects in steel plates as well as detecting subsurface defects in composite plates [9, 10]. In addition to IRT, ultrasonic Vol. 10, Issue 2 September 2011

measurements are being used for the assessment of bonding defects in different composite materials [11-17]. Both these techniques have advantages as well as some limitations in terms of its applicability. Thermography has advantages like its non contact nature, fast inspection rate and less health hazards [18]. On the other hand, there are few disadvantages like difficulty to deposit uniformly a large amount of energy in short period of time over a large surface, effect of thermal losses, emissivity problems and capability to detect only subsurface defects, resulting in a measurable change of thermal properties. Ultrasonic techniques are better suited for quantification of defects though they are slower than to thermal imaging techniques and are contact type in nature. In this paper, we explore the possibility of using infrared thermography (lock-in thermography and pulse-phase thermography) and studied these technique vis-à-vis ultrasonic techniques to detect the debonding in ZTA/Ti bonded composites. Composite panels of ZTA/Ti with simulated debond have been tested to assess the feasibility of thermography technique vis-àvis ultrasonic technique as a NDE method to detect debond.

2. EXPERIMENTAL DETAILS Two specimens of simulated debonds have been tested to study the feasibility of ultrasonic vis-à-vis thermography as a NDE method for characterization of debond. The details of the specimens are given in Table 1 and the photographic images are shown in Figure 1. Thermal imaging of the two specimens was carried out using a medium wave infrared camera (M/s. Cedip, France) with InSb focal plane array detector. The camera has spectral sensitivity in the range of 3-5 μm with a spatial resolution of 5.4 μm. The temperature sensitivity of the camera is 20 mK at 25oC. Image acquisition was done in the frequency range of 50 - 380 Hz at a full frame view of Journal of Non destructive Testing & Evaluation

Technical Paper

Fig. 1 : Photographic image of Specimens

320 x 256 pixels. A fully automated software system was employed for capturing and processing of images. Both lock-in as well as the pulsed phase techniques was used and the schematic of the experimental setup are shown in Figure 2 (a) & (b) respectively. Table 1: Details of specimens Nomenclature

Type of materials

Specimen â&#x20AC;&#x201C; 1

50 mm x 50 mm x 5 mm ceramic tile bonded to metal plate of 84 mm x 84 mm x 5 mm

Specimen â&#x20AC;&#x201C; 2

30 mm x 30 mm x 5 mm ceramic tile with a centre hole of 7 mm dia bonded to metal plate of 70 mm x 95 mm x 5 mm

The experiment was performed at room temperature in which the detector and heating sources are kept on the opposite sides. The images were acquired from the metal side. Both the specimens were heated using two Halogen lamps of 1 kW power. The lamps were kept at a distance of ~ 0.4 m and the IR camera was kept at a distance of ~ 0.4 m from the specimens. The two heating sources

(a) Lockin thermography

were kept at an angle of 75o in order to ensure uniform heating of the specimens. The distances among the specimen, IR camera and heating sources were kept constant throughout the experiment. In pulsed phase technique (PPT) specimens were heated with a square pulse of ~ 2 sec duration and images were captured after the heat pulse. Images thus captured were processed to obtain phase images within a range of chosen frequency depending on the type and depth of debond. In lockin technique, the procedure consisted of acquiring phase image at lock-in frequency while the specimen surface was thermally stimulated with a sinusoidal lockin heat pulse. Specimens were heated with sinusoidal modulation for 4-5 cycles and the resulting oscillating temperature field in the stationary regime (that is after the transient regime) was remotely recorded. A Hameg function generator was used to generate the thermal modulation. The frequency of modulation coupled with the lockin option was chosen from the analysis of phase images from pulsed phase experiments. Ultrasonic measurements have been performed using contact transducer of 5 MHz frequency in pulse echo mode, at a gain of 50 dB. Water was used as a couplant between the transducer and the specimen. During the measurements, a constant pressure was maintained between the transducer and the specimen. These measurements of back wall echo amplitudes were made in the far field region. Multiple rectified back wall echo pattern has been studied both from the debond and good regions in the specimens. These echo patterns were compared with the reference echo taken only from metal, i.e. metal air interface.

RESULTS

3.1 THERMOGRAPHY RESULTS The temperature-time evolution pattern for specimen -1 at four chosen spot on the surface has been shown in Figures 3(a) and 3(b). Though the temperature evolution pattern shows some indication but it could not conclusively

(b) Pulsed Phase thermography are missing.

Fig. 2 : Experimental set up Journal of Non destructive Testing & Evaluation

Vol. 10, Issue 2 September 2011

Technical Paper

reveal the presence of debond. The problem of nonuniform heat deposition masks the temperature evolution pattern. To overcome this problem the acquired images were processed in the frequency range of 0.01 Hz to 0.1 Hz. The frequency-phase images were analyzed for detection of debond/hole in the specimens. Figure 4a shows pulse phase image at 0.03 Hz for specimen-1 which clearly reveals debond and Figure 4b shows pulse phase image at 0.055 Hz for specimen-2 which shows the presence of hole in the ZTA plate seen from the metal side. It can be seen from the image that the hole is seen at the centre of the plate but the size of the hole is not to the exact scale. For better detectability, lockin thermography has been performed at the frequency of pulse phase for the specimens. But the frequency of lockin process does not match exactly with that of the frequency of pulse phase. Lockin images are obtained at slightly different frequencies. Figure 5a shows lockin image at 0.385 Hz for specimen-1. The image not only reveals the debond region but also reveals the quality/uniformity of bonding in other regions. The four corners are bonded more as compared to the sides. Figure 5b shows lockin image at 0.037 Hz frequency for specimen-2. All these images show an improvement in terms of detectability over the pulsed phase images.

Fig. 3 : (a) Thermogram showing four spots (specimen-1)

Fig. 4 :

(a) Pulse Phase image at f = 0.03 Hz (specimen-1)

Vol. 10, Issue 2 September 2011

3.2 Ultrasonic Results

Specimens were also tested using ultrasonic technique. Figures 6(a-c) and 7(a-c) shows the multiple echo pattern (Amplitude Vs time) taken from the metal surface from the good region and debond region for specimen-1 and specimen-2 respectively. The scatter in the attenuation measurements is ±10%.

4. DISCUSSION 4.1 Thermography

Theory: When a uniform heat is deposited periodically with a modulation frequency of ‘ω’ on a semi infinite planar surface, notwithstanding the three dimensional heat flow, temperature evolution on the surface as function of depth (z) and time (t) for a fixed location (x, y) can be expressed as [19]:

(1)

where

is the thermal diffusion length; k is thermal

conductivity; ρ is density; C is specific heat. From the above equation, it can be seen that the temperature for a particular point on the surface (z = 0) of the specimen

(b) Time-temperature graph

(b) Pulse Phase image at f = 0.055 Hz (specimen-2) Journal of Non destructive Testing & Evaluation

Fig. 5

Technical Paper

: (a) Lockin image at f = 0.385 Hz (specimen-1)

(b) Lockin image at f = 0.037 Hz (specimen-2)

Fig. 6 : Signal amplitude for specimen-1

Fig. 7 : Signal amplitude for specimen-2

changes periodically in time and corresponding phase of that depends on the thermal properties of the material along which heat diffuses beneath the point. Variation in the thermal properties, along the path with respect to the path beneath the surrounding points, can lead to a differential phase contrast on the surface of the specimen. This is due to interference in time of the deposited heat with the diffusing thermal front. Now in lockin thermography the exact time dependence between the output signal and the reference input signal (i.e. the oscillating - also called modulated heating) is monitored. The resulting oscillating temperature field (following the oscillating thermal stimulation) in the stationary regime (that is after the transient regime) is remotely recorded through its thermal infrared emission. For a particular (lock in) frequency, phase image is map of f as a function of (x, y). Images are obtained in stationary mode and total number of images is a function Journal of Non destructive Testing & Evaluation

of total periods of heating and frame rate. Images captured during a complete period are sampled, for the lock in frequency, using Fourier analysis and a single phasefrequency image corresponds to the summation over the total number of periods. Pulsed Phase Thermography (PPT) is a processing technique which combines advantages of both PT (operating in the transient regime) and LT (operating in the stationary regime). In PPT deployment [20, 21], the specimen is pulse-heated as in PT and the mix of frequencies of the thermal waves launched into the specimen is unscrambled by performing the Fourier transform (FT) of the temperature decay on a pixel by pixel basis thus enabling computation of phase images as in LT. In the analysis, for each pixel (i, j), the temporal decay f (x) is extracted from the image sequence (where x is the index in the image sequence). Next, from f (x), the discrete Fourier transform F (u) is computed (u being the frequency variable). Finally, from the real R (u) and imaginary I (u) components of F (u), the phase is computed Vol. 10, Issue 2 September 2011

