Journal of Non Destructive Testing and Evaluation by LifeCell International

Volume 11 issue 2 September 2012

from the Chief Editor

This years annual get-together of ISNT, the NDE2012, to be held in Ghaziabad (Delhi), will bring forth the complete flavor of the ISNT community. The Journal of Nondestructive Testing and Evaluation team welcomes all delegates to this gala technical festival. The NEW Editorial Board joins me in congratulating all the ISNT Award Winners for the 2012. I would like to recognize the contributions of the out-going members of the Editorial Board; Drs. D.K. Bhattacharya, P.Kalyanasundaram, T. Jayakumar, K. Viswanathan, K. Rajagopal, MVMS Rao, J. Lahiri, KRY Simha, Shri. Vaidyanathan, Shri Ramesh Parikh, Shri. Srinivasa Rao, Shri. G. Ramachandran. The Journal also welcomes the new members of the Editorial Board; Dr. CV Krishnamurthy, Dr. O. Prabhakar, Dr. MT Shyamsunder, Dr. B. Venkataraman, Dr. H. Wolf, Dr. K. Srinivas and Shri. P. Nanekar. In addition, several new international experts in the fields of NDT have agreed to serve in the board. It is envisaged that the newly constituted board will further enhance the quality and readership of the Journal. In addition, efforts are underway to include the Journal in the different citations indices. In this edition, the BASICS section is focused on Theoretical Modeling Methods and how it applies to NDT. Today, the theoretical models have taken the form of VIRTUAL NDT that the new generations of NDT Engineers feel comfortable and may immensely benefit. The HORIZONS describes the use of a hybrid method of using high power ultrasound to vibrate and consequently heat the crack surfaces inside materials and observe the indications using infra-red camera in a transient mode. The technical articles in this issue cover a wide range of topics in NDT including adhesive bond inspection using ultrasonics, solutions to the eddy current benchmark problems as posed by the World Federation of NDE Centers, composite inspection using guided ultrasonic wave modes, and weld inspection for rails using phased array technology. I would like to touch base on one of the topics of interest to all of us i.e. from who will we buy our NDT instruments and products? Unlike the past, few India entrepreneurs are coming forefront with products that can compete with the international brands viz. Technofour, Modsonic, PulseEcho, Electro-Magfield Controls, Dhvani Research, EEC, etc. These companies face severe adversities such as limited market for their products in India, competition from international brand that have high marketing budgets, lack of echo systems to foster innovation and invention, a Government policy that is suspicious of small businesses, etc. However, it may be the right time to ask ourselves a few questions. z

Are Indian products getting due recognition from the NDT users in India?

Is there a case to be made for creating an eco-system for encouraging entities that develop indigenous products?

Should ISNT an ISNT members take a lead in incubating new companies that can provide quality Indian made products?

Should ISNT lobby the Government and the stakeholders to provide preferential treatment of Indian made products?

What is the right mix between Indian made products vs. imported ones?

May be the time to ponder is NOW!

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

I S N T - National Governing Council Chapter - Chairman & Secretary President Shri P. Kalyanasundaram President Elect Shri V. Pari Vice-Presidents Shri D.J. Varde Shri Swapan Chakraborty Shri N.V. Wagle Hon. General Secretary Shri R.J.Pardikar Hon.Jt.Secretaries Shri Rajul.R.Parikh Dr.B.Venkatraman Hon. Treasurer Shri S.Subramanian Hon. Co.Treasurer Shri Sai Suryanarayana Immediate Past President Shri K.Thambithurai Past President Shri Dilip.P.Takbhate 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 Prof. G.V.Prabhugaunkar Shri B.K.Pangare Shri M.V.Rajamani Shri Samir K. Choksi Shri B.K.Shah Shri S.V.Subba Rao Shri Sudipta Dasgupta Shri R.K.Singh Shri A.K.Singh(Kota) Shri C.Awasthi Shri Brig. P.Ganesham Shri Prabhat Kumar Shri V.Sathyan Shri P.Mohan Shri R.Sampath Ms. Hemal Thacker Shri A.K.Singhi Shri T.V.K.Kidao Shri B. Prahlad Dr. BPC Rao Dr.Sarmishtha Palit Sagar Permanent Invitees Shri V.A. Chandramouli Prof. S. Rajagopal Shri G. Ramachandran and All Past Presidents, All Chapter Chairmen / Secretaries Ex-officio Members Chairman NCB, Secretary NCB, Treasurer NCB, Controller of Examination NCB, President QUNEST, Secretary QUNEST, Treasurer QUNEST

vol 11 issue 2 September 2012

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 QAS, RAPS - 5 & 6, PO Anushakti Rawatbhata 323 303 abahl@npcil.co.in Shri S.K. Verma, Hon. Secretary, TQAS, RAPS - 5 & 6, PO Anushakti Rawatbhata 323303. surendrakverma@npcil.co.in