Technical Paper

[22]. In PPT as in LT, it is possible to explore the various frequencies u. However, differences exist since analysis in PPT is performed in the transient mode while in LT, the signal is recorded in the stationary mode. This means, for instance, the quality of images will be better in LT due to the summation process involved in the computations. On the other hand, a single pulsed experiment is needed in PPT while LT sometimes requires more. The FT is used to extract various frequencies and is expressed as [23] (2)

where j is an imaginary number, n designates the frequency increment, N is the total number of thermograms and Re and Im are respectively the real and the imaginary parts of the transform. Amplitude A and phase Ď&#x2020; data are available as follow: (3)

It can be seen that though the presence of debond and hole have been detected for the specimens using both the pulsed phase and lockin techniques, the lockin images obtained are at slightly different frequencies. In the case of specimen-1, i.e. specimen with debond at the centre, pulsed phase and lockin images are almost at same frequency. In specimen-2, there is a finite frequency difference between pulsed phase and lockin images. Pulsed phase images are at higher frequencies. From the relationship of thermal diffusion length, as discussed above, it can be seen that Îź is inversely proportional to Ď&#x2030;. This suggests that with decrease in the modulation frequency, greater depth can be probed inside a material. But, this has a limit in terms of detectable contrast (depends on the temperature sensitivity of the camera) arising out of the temperature difference between the neighboring points (depends on the spatial resolution of the camera) on the surface of the specimen. In the present study, phase contrast arising out of the difference in the thermal properties of the glue and air (at a depth of 5 mm) is getting reduced due to the high rate of three dimensional heat diffusion in metal. In case of pulsed phase, this becomes an impediment towards the detection of defect either too small. In the case of smaller defects, temperature sensitivity becomes a matter of concern. To overcome these problems and as first step towards the detection of debond, specimens were heated for higher time (~ 2 sec) and images were analyzed at higher frequencies. Higher frequency image indicates the presence of debond at lower depth. In this case, it is seen as an impression of debond, as higher heating time gives rise to partition of more energy towards lower frequency components. These lower frequency components interfere incoherently with the original thermal wave front from the defect and gets manifested in the form of noises in the pulsed phase images wherein the images do not reveal the proper size of the debond. Nevertheless, pulsed phase analysis has given a Vol. 10, Issue 2 September 2011

firsthand indication of the presence of debond in the specimens with a rough estimate of the modulation frequencies for further analysis with lockin signal. In lockin process, the problem of decreasing phase contrast due to faster heat transfer in metals is somewhat nullified and defects at greater depth can be detected. Periodic heating with lock in modulation frequency, is chosen according to the thermal perturbation caused by the presence of defects, leading to the physical interference in time. Interference in time of thermal front along with the overall summation principle discussed above leads to a better phase contrast in case of lockin images for the specimens. One of the aspects of the phase images over amplitude images is that phase images are less susceptible to the infrared surface features such as emissivity variation. This leads to better depth probing as compared to the conventional pulsed thermography with IR camera of similar characteristics. Using the phase information

obtained from

the thermograms and the thermal diffusion length calculated from the modulation frequency, depth of the defects can be roughly estimated. But lockin and pulsed phase techniques still have some limitations in terms of exact defect quantification, as in this process the time information is lost. Further, experiments along with image processing with wavelet transform for quantification of debond is currently being studied. 4.2 Ultrasonics

From the echo pattern it can be seen that exponential decay from the good bonding region is markedly different from the debond region. By comparing these echo patterns with the echo pattern from the metal reference, it can be inferred that echo from the debond region resembles the echo pattern of the metal reference. This is due to the fact that in case of debond; the back wall echo originates from the metal air interface instead of metal glue interface. Change in the nature of interface causes change in the reflection coefficient at the interface. As the ultrasonic parameters were same for all the measurements, drop in the amplitude of the back wall echoes from the metal glue interface was more owing to the low reflection coefficient at the interface. This method shows the promise of detecting debond very effectively. However, C-scan imaging can be used for automatic and faster inspection of large plates and hence the study can be extended further for detection/sizing of debond in case of large components.

5. CONCLUSIONS It could be concluded from the above experiments that both the techniques: Thermography and Ultrasonics can Journal of Non destructive Testing & Evaluation

52 be adopted for detecting debond in the ZTA/Ti alloy bonded composite plates. It has been shown that thermography technique can be used for detection of debonding as well as to check the quality/uniformity of bonding. Besides this, thermography can serve as a fast non-contact method for detecting debond and has the potential of automation as well as it can serve the purpose of quality checking.

ACKNOWLEDGEMENTS The authors would also like to thank DRDO, India for funding this activity. The constant support and encouragement from Director, Defence Metallurgical Research Laboratory (DMRL), Hyderabad is gratefully acknowledged. We also thank all the technical staff of DMRL for their valuable contributions.

REFERENCES 1. N.P. Avdelidis, A. Moropoulou and Z.P. Marioli Riga, “The technology of composite patches and their structural reliability inspection using infrared imaging”, Progress in Aerospace Sciences, 39 (2003) 317-328. 2. Maurice Cummings, Paul Biagioni, Philip John-Lamey and Donald J. Burden, “Thermal image analysis of electrothermal debonding of ceramic brackets; an in vitro study”, European Journal of Orthodontics, 21 (1999) 111-118. 3. S. Bagavathiappan, Y. Siva Sankar, M.C.S. Kumar, John Phillip, T. Jayakuamr and Baldve Raj, “Active infrared thermal imaging for quantitative analysis of defects and delaminations in composite materials”, Journal of Non destructive Testing & Evaluation, 8 (2009) 28-36. 4. V. Dattoma, R. Marcuccio, C. Pappallettere and G.M. Smithm, “Thermographic investigation of sandwich structures made of composite materials”, NDT & E International, 34 (2001) 515-520. 5. M. Menaka, S. Bagavathiappan, B. Venkataraman, T. Jayakumar and Baldev Raj, “Characterisation of adhesively bonded laminates using radiography and infrared thermal imaging techniques”, Insight 48 (2002) 606-612. 6. K. Srinivas, A.O. Siddiqui and J. Lahiri, “Thermographic inspection of composite materials”, Proceedings of National Seminar on Non destructive Evaluation (2006) 131-142. 7. W. Bai and B.S. Wong, “Evaluation of defects in composite plates under consecutive environments using lock-in thermography”, Meas. Sc. Technology, 12 (2001) 142-150. 8. N. P. Acdelidis, B.C. Hawtin and D.P. Almond, ‘Transient thermography in the assessment of defects of aircraft composites”, NDT & E International, 36 (2003) 433- 439. 9. C. Wallbrink, S.A. Wade and R. Jones, “The effect of size on the quantitative estimation of defect depth in Journal of Non destructive Testing & Evaluation

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steel structures using lock-in thermography”, Journal of Applied Physics, 10 (2007) 101-108. Sung Quek, Darryl Amond, Luke Leson and Tim Barden, “A novel and robust thermal wave signal reconstruction technique for defect detection in lockin thermography”, Meas. Sci. technology, 16 (2005) 1223-1233. C. Scarponi and G. Briotti, “Ultrasonic technique for the evaluation of delaminations on CFRP, GFRP, KFRP composite materials”, Composited, Part B 31 (2000) 237-243. T. Meitzler, G. Smith, I. Wong, M. Charbeneau, E. Sohn, M. Bienkowski and A. Meitzler, “Crack Detection in Armor Plates Using Ultrasonic Techniques,” American Society for Nondestructive Testing, Materials Evaluation, 66 (2008) 555-559. F. Bastianini, A. Di Tommas and G. Pascale, “Ultrasonic non-destructive assessment of bonding defects in composite structural strengthenings”, Composite Structures, 53 (2001) 463-467. W. Hillger, “Ultrasonic imaging of internal defects in composites”, NDTnet, 2 (1997). C.V. Subramanian, M. Thavasimuthu, P. Palanichamy. D.K. Bhattacharya and Baldev Raj, “Evaluation of bond integrity in sandwiched structures by dry couplant ultrasonic techniques”, NDT & E International, 24 (1991) 29-31. L.S. Chang, T.H. Chuang and W.J. Wei, “Characterisation of alumina ceramics by ultrasonic testing”, Materials Characterisation, 45 (2000) 221226. M.Lethiecq and M. Perdrix, “Automatic discrimination techniques for NDT of metal-ceramic bonds”, NDT & E International, 24(1991) 307-311. Xavier Maldague, “Applications of infrared thermography in nondestructive evaluation”, New York: Willey. X. Maldague, F. Galmiche, A. Ziadi, “Advances in pulse phase thermography”, Infrared physics and technology, 43 (2002)175-181. X. Maldague, “Introduction to NDT by Active Infrared Thermography”, Materials Evaluation, 6 (2002) 10631073. H. Kaplan, “Practical Applications of Infrared Thermal Sensing and Imaging Equipment”, Second Edition, Proc. Soc. of Photo-Opt. Instrumentation Eng. (SPIE), TT34 (1999) 160. X. Maldague and S. Marinetti, “Pulse Phase Infrared Thermography,” Journal of Applied Physics, 79 (1996) 694-2698. C. Ibarra-Castanedo, F. Galmiche, A. Darabi, M.Pilla, M. Klein, A. Ziadi, S. Vallerand, J.F. Pelletier and X. Maldague, “Thermographic nondestructive evaluation: Overview of recent progress’, Thermosense XXV (SPIE), 5073. Vol. 10, Issue 2 September 2011

Technical Paper

NDE Technique for Reactor Core Vibration Measurement in FBRs R. Ramakrishna, P. Anup Kumar, M. Anandaraj, M. Thirumalai, V. Prakash, C. Anandbabu and P. Kalyanasundaram Fast Reactor Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, India Email: ramakrishna@igcar.gov.in, prakash@igcar.gov.in

ABSTRACT The Prototype Fast Breeder Reactor (PFBR) which is under construction at Kalpakkam, India, is a 500 MWe sodium cooled pool type reactor. The core of the PFBR consists of 1758 free standing subassemblies, out of which 181 are fuel subassemblies, supported on the grid plate. Coolant sodium flows axially from the bottom of the subassembly to top and it is in turbulent regime, which can excite flow induced vibration (FIV) of fuel subassemblies. Flow induced vibration is not desirable as it can cause failure of the fuel element clad tubes from fatigue, wear and vibration induced fretting. Excessive vibration can also cause reactivity fluctuations, rattling and power control problems. During commissioning of PFBR, it is planned to measure the subassembly vibration in sodium at isothermal condition at 2000C. Conventional contact type vibration sensors such as accelerometers, strain gages and displacement probes are not suited for high temperature sodium environment and due to difficulty in sensor mounting. Ultrasonic technique is the only possible solution for non-contact type vibration measurement in the primary sodium pool of fast reactors. Feasibility experiments were carried out in water to measure the subassembly vibration using ultrasonic technique. A full scale dummy fuel subassembly is erected in a water test loop and an electro-dynamic exciter is used to excite the subassembly with known frequency and amplitude. Ultrasonic sensor is mounted near the target (subassembly surface) and is operated in pulse-echo mode. An LVDT mounted directly on the subassembly is used as the reference sensor to validate the results of ultrasonic technique. Ultrasonic A-scan signals are acquired during subassembly vibration. The vibration of the subassembly will cause a variation in the time delay between the transmitted pulse and received echo which is extracted from the ultrasonic A-scan signals. The measured vibration amplitude and frequency from ultrasonic technique is compared with reference LVDT signal and the error was found to be less than 3%. The developed ultrasonic technique proved to be a potential NDE method for subassembly vibration measurement during the commissioning of PFBR. In this paper the subassembly vibration measurement using ultrasonic technique is discussed with experiment details and results.

INTRODUCTION

Prototype Fast Breeder Reactor (PFBR), which is under construction at Kalpakkam, is a sodium cooled pool type reactor. The PFBR core consists of 1758 core subassemblies which are supported in the grid plate. There are 181 fuel subassemblies in PFBR core with 217 fuel pins in each subassembly, vertically held in the form of bundle within a hexagonal wrapper tube (hexcan). The pins are separated by spacer wires wound around the pins helically. The total height of the subassembly is 4.5 m out of which 3.9 m is above the grid plate. The nominal flow through the maximum rated subassembly is 36 kg/s. Coolant sodium flows axially from the bottom of the subassembly to top and it is in turbulent regime, which can excite flow induced vibration (FIV) of fuel subassemblies [1]. Flow induced vibration is not desirable as it can cause failure of the fuel element clad tubes from fatigue, wear and vibration induced fretting. Excessive vibration can also cause reactivity fluctuations, rattling and power control problems. During commissioning of PFBR, it is planned to measure the vibration of subassembly in sodium at isothermal condition at 200oC. Vibration measurement of components in general is carried out using accelerometers, strain gages etc. Accelerometers and strain gages must be Vol. 10, Issue 2 September 2011

mounted directly on the component surface for vibration measurement, which is not possible in case of sodium immersed components in a reactor. Hence ultrasonic technique is the only viable solution for under sodium vibration measurements. Feasibility of using ultrasonic technique for the vibration measurement was studied earlier in water. Ultrasonic Bscan imaging has been employed to determine the vibration amplitude and frequency. In this measurement the ultrasonic sensor is kept stationary and the target (subassembly) is vibrated using an exciter. Fig.1 shows the B-scan image recorded for a subassembly excitation frequency of 2 Hz and 0.6 mm amplitude. The measured frequency from the recorded ultrasonic signal is 2.08 Hz and the amplitude is found to be 0.59 mm [2].

Fig. 1 : B-Scan Image (Excitation 2 Hz; 0.6 mm) Journal of Non destructive Testing & Evaluation

54 Based on this encouraging result obtained from feasibility studies, extensive experiments were carried out and signal processing techniques were developed to extract vibration signal from the ultrasonic A-scan signals. This paper discusses the details of the test section, instrumentation, and ultrasonic methodology, signal processing technique and its test results and discussion.

2. PRINCIPLE OF OPERATION AND INSTRUMENTATION Ultrasonic sensor operating in pulse-echo mode is used to detect the movement of subassembly. Ultrasonic sensor is mounted near the target (subassembly surface) and the time delay between the transmitted pulse and reflected echo is used to calculate the distance between the sensor and the target. The movement of the subassembly will cause a variation in the time delay, which will be a representation of the subassembly movement.

Technical Paper

Ultrasonic sensor (Lithium-Niobate) operating at 4 MHz is used for the measurement. An electro-dynamic exciter attached to the subassembly is used to excite the subassembly with known frequency and amplitude. Ultrasonic sensor is mounted near the target (facing subassembly surface) and the time delay between the transmitted pulse and reflected echo is used to calculate the distance between the sensor and the target. For validating the results obtained from ultrasonic technique, a Linear Variable Differential Transformer (LVDT) and an accelerometer were attached to the subassembly. Fig.2 shows instrumentation schematic employed.

EXPERIMENTAL PROCEDURE

A test loop was made of stainless steel, simulating the geometrical arrangement as in PFBR. Full scale dummy fuel subassembly was fabricated and erected in the grid plate in the test section. Perspex embedded SS window was assembled in the test loop to view the movement of top end of subassembly during measurements (Fig.3).

Fig. 2 : Instrumentation Schematic Setup

Fig. 3 : Test loop Perspex window Journal of Non destructive Testing & Evaluation

Fig. 4 : Displacement Spectra Vol. 10, Issue 2 September 2011

Technical Paper

Flow induced vibration (FIV) measurements were done in water earlier to determine the vibration characteristics of the subassembly [3]. The first natural frequency of the subassembly was identified as 3.6 Hz. Fig.4 shows the vibration spectra (displacement) recorded during the FIV studies. These results gave an insight to the expected maximum amplitude of vibration of the subassembly and its natural frequencies. In the current experiment, ultrasonic sensor is kept immersed in water near top of the subassembly and it is fired continuously at a pulse repetition rate of 31 pulses/ sec. Using the signal generator, power amplifier and electrodynamic exciter, subassembly is excited and experiments were conducted with different excitation frequencies ranging from 2 Hz to 6 Hz. The amplitude of subassembly vibration was also varied from 0.1 mm to 1.2 mm. Experiments were repeated with different pulse repetition frequencies to study the effect of signal distortion. The output signals from the ultrasonic sensor and the reference LVDT were recorded using digital storage oscilloscope.

DEVELOPMENT OF SIGNAL PROCESSING TECHNIQUE AND TEST RESULTS

As the ultrasonic sensor is fixed as the reference point, the movement of the subassembly with respect to the

reference will cause variation in the transit time between the transmitted pulse and received echo. This variation in transit time represents the movement of the target (subassembly). Fig.5 shows typical time signal plot (series of A-scans) recorded from the ultrasonic sensor, depicting the variation in time delay between the transmitted pulse and reflected echo. Signal processing technique to extract the vibration signal from the ultrasonic pulse-echoes has been developed. The ultrasonic A-scan signal generated during the vibration measurement consists of transmitted pulse and received echo. The time difference between the peak value of transmitted pulse and received echo is a measure of vibration (displacement) of subassembly. The received echo is a signal with sinusoids of different amplitudes. It is very difficult to read the peak values of transmitted pulse and received echo, directly from the raw time signal. Signal processing technique based on Hilbert transform is employed here to extract the envelope of the ultrasonic Ascan signals. Consider x(t ) is the ultrasonic A-scan time signal, then the Hilbert transform, y(t)=H{x(t)}, is defined as (1)

Using the signal, and its Hilbert transform, an artificial complex signal, called analytic signal is generated (given in eq.(2)) whose real part is the original time signal and

Fig. 5 : Ultrasonic time signal with pulse and echo trains

Fig. 6 : Envelope extraction using Hilbert Transform; a) Ultrasonic A-scan signal, b) Envelope of A-scan signal Vol. 10, Issue 2 September 2011

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Technical Paper

Fig. 7 : LVDT reference signal

Fig. 8 : Vibration signal extracted from the ultrasonic A-scan signals

whose imaginary part is the Hilbert Transform of original time signal. z(t) = x(t) + jy(t) = E(t)ejΨ(t)

(2)

Where, E(t) is the instantaneous envelope which is the magnitude of the complex analytic signal, and Ψ(t) is the phase angle of analytic signal. Fig.6 shows the envelope extraction of a typical ultrasonic A-scan signal acquired during subassembly vibration at frequency of 2 Hz and amplitude of 0.5 mm (pk-pk). After envelope extraction, the time difference (transit time) between the peak value of transmitted pulse and received echo is found. As the velocity of ultrasound in water is known, the displacement of target (subassembly) is calculated from the transit time. When subassembly vibrates, displacement of target changes and hence transit time changes. The transit time is calculated for all the ultrasonic A-scan signals and the displacement signal is generated. Fig.7 shows the vibration (displacement) signal extracted from the ultrasonic signals and Fig.8 shows the reference signal recorded from the LVDT for an excitation frequency of 2 Hz and amplitude 0.5 mm (pk-pk). The amplitude and frequency of vibration of subassembly measured using ultrasonic technique were 0.51 mm (pk-pk) and 2.04 Hz respectively. This vibration signal is matching well with the reference signal from LVDT. The signals from the ultrasonic sensor and the reference LVDT sensor were analyzed for various subassembly excitation frequencies and amplitudes, and the overall error in this measurement technique is found to be less than 3 %. Journal of Non destructive Testing & Evaluation

5. CONCLUSION Ultrasonic technique and its signal processing algorithm have been developed for the subassembly vibration measurements in PFBR. Experiments were carried out in a water test loop using a PFBR full scale dummy fuel subassembly. Amplitude and frequency of vibration of fuel subassembly measured using ultrasonic technique is compared with a reference LVDT sensor and the error was found to be less than 3%. The developed technique proved to be a potential and promising NDE method for vibration measurement, where the conventional contact type sensors cannot be used.

REFERENCES 1. V. Prakash et.al, Experimental qualification of subassembly design for Prototype Fast Breeder Reactor, Nuclear Engineering and Design, Vol. 241, No.8, pp.3325– 3332, August 2011. 2. R.Ramakrishna et.al, Flow Induced Vibration measurement using Ultrasonic Technique, International Conference on Sensors & Related Networks (SENNET 2009), VIT, Vellore, India, Dec 07-10, 2009. 3. M. Anandaraj et.al, Flow induced vibration studies on PFBR fuel subassembly, 2nd International Conference on Asian Nuclear Prospects, ANUP 2010, Chennai, India, Oct 10-13, 2010.

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Technical Paper

On the conversion of multi-frequency “apparent” conductivity data to actual conductivity gradients on peened samples Veeraraghavan Sundararaghavan1 and Krishnan Balasubramaniam2 Center for Non-Destructive Evaluation, Machine Design Section, Department of Mechanical Engineering, Indian Institute of Technology-Madras, Chennai 600036, India. 2 E-mail: balas@iitm.ac.in 1 Currently with Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI, USA

ABSTRACT This paper addresses the interpretation of “apparent” conductivity measurements as a function of frequency in order to determine the actual conductivity profile as a function of depth in a conducting material. Simulations show that “apparent” conductivities are not indicative of actual conductivity gradients because of inherent constant conductivity approximation that is assumed at every frequency. This paper focuses on facilitating the conversion of the multi-frequency “apparent” conductivity data conductivity depth profiles through a Model based inversion scheme. The inversion uses a multi-layer axi-symmetric finite element model as the forward model and uses an optimal skin depth approximation for isolating the integral effects of the conductivity gradients on the multi-frequency “apparent” conductivity measurements. Unlike the inductance inversion method that has been reported elsewhere, this method does not depend on the sensor coil parameters and is robust enough to accommodate for some common measurement uncertainties. Also, commercial multi-frequency conductivity measurement instruments can be used to obtain input data for the inverse model. Possible application of the model towards characterization of residual stresses in peened specimens is also addressed. Keywords: Multi-Frequency Eddy Current Testing; Inverse Model; Conductivity Gradient Measurements

1. INTRODUCTION Eddy current NDT techniques are well developed and have been primarily applied as a means of detecting near surface discontinuities. The changing voltage in the sensor coil induces eddy currents in a nearby conductor that in turn loads the coil and changes its impedance and phase. The depth of penetration of eddy currents can be controlled by the frequency of testing, due to the skin-depth effect, and hence, can be used to test components over different depths. Eddy current testing methodology has been successfully applied over the years to address problems like crack detection, material thickness measurements, heat damage detection, “apparent” conductivity measurements and for monitoring a variety of processes (ASNT (2004)). A method for assessing “apparent” conductivity involves measurement of the impedance of coils, driven by a constant amplitude alternating current, placed above the sample surface. Material is assumed to have a constant conductivity and the net conductivity is measured at various frequencies using eddy current absolute-coil configuration (ASTM E 1004-99). A conductivity measurement at a particular frequency has information from the surface to a particular depth of penetration that is related to the skin-depth (d) in that material. Also, it represents an integrated value of conductivity over this depth. Hence, when making measurements of “apparent” conductivity at different frequencies, these measurements are not independent of Vol. 10, Issue 2 September 2011

each other, since any measurement at a lower frequency has the information already existing in all of the measurements made using higher frequencies. Any NDE process may be considered to involve three systems, each having a unique set of parameters that define its characteristics viz. (a) The Input to the material, (b) The material itself, and (c) The output response measured by the NDE system. Traditionally, the input and the material parameters are assumed known and numerous Forward Models have been developed that predict or estimate the output response function. Over the years, forward models are very well established and serve the key purpose, for improved interpretation as well as to optimize the input parameters to obtain the desired output response. The other two scenarios i.e. if the output response function in the form of measured data is available, to obtain one of system parameters, i.e. either the input function or the material properties while the other one is assumed to be known are classified as Inverse Problems. Due to the availability of computational resources, the inverse problem solutions are becoming increasingly feasible. The traditional difficulties with the ill-posedness of the inverse solution (which includes lack of uniqueness or stability of the solution process) are increasingly becoming solvable. Typical applications include measurement of material properties such as modulus, viscosity, temperature, hardness and stress profiles, etc. The formulation includes both numerical and analytical solutions in ultrasonics, eddy current and thermal imaging Journal of Non destructive Testing & Evaluation

58 and the inverse solution process utilizes a variety of techniques such as Neural Networks, Genetic Algorithms, Maximum Entropy Methods, etc (Liu, 2003). Popular methods to invert eddy current impedance data include the use of regressive tools like neural networks or numerical inversion schemes based on analytical or finite element based forward models. The use of neural networks for conductivity inversion has been reported by several authors (Rekanos et al., 1997; Katragadda et al., 1997; Glorieux et al., 1999) with inverse models being trained using either experimental data or data from numerical models. The proposed “apparent” conductivity inversion scheme is primarily a numerical inversion scheme based on a Finite element based forward model. Seminal work on numerical inversion of multi-frequency eddy current impedance measurements for characterizing coating thicknesses and conductivities in layered materials can be found in Moulder et al. 1992. More advanced numerical inversion models (Bowler and Norton, 1993; Liu et al., 2000; Sun et al., 2002 etc.) primarily work by iteratively adjusting relevant parameters in a forward model until measured signal value is reached. Such techniques require accurate calibration of coil design parameters for use in the forward model. The proposed “apparent” conductivity inversion model is different from other existing numerical models in that it does not employ any form of iterative refinement and further, does not depend on coil design parameters, hence, can be used with commercial conductivity meters. “Apparent” conductivity measurements using the eddy current sensor have been widely used as a basis for characterizing surface-treated specimens. This work was motivated by a study of finite element simulations that showed that the “apparent” conductivity does not follow the trends followed by the actual conductivity profiles. Specifically, an attempt is made here to study the effect of peening on true conductivity profiles of a specimen using the proposed inversion scheme. Studies have been performed (Blodgett et al., 2003; Lavrentyev et al., 2000) to analyze the effect of peening on a material using change of measured “apparent” conductivity with frequency. The conductivity of the peened specimen continuously changes as a function of depth due to interacting effects of several factors including cold work gradients, surface roughness, and stress gradients. However, “apparent” conductivity profiles are only able to provide a depth-averaged response

Technical Paper

of a peened specimen. Hence, there is a need for an appropriate inversion scheme that can recover the information-rich actual conductivity profiles from such measurements. This paper extends the inductance inversion technique already proposed (Sundararaghavan and Balasubramaniam, 2004; Sundararaghavan et al. 2005) to invert multi-frequency “apparent” conductivity data measured over nonmagnetic metals. The true conductivity profiles show several promising trends that would possibly allow characterization of residual stresses in peened specimens.

2. METHODOLOGY OF INVERSION The critical input for the inversion model comes in the form of “apparent” conductivity measurements at various frequencies. The “apparent” conductivity measurement is based on the notion that the material has a single conductivity that contributes to the measured inductance of the coil placed over the material. Given the “apparent” conductivity, the expected inductance change of any coil placed over the material can be found using existing eddy current forward models which are either analytical (Dodd et al., 1970) or based on the Finite element method. In this paper, finite element forward model (FEFM) (Palanisamy, 1980) is used to estimate the inductance values of a simulated coil placed over the material with the given “apparent” conductivity at a particular excitation frequency. For applications involving conductivity variation with depth in axi-symmetric testing situations, apart from Finite element based models, numerical models such as those reported by Uzal et al. (1993) can also be used as the forward model. The proposed “apparent” conductivity processing procedure is depicted in Figure 1. Finite element technique using the energy functional approach (Palanisamy, 1980) is used to solve the axi-symmetric eddy current governing equation for the magnetic vector potentials (A) in the discretized domain consisting of coil, material and air (Figure 2). The governing equation for axisymmetric geometries is given by, (1)

Given the “apparent” conductivity (σa) of the material at frequency, f, the current density (Js) in the coil and known geometrical parameters (ro,ri,h,lo,d) as shown in Figure 2, the magnetic vector potentials (A) can be calculated over all nodes in the discretized domain. Only nonmagnetic materials are considered, hence we use the permeability of free space, μ0. The impedance (Zcoil) of the coil whose cross section is discretized into N triangular finite elements is then calculated as, (2)

Fig. 1 : Proposed “apparent” conductivity inversion procedure Journal of Non destructive Testing & Evaluation

where j is the complex operator, Ns is the turn density of the coil (turns/m2), Is is the current in the coil, ω is the Vol. 10, Issue 2 September 2011

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Fig. 2 : Finite element model configuration

Fig. 3 : Multi-frequency inductance inversion scheme

angular frequency of the excitation current, and rcj, Acj, Δcj are the centroidal radius, centroidal magnetic vector potential and the area of the jth triangular element in the finite element mesh respectively. The FEFM was verified by comparing the results with the analytical model reported by Dodd and Deeds (1968). The self-inductance (L0) of a coil, whose height, outer radius, inner radius and number of turns are 6.35 mm, 9.525 mm, 3.175 mm, and 200 respectively, was calculated using both techniques. Analytical solution yields a self-inductance of 3.217x10-4 Henries while the FEFM solution gives a self-inductance of 3.216 x 10-4 Henries, a net error less than 0.1 %. Based on these results, the confidence in the FEFM for axisymmetric cases was established.

substrate can be used to find the unknown conductivity by following a procedure similar to the first solution step of the inversion process. In the nth step, an n+1-layered model is used. The inversion method generates the depthconductivity profile within ‘n’ steps. This technique can be applied for measurements peened samples on which the stresses are inherently axi-symmetric (Sundararaghavan et al. (2005)).

Once the measured “apparent” conductivities are converted to inductance values of a simulated coil, a multi-frequency inductance inversion model (Sundararaghavan and Balasubramaniam, 2004; Sundararaghavan, et al., 2005) is used to obtain the conductivity profiles. The conductivity profile is assumed to be discontinuous, piecewise constant and each constant conductivity layer is modeled by several rows of triangular elements. The multi-frequency inductance data inversion scheme is depicted in Figure 3. During the inductance inversion process, frequencies are first sorted in the descending order. The highest frequency, corresponding to the least depth of penetration according to the optimal skin depth approximation, is used in the first solution step of the inverse model. Since the substrate conductivity is known, a two-layer model (optimal skin depth at the highest frequency and the substrate) can be used to separate the conductivity of the topmost layer. During this step, the finite element forward model assigns a range of conductivity values to the topmost layer and calculates the inductance of the coil. The actual inductance value is then matched to a particular value of conductivity by rational interpolation of the conductivity-inductance data. In the subsequent step of the inversion scheme, a lower coil excitation frequency is used as the input to the inverse model. In this case, the depth of penetration is higher than that of the first frequency input, and the eddy current penetrates the top layer whose conductivity was already found during the first step. A three-layer model with this top layer, a second layer of unknown conductivity and the Vol. 10, Issue 2 September 2011

Since the inverse model uses the apparent conductivity data directly, only test specimen related parameters like base conductivity and material thickness need to be provided to the inverse model. The measurement is performed at each frequency at two levels as discussed before, (1) Experimental measurement of “apparent” conductivity using a conductivity meter and (2) a computer simulated measurement of inductance of a simulated coil over the material using the set up shown in Figure. 2. In the simulated measurement, the virtual test specimen is given a uniform conductivity corresponding to the effective conductivity of the specimen as measured by the conductivity meter in step (1). The second step employs a simulated air-core coil whose parameters are given in Table. 1. This is an advantage over the multi frequency inductance inversion scheme where customized coils were fabricated and the coil parameters and lift off have to be measured accurately and fed into the inverse model (Sundararaghavan et al., 2005). Since the proposed model works with apparent conductivities as an input, the inductance measurement is performed on computer using the Finite element model.

3. MODEL VALIDATION The forward model is used to generate the “apparent” conductivity inputs for the inversion model given any conductivity profile. The methodology for the forward problem is shown in Figure 4. Piecewise constant conductivity profiles were simulated on the material with a conductivity of 28 MS/m and a relative permeability of 1 assigned to the substrate for all simulations. The FEFM is used to measure the inductance of a coil placed over the material with the known conductivity profile. The geometrical parameters of the simulated coil used for the simulations are specified in Table-1. A conductivityJournal of Non destructive Testing & Evaluation

Technical Paper

Table – 1 : Properties of the simulated pancake coil Property

Fig. 4 : Forward Problem: Obtaining “apparent” conductivity from the conductivity profile

inductance calibration curve is used to calculate the “apparent” conductivities corresponding to the measured inductances at various frequencies. The calibration curve for any particular measurement configuration can be generated using the finite element forward model. Two different piecewise continuous conductivity profiles were simulated on the material and are plotted in Figure 5. The “apparent” conductivities at 6 different frequencies for each of these profiles are shown alongside the simulated profiles. The “apparent” conductivity-frequency data was inverted using the proposed algorithm using an optimum skin depth of 2d.

Value

Number of Turns

240

Outer radius (ro mm)

4.3

Inner radius (ri mm)

2.5

Lift Off (lo mm)

Thickness (h mm)

A comparison of the actual conductivity profile and the “apparent” conductivity measurements in Figure 5 reveals that the trends in “apparent” conductivity are not indicative of the actual conductivity gradients in the material. For example in Figure 5(a), the actual conductivity gradient is monotonically increasing whereas the “apparent” conductivity plot in Figure 5(b) does not show any such trend. It must also be noted here that for a monotonic change in actual conductivity of about 25%, the corresponding change in the “apparent” conductivity is of the order of only about 1%. Also, the “apparent” conductivity measurements are significantly influenced by the top most layer conductivity. Similarly, for case in Figure 5b, a 10% change in actual conductivity brings about changes in the “apparent” conductivity of the order of only 1%. This FEFM result clearly shows the reason for the lack of sensitivity of the “apparent” conductivity measurement to conductivity depth profiling during

Fig. 5 : Simulated conductivity profiles and “apparent” conductivity measurements at 6 different frequencies Journal of Non destructive Testing & Evaluation

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measurements of material properties such as stress as reported elsewhere (Lavrentyev et al., 2000; Blodgett et. al. (2003)). The “apparent” conductivity data was then used an input into the inversion model and the reconstructed actual conductivity profiles are compared with the input actual conductivity profiles in Figure 6. The “apparent” conductivity inversion methodology was implemented using C++ solvers for the eddy current forward FEM problem and a LabVIEW graphical user interface. The method is computationally efficient and consumes 0.1 seconds for each “apparent” conductivity measurement on a 2.2 GHz Pentium IV PC. Hence, the methodology has scope for applicability in online monitoring where the profiles are expected to be axisymmetric, in processes such as peening and heat treatment. An analysis of Figure 6 indicates that the correlation between the actual conductivity and the reconstructed conductivity is excellent near the surface, but the reconstruction error progressively increases with depth. The increase in reconstruction error with depth can be attributed to the skin depth approximation used in the proposed incremental layer approach. The skin depth (d)

in the proposed model was calculated based on the substrate conductivity since the actual conductivities are not known prior to the reconstruction. Future work might involve optimization of the skin depths based on an iterative framework using an initial guess based on the substrate conductivity for the reconstruction. It must also be noted that substrate conductivity is not easily obtained. These may be coarsely approximated by the “apparent” conductivity measured over surface unaltered specimens at low frequencies or by using the intrinsic volumetric conductivity of the material.

4. SENSITIVITY OF “APPARENT” CONDUCTIVITY BASED INVERSION It is difficult to quantify the error in the multi-frequency inductance inversion scheme since the procedure is dependant on accurate measurement of multiple coil geometry parameters. There is a need for an inversion procedure that works regardless of the measurement sensor design. In the “apparent” conductivity inversion technique, the only eddy current measurement input is the “apparent” conductivity. These values can be measured using several available commercial conductivity meters within an accuracy of +0.5% IACS by calibrating against reference

Fig. 6 : Simulated conductivity profile and the profile inverted from the “apparent” conductivity data

Fig. 7 : (a) RMS error in the reconstructed profile due to error in apparent conductivity input (b) Change in correlation of the reconstructed profile with the actual profile due to error in apparent conductivity input Vol. 10, Issue 2 September 2011

Journal of Non destructive Testing & Evaluation

62 standards at working frequencies. The success of any inversion scheme depends on its stability within this range of uncertainty. In order to test the robustness of the proposed inversion method, linear conductivity profiles were reconstructed using “apparent” conductivities with simulated measurement errors (“noise”) as input. Figure 7(a) shows the typical change in RMS error in the conductivity profile reconstructed using the proposed inversion scheme when different amounts of “noise” are added to the “apparent” conductivity inputs. Figure 7(b) shows the change in correlation of the reconstructed profile with the actual conductivity profile due to error in the “apparent” conductivity input. The “apparent” conductivity inversion scheme gives a RMS error of +0.09 MS/m or +0.3% of the substrate conductivity (s0) for an error in “apparent” conductivity measurement of +0.5% IACS. The quality of correlation of the reconstructed profile with the actual profile is excellent even for errors as large as 10% IACS. Hence, the proposed “apparent” conductivity inversion scheme is robust enough to accommodate for reasonable measurement uncertainties.

5. INVERSION FOR SHOT PEENED SAMPLES Fatigue cracks typically initiate from the surface since the operating stresses are often maximum at the surface. One of the most popular methods to prevent crack initiation is to induce compressive residual stresses at the surface by means of peening. In the process of shot peening, a highvelocity stream of beads are used to plastically deform the surface of a specimen. Within the plastically deformed layer, compressive residual stresses are locked in and are balanced by tensile residual stresses in the unaffected base metal. Over a uniformly peened base metal, these residual stresses are known to be axisymmetric in nature allowing analysis of “apparent” conductivity data using the proposed model. The electrical conductivity of the cold worked layer is lower than the conductivity of the underlying base metal. Eddy current apparent conductivity measurements over a range of frequencies in most peened materials usually display a trend of decreasing “apparent” conductivity with increase in frequency (with the exception of certain alloys with piezoresistive properties). At high frequencies the depth of penetration is low and the measurement is dominated by the effect of the cold worked layer that effectively decreases the apparent conductivity. Due to large depth of penetration at lower frequencies, the effect of the cold worked layer near the surface is diminished and measured apparent conductivity approaches the substrate conductivity. On the other hand, it is well known that compressive stresses result in an increase in conductivity. The effect of cold work is predominant close to the surface and the maximum compressive stresses (thus, maximum increase in conductivities) are obtained at a certain depth Journal of Non destructive Testing & Evaluation

Technical Paper

below the surface. The effect of compressive stresses can be, in principle, observed by taking “apparent” conductivity measurements at multiple frequencies that would selectively reach required depths in the sample. However, the intensity of eddy currents is largely affected due to the layers of decreased conductivity close to the surface than the sub surface compressive stresses due to the skin effect. “Apparent” conductivity measurement averages out the conductivities over the depth of penetration and hence, does not reflect the true underlying conductivity profile. Further, effect of shot peening on measured “apparent” conductivity is quite modest at about 1 to 2%, making it difficult to quantify the change due to compressive stresses. However, it would be interesting to observe the trends in the actual conductivity profiles since the effect of cold work and compressive stresses on the conductivity at every depth inside the specimen can be quantitatively ascertained. In our previous study on water jet peened samples, the effect of cold work was found to be low and true conductivities provided trends consistent with the effect of residual stresses (Sundararaghavan and Balasubramaniam, 2004; Sundararaghavan et al., 2005). It is possible that similar trends might be observed in the profiles obtained from shot peening although we expect cold work to be far larger under solid impact. As discussed before (see Fig. 5), small changes in measured “apparent” conductivity can result from large changes in true conductivity over small depths, which can possibly provide sufficient resolution to capture the effect of compressive stresses. In addition to cold work and residual stresses, surface roughness effect (Blodgett et al., (2003)) and texture also causes a distortion to “apparent” conductivity measurements. The effect of crystallographic anisotropy (texture) on conductivity is assumed to be negligible and is not considered in the present study. Grid measurement methods for independent conductivity and lift-off measurements have been reported (Washabaugh et al., 2000) that makes the measurements insensitive to surface roughness effect. Figure 8 shows one such result of multiple frequency apparent conductivity measurement (Washabaugh et al., 2000) with grid measurement methods for Al 2024 samples shot peened to Almen intensities of 0.005, 0.012, and 0.017, Scale A. In these “apparent” conductivity measurements, the unpeened sample conductivity was essentially constant with frequency which validates the quality of the reference specimen and provides the value of conductivity of the unaffected substrate (17.407 MS/m). The data shows a clear trend of decreasing conductivity with frequency, displaying the predominant effect of cold working. True conductivity profiles up to a depth of 0.384 mm were obtained from this data set by inverting the “apparent” conductivity measurements at four frequencies of 2 MHz, 1 MHz, 0.6 MHz and 0.4 MHz. These frequencies correspond to optimal depths of 0.168 mm, 0.24 mm, 0.312 mm and 0.384 mm. The results are shown in Fig. 9. Since the difference in depths between successive layers is less than 0.1 mm, the assumption of constant Vol. 10, Issue 2 September 2011

Technical Paper

Fig. 8 : Multiple frequency measurement results for Al 2024 alloy shot peened to Almen intensities of 0.005, 0.012, and 0.017, Scale A as reported in Washabaugh et al. (2000). Dotted lines represent the apparent conductivity profiles reconstructed using specimens with conductivity profiles shown in Fig. 9.

Fig. 9 : Depth profiles of conductivity reconstructed from apparent conductivity data in Fig. 8. using the proposed inversion algorithm

conductivity layers is expected to provide a good approximation. However, since “apparent” conductivity data was not provided above 2 MHz, the true profile up to a depth of 0.168 mm was not known and was approximated by a single layer of constant conductivity.

allow accurate characterization of conductivity profiles and possible calibration of residual stresses or the depth of maximum compressive stresses in peened specimens. Although the measurement beyond 0.312 mm carries significant reconstruction error for specimens with higher peening intensities, the trend of a drop in conductivity at depths between 0.312 and 0.384 mm is consistent for all the peened samples. This might be due to the effects of both tensile stresses in the substrate as well as the plastic strains that might exist at these depths. It is interesting to note that significant changes in the true conductivity profiles are obtained even though percentage change in the apparent conductivities at these frequencies are small, which might provide the resolution required to capture the effect of residual stresses on conductivity.

To validate the inverted conductivity profiles, apparent conductivities at 2-0.4 MHz were obtained from profiles in Fig. 9 using FEFM and are shown as dotted lines over the experimental data in Fig. 8. The results show that the reconstruction accurately represents the trend in apparent conductivity up to a frequency of 0.6 MHz (0.312 mm optimal depth). The reconstruction error becomes significant over larger depths (lower frequencies) due to the optimal skin depth approximation using the assumed substrate conductivity. For example, in the case of the high peening intensity sample (0.017 A), dotted lines in Fig. 8 show a large deviation at 0.5 MHz since the optimal depths are expected to be higher than those used in the inversion. This is because the effective conductivity of the substrate is lower than the assumed substrate conductivity due to a larger cold worked layer. On the other hand, the assumed substrate conductivity provides a good approximation for the 0.005A sample that has a smaller cold worked layer. In all the peened samples, a large drop in conductivity is observed close to the surface up to about 0.24 mm and the drop in conductivity closely relates to the peening intensity, the largest drop occurring for the sample with highest peening intensity. This effect can be attributed to the dominant effect of cold working over the near surface layers. The trend reverses for the 0.012A and 0.017A samples, wherein a definite increase in conductivity is observed from a depth of 0.24 mm, however the conductivity is still below substrate conductivity. The effect of cold work that decreases the conductivity is possibly offset due to the opposing effect of compressive stresses causing this increase. This effect increases with increasing peening intensities as seen from the inverted profiles. Further experimental study of this effect (including measurements at significantly higher frequencies) would Vol. 10, Issue 2 September 2011

6. CONCLUSION Motivation of this work was a study of the use of “apparent” conductivity as a basis for characterizing surface-treated specimens from processes like peening, heat treatment, cladding, coating etc. Simulations show that the “apparent” conductivity does not follow the trends followed by the actual conductivity profiles. Also, for large changes in the actual conductivity with depth, the “apparent” conductivity measurements show relatively poor sensitivities. Hence, there is a need to further process the measured “apparent” conductivity in order to obtain the information-rich conductivity gradients. A new “apparent” conductivity inversion methodology has been presented in this paper as an extension to the multi-frequency inductance inversion methodology proposed by the same authors (Sundararaghavan and Balasubramaniam, 2004). The method is specific to planar layered geometries, is computationally efficient and is applicable for online testing. A study of sensitivity of the technique indicates that the scheme is robust over a range of measurement uncertainties. An advantage that this methodology enjoys is that it does not require extensive probe calibration or design and can be used along with any commercial sensor or conductivity meters that can work over a range of Journal of Non destructive Testing & Evaluation

64 frequencies. Inversion of “apparent” conductivity measurements of shot peened samples show large changes in conductivities near the surface even though the changes in “apparent” conductivities are quite modest. A decrease in conductivity due to cold working is possibly offset due to the effect of compressive stresses at particular depths and the trend is stronger with increasing peening intensities. This interesting observation might allow possible characterization of residual stresses from peening. Dependence on substrate conductivity for calculating the optimal depth of penetration of eddy currents leads to erroneous calculation of optimal depths in applications where large changes in conductivities are observed over the depth of the specimen. Hence, the reconstruction error progressively increases with decreasing frequencies used in reconstruction. Future work in this area involves improvement of the inverse model by using an iterative framework to eliminate the need for using optimal skin depth approximation based on substrate conductivity. This would allow accurate inversion over a larger range of frequencies. Further experimental work needs to be carried out to test the applicability of the technique for monitoring residual stresses over peened specimens.

REFERENCES 1. American Society for Nondestructive Testing, “Electromagnetic Testing” Nondestructive Testing Handbook, Vol. 5, Udpa, S.S. and P.O. Moore, eds., Columbus, Ohio, ASNT, 2004. 2. ASTM International, ASTM E 1004-99, Standard Practice for Determining Electrical Conductivity Using the Electromagnetic (Eddy-Current) Method, West Conshohocken, Pennsylvania, ASTM International, 1999. 3. Blodgett, M.P., C.V. Ukpabi, and P.B. Nagy, Surface Roughness Influence on Eddy Current Electrical Conductivity Measurements, Materials Evaluation, Vol. 61, 2003, pp.766-772. 4. Bowler, J.R. and S.J. Norton, “Theory of eddy current inversion,” Journal of Applied Physics, Vol. 73, No. 2, 1993, pp. 501-512. 5. Dodd, C.V. and W.E. Deeds, “Analytical solutions to eddy-current probe-coil problems” Journal of Applied Physics, Vol. 39, No. 6, 1968, pp. 2829-2838. 6. Glorieux, C., J. Moulder, J. Basart and J. Thoen, “The determination of electrical conductivity profiles using neural network inversion of multi-frequency eddycurrent data,” Journal of Physics D: Applied Physics, Vol.. 32, 1999, pp. 616-622. 7. Katragadda, G., J. Wallace, J. Lee, and S. Nair, “Neural network inversion for thickness measurements and conductivity profiling,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 16A, D.O. Thompson and D.E. Chimenti, eds., Melville, AIP, July 1997, pp. 781-788. Journal of Non destructive Testing & Evaluation

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8. Lavrentyev, A.I., P.A. Stuky and W.A. Veronesi, “Feasibility of Ultrasonic and Eddy Current Methods for Measurement of Residual Stress in Shot Peened Metals,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 19B, D.O. Thompson and D.E. Chimenti, eds., Melville, AIP, July 2000, pp. 1621-1628. 9. Liu, G., Y. Li, Y. Sun, P. Sacks, and S. Udpa, “An iterative algorithm for eddy current inversion,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 19A, D.O. Thompson and D.E. Chimenti, eds., Melville, AIP, July 2000, pp. 497-504. 10. Liu G.R. and X. Han, Computational Inverse Techniques in Nondestructive Evaluation, Boca Raton, CRC Press 2003. 11. Moulder, J.C., E. Uzal and J. H. Rose, “Thickness and conductivity of metallic layers from eddy current measurements,” Review of Scientific Instrument., Vol. 63, No. 6, 1992, pp. 3455-3465. 12. Palanisamy, R. “Finite element modeling of eddy current non-destructive testing phenomena,” PhD. Thesis, 1980, Colorado State University, Fort Collins, U.S.A. 13. Rekanos, I.T., T.P. Theodoulidis, S.M. Panas, T.D. Tsiboukis, “Impedance inversion in eddy current testing of layered planar structures via neural networks,” NDT&E International, Vol. 30, No. 2, 1997, pp.69-74. 14. Sun, H., J.R. Bowler, N. Bowler and M.J. Johnson, “Eddy current measurements on case hardened steel,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 21B, D.O. Thompson and D.E. Chimenti, eds., Melville, AIP, 2002, pp. 1561-1568. 15. Sundararaghavan, V., K. Balasubramaniam and N.R. Babu, “A multi-frequency eddy current inversion method for characterizing water jet peened aluminum alloys,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 23A, D.O. Thompson and D.E. Chimenti, eds., Melville, AIP, 2004, pp. 651658. 16. Sundararaghavan, V., K. Balasubramaniam, N.R. Babu and N. Rajesh, “A Multi-Frequency Eddy Current Inversion Method for Characterizing Conductivity Gradients on Water Jet Peened Components,” to appear in Int. J. of NDT&E, 2005. 17. Uzal, E., J.C. Moulder, S. Mitra and J.H. Rose, “Impedance of coils over layered metals with continuously variable conductivity and permeability: Theory and experiment”, Journal of Applied Physics, Vol. 74, No. 3, 1993, pp. 2076-2089. 18. Washabaugh, A., V. Zilberstein, D. Schlicker and N. Goldfine, “Absolute Electrical Property Measurements Using Conformable MWM Eddy-Current Sensors for Quantitative Materials Characterization,” In: Proceedings of ROMA 2000 - 15th World Conference on Non Destructive Testing (WCNDT), Roma, Italy. AIPnD, 2000 (http://www.ndt.net/article/wcndt00/). Vol. 10, Issue 2 September 2011

Ultrasonic Non-Destructive Evaluation (NDE) based internal inspection of pressure vessels for better maintenance practice S.K.Nath1 and B.H.Narayana1 1

Central Power Research Institute, Thermal Research Centre, Nagpur-441 111, Maharastra, India E-mail address: sknath2000@yahoo.com

ABSTRACT This paper discusses about the possibility of detection of entrapped foreign object in a pressure vessel by ultrasonic inspection technique. The inspection plan is designed and illustrated here. Successful in-situ implementation of this technique will help in achieving better maintenance practice for the plant components.

INTRODUCTION Availability of any subsystem in a thermal power plant plays a very important role in uninterrupted generation of electricity. The plant consists of number of units based on the total generating capacity. The typical capacity of one such unit may be from 110 MW to 660 MW. The steam generator of a unit of a power plant namely the boiler is one such important subsystem. A typical boiler consists of various pressure vessels e.g. headers, drums meant for containing the working fluids namely water and steam of varying temperature and pressure in the circuit. Frequent forced outages reduce its availability. One of the major causes of forced outages of the boiler is the entrapment of foreign objects inside such closed pressure vessels i.e. headers. The objects could be anything like welding rods, iron files, insulation materials, wooden pieces etc. These unwanted foreign objects restrict the flow of the working fluid causing starvation in the tubes which eventually leads to leakage. Detail failure investigation of such tubes confirms the presence of foreign objects inside the headers. Thus detection and retrieval of the foreign objects in the vessel is of prime importance to reduce the forced outages of the boiler causing huge generation loss. Generally fibroscopic inspection is carried out for detecting the entrapped foreign material inside the headers. Inspection nipples or stub joints are cut for creating the opening for inserting the fibre-optic probe inside the otherwise inaccessible location of the headers. The inspection will detect the foreign material if it is physically present inside; otherwise no such detection will be made. However, irrespective of the detection or not, the openings created for this inspection need to be closed before restart of the plant. This requires re-welding of the cut portion, stress relieving based on thickness and radiography to check the weld quality. In case of absence of any foreign material, these are additional work serving no meaningful purposes and the same can be avoided if the information that â&#x20AC;&#x153;no foreign material is inside the headerâ&#x20AC;? is known by some

other means. Thus a priori information regarding the presence or absence of the foreign material will help in optimizing the cutting, welding, radiography work. An ultrasonic inspection plan is developed in the present investigation which will provide a priori information regarding the presence or absence of the foreign material inside the headers. The inspection will help in better maintenance planning with respect to optimum utilization of time and resource of plants and industries. Inspection plan

A schematic diagram of the inspection plan is illustrated in Figure 1 below.

Fig. 1 : Schematic diagram of longitudinal inspection plan

The pressure vessel containing an unwanted foreign object is partially filled with stagnant water. The object could either be in suspension in water or resting in the bottom portion of the vessel depending on its specific gravity. Two ultrasonic probes; one transmitter and another receiver are placed on the surface of the vessel in a pitch-catch arrangement and scanning is performed along the longitudinal direction as in ultrasonic Time of Flight Diffraction (TOFD) inspection. However, in TOFD, the diffracted beam from the flaw is considered for its detection and sizing. Here in this case we are using the

Technical Paper

reflected beam from the reflector (i.e. the foreign object). The space (S) between the two probes is optimized based on the metal thickness of the vessel and the water level inside it. Moreover, the space is determined in such a way that the central axes of both transmitter and receiver probes meet at the water-air interface. Central axes of the beams are of maximum energy levels [1]. Thus the same are considered with an objective of maximizing transmitted and received energy. Longitudinal wave probes are used because of the obvious reason of having liquid medium wherein shear mode of ultrasound cannot propagate. Additional advantage of longitudinal wave is its more energy content as compared to shear wave [2]. Details of the ultrasonic wave propagation, its interaction with different interfaces having varying media are illustrated in Figure 2 (a and b). Probe centre spacing (S), transmitted beam path (B) based on the simple trigonometric relationship are determined by solving Eq. (1)-(5). The metal thickness‘t’ and the water level ‘h’ used in the equations are determined separately by conventional normal beam pulse echo method. (1)

S1 = t tanθ; S2 = h tanϕ

(2)

S = 2 * (S1 + S2)

(3)

B1 = t Secθ; B2 = h Secϕ

(4)

B = 2 * (B1 + B2)

(5)

θ and ϕ = Angle of incidence and transmission respectively V1 and V2 = Ultrasonic velocity in metal and water respectively t and h = Metal thickness and water level respectively S = Spacing between transmitter and receiver probes B = Total beam path between transmitter and receiver probes The probe pair is scanned on the outer bottom surface along a line marked by a vertical plane through the centre line of the vessel. During scanning part of the incident ultrasound beam will travel from the transmitter to receiver probe as creeping/surface wave [3]. As long as there is no obstruction in the path as illustrated by position ‘A’ and ‘B’ in Figure 1, rest of the incident beam will propagate from metal to water, reflect from the water-air interface, the reflected beam will subsequently get transmitted back into the metal and finally received by the receiver probe. The various signals generated during scanning are schematically illustrated in Figure 2 (b). In the above Figure 2 (b) there are two clearly visible signals, one is the lateral wave (LWE) or creeping wave and the other is the reflected (from water-air interface) and subsequently transmitted (at water-metal interface) signal (BWE). As long as there is no obstruction in the

Fig. 2 : Schematic illustration of (a) ultrasonic wave propagation in varying media (b) signals in case of absence of foreign object (c) signal in case of presence of foreign object Journal of Non destructive Testing & Evaluation

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path of the interrogating beam, both the signals will appear. The moment the beam encounters an obstruction e.g. foreign object as illustrated by position ‘C’ in Figure 1; there will be unpredictable and irregular reflection, dispersion of the beam and in all probability no beam will reach the receiver. Thus in such a case, though there will be the 1st signal i.e. lateral wave (LWE) as always, no 2nd signal i.e. the reflected wave (BWE) will appear. Figure 2 (c) illustrates this case. During inspection the operator should look for the presence of both the signals (LWE and BWE) which confirms the absence of any foreign object. However, absence of the 2nd signal i.e. BWE prima facie confirms the presence of the foreign object.

DISCUSSION The present study shows the possibility of ultrasonically detect the entrapped foreign objects inside a pressure vessel. The inspection plan designed here is for partially filled vessel. For completely filled vessel, the inspection plan will be more or less same with minor variation in probe centre spacing (S) and beam path (B) calculation. The metal wall of the vessel is assumed to be defect free; otherwise any such defect here also will cause the absence of the 2nd signal leading to the misinterpretation of presence of foreign object inside the vessel. Thus before actual inspection the metal wall should be separately examined to pre-empt any such possibilities. There could be various other signals due to multiple reflections within the metal wall or the water. In actual laboratory experiment or in-situ application these signals should be clearly identified and filtered out to avoid any false interpretation. The inspection plan under discussion is applicable for a partially water filled pressure vessel. Requirement of inspecting dry vessels also may arise. In such case, different inspection plan probably deploying the technique of air-borne ultrasound can be developed. However, this is not in the purview of the present investigation. As per the scanning plan so far discussed, the probe pair with a fixed spacing between them is moved longitudinally on the outer bottom surface along a line marked by a vertical plane through the centre line of the vessel. Thus the foreign object, if any lying in the inspection region along the length of the vessel will be detected. However, the possibility of the object lying on either side of this inspection region is not ruled out and in such cases, the same is likely to be missed. A circumferential scan as illustrated in Figure 3 can overcome this situation. Inspection with different spacing between the two probes can be performed to scan the entire volume of water body. The inspection plan developed here may be validated by a laboratory experiment. Once validated experimentally the Vol. 10, Issue 2 September 2011

Fig. 3 : Schematic illustration of circumferential inspection plan

same can be successfully implemented during site inspection. The main perceivable advantage is that this inspection will confirm the presence or absence of any foreign object inside a vessel. In case of confirmation regarding the presence of an object by this inspection, the inspection nipple or the tube stub joint can be cut open for inserting the fibreoptic probe for further verification and retrievable purpose and subsequently the cut portion rewelded, stress relieved and radio-graphed. It is reiterated here that in case of confirming the absence of a foreign object by the present ultrasonic inspection, the entire cutting/re-welding/stress-relieving/radiography works can be judiciously avoided evolving a better maintenance practice.

ACKNOWLEDGEMENT The authors are thankful to the management of CPRI for constant encouragement, motivation and support for research and developmental activities.

REFERENCES 1. Charlesworth JP, Temple JAG. Engineering Applications of Ultrasonic Time of Flight Diffraction. 2nd. Ed., 2001, Research Studies Press Ltd., England. 2. Ogilvy JA, Temple JAG. Diffraction of Elastic Waves by cracks: Application to Time of Flight Inspection. Ultrasonics 1983; 7: 259-269. 3. Nath S.K., Balasubramaniam K., Krishnamurthy C.V. and Narayana B.H., ‘Sizing of surface-breaking cracks in complex geometry components by ultrasonic Timeof-flight Diffraction (TOFD) technique’, Insight, 49(4), 200-206, (2007). Journal of Non destructive Testing & Evaluation

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PROBE

This article commences with a slight change in the list of SEVEN enumerated in the earlier issue. The new list is 1. Prioritization, 2. Perception, 3. Possibility Thinking, 4. Process, 5. Procrastination – A strict NO NO, 6. Pondering over the success, 7. Patience and Perseverance. This article will deal with Prioritize. Prioritization sets the direction of life. For many of us success in life is equated to possession of material things. The more and expensive the gadget the better it is. It is construed that The more the possession the more is happiness and hence success. We assume that Success & Happiness are directly proportional to the amount of material possession. But in reality it is not so. The more the possession the more is the fear and worry. Since we get used to a particular kind of possession, the absence causes more pain (Air conditioning). Acquiring material possession provides a momentary “KICK” and is accompanied by pain. Remember “ NO pain No gain”. Once you have acquired the material object, the mind is un satiated. It craves for more and more. There is an un satiated longing or in other words “GREED” sets in. We must be able to differentiate between “Need and Greed”. Life shall be need based rather than greed based like inspection. For example take an obese person or an alcoholic. It all starts with an extra morsel of food or a peg of alcohol. A smoker is converted into a chain smoker because the mind that is possessed by the greed converts the smoking into a need for the body. Let us realize that mind controls the body and not the other way. Mind is superior to body. Materials possession is an acceptance criteria of success set by the mind. Be it known that we are neither the mind nor the body but the inner consciousness the primordial energy (the life) is supreme. This consciousness may be called by any name. After all what is there in a name. A rose by any name will always be a rose. This energy is God particle (Boson). This God particle is all pervading and we should be guided by it as it is the core of a being and the being is built around it. Now the question is how do we do it? By sitting still, keeping both the body and mind still this can be achieved. That is action in inaction. This may sound contradictory and prove to be difficult in the beginning but practice maketh a man perfect. By being still you will dig deep into yourself and start listening to your inner self. Once you start listening to your inner voice the inner voice will guide you. Basically it is based on the principle of “What feeling you get when you rewind your innings of life at the autumn of your life? Is it of satisfaction or is it of regret! As Stephen Covey the author 7 Principles of great men says 4 Ls - Live, Love, Learn and Leave a Legacy shall govern your life. If you consider each L as a circle then the more the 4 circles merge with each other then more will be the satisfaction in life. Live- The physical Needs, Love- The emotional Need (acceptability by the society), Learn – The need of the intelligence and Leave a legacy- the need of the self. Try this, you have nothing to loose , except probably anger, jealousy, lust, sorrow, worries and all other negativities including ego (the ones that causes secretion of unwanted chemicals in your body) but a lot to gain including tranquility, peace, happiness, satisfaction and may be self actualization. Recently we witnessed the uprising of masses against corruption lead by a simple person which probably can be equaled to the one lead by Mahatma Gandhi.. The motto shall be to learn to prioritize. Journal of Non destructive Testing & Evaluation

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