Bangalore

Shri.S.P.Srivastva, Chairman 303, Lok Centre, Marol Maroshi Road, Andheri (East), Mumbai 400 059 Email: spsrivastava63@rediffmail.com , offc@isnt.org ; isntmumbai@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

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

Chennai Shri R.Sundar, Chairman, First Floor, North Wing, PWD Office Complex, Chepauk, Chennai - 600 005 tnboilerdirector@gmail.com, eramsun@gmail.com Shri RG. Ganesan, Hon. Secretary, Chief Executive, BETZ Engineering & Technology 49, Vallalar Street, Adambakkam, Chennai - 600 088 betzzone@vsnl.net ; rg_ganesan@yahoo.com

Delhi Shri A.K Singhi, Chairman, MD, IRC Engg Services India Pvt. Ltd 612, Chiranjiv Tower 43, New Delhi 110019 ashok.ircengg@gmail.com Shri M.C. Giri, Hon.Secretary, Managing Partner, Duplex Nucleo Enterprise New Delhi 110028 munish.giri@yahoo.com

Mumbai

Nagpur 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

Pune Shri BK Pangare, Chairman Quality NDT Services, Plot BGA, 1/3 Bhosari, General Block, MIDC, Bhosari, Pune- 411 026 ndtserve@pn3.vsnl.net.in Shri BB Mate, Hon Secretary, Thermax Ltd., D-13, MIDC Ind. Area, RD Aga Road, Chinchwad, Pune- 411 019 bmate@thermaxindia.com

Hyderabad

Sriharikota

Shri N. Saibaba, Chairman, Chief Executive, Nuclear Fuel Complex, ECIL PO, Hyderabad-500062 cenfc@nfc.gov.in ; nsaibaba54@yahoo.com Shri M.N.V. Viswanath, Secretary, Dy. Manager, Quality Assurance-Fuels, CFFP Building,Nuclear Fuel Complex, ECIL PO, Hyderabad-500062 viswanathmnv@yahoo.com ; mnvv@nfc.gov.in

Shri V Ranganathan, Chairman, Chief General Manager , Solid Propellant Plant, SDSC – SHAR, Sriharikota – 524124 vranga@shar.gov.in Shri B Karthikeyan Hon. Secretary, ISNT Sriharikota Chapter Sci/Eng. NDT/SPROB, SDSC – SHAR, Sriharikota – 524124 karthikeyan.b@shar.gov.in

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

Tarapur

Kalpakkam

R.J. Pardikar AGM, (NDTL) BHEL Tiruchirapalli 620 014. rjp@bheltry.co.in Shri L. Marimuthu, Hon. Secretary, HA-95, Anna Nagar Tiruchirapalli 620 026. lmmuthu@bheltry.com

Shri PG Bhere, Chairman, AFFF, BARC, Tarapur-401 502. pgbehere1@rediffmail.com Shri Jamal Akhtar, Hon.Secretary, TAPS 1 & 2, NPCIL, Tarapur. jakhtar@npcil.co.in

Tiruchirapalli

Dr. B. Venkatraman, Chairman Associate Director, RSEG, & Head, QAD, IGCAR, Kalpakkam 603 102 bvenkat@igcar.gov.in Shri B. Dhananjaykumar, Hon.Secretary Reprocessing Group, IGCAR, Kalpakkam – 603 102 mdjkumar@igcar.gov.in

Vadodara

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 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, ISRO, Thiruvananthapuram 695022 s_annamala@vssc.gov.in Shri. Binu P. Thomas Hon. Secretary, Holography section, EXMD/SDEG, STR Entity, VSSC, Thiruvananthapuram 695 022 binu_thomas@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 11 issue 2 September 2012

Contents Chapter News

9 10

Basics - Modelling and simulation for Nondestructive Evaluation

Horizon - Sonic Infrared Imaging â&#x20AC;&#x201C; Emerging NDE Tool

NDE Events NDE Patents

26 Computed Tomography (CT) using X-rays has become very popular in many field including medical, science, and NDT. In conventional CT technique, the Xray or Gama-ray sources are used to penetrate the object under examination and the data collected by moving the sample is used to reconstruct the 3D image of the object. Recently, the use of emission tomography has been shown to have several advantages for some key applications. In emission tomography the radiation from within the body is used to reconstruct the 3D image of the object. In the cover page, the emission computed tomography (CT) reconstruction of bowl is shown indicating blockages in

NDE Puzzle

34 Technical Papers Quality Assessment of Composite Adhesively Bonded joints by Non-linear Ultrasonic Method

R.L. Vijayakumar,

M.R. Bhat and

CRL Murthy

Solution to the third eddy current benchmark problem of WFNDE centers

S. Thirunavukkarasu, B. Purna Chandra Rao, S. Shuaib Ahmed and T. Jayakumar

Characteristics of turning Lamb modes in composite sub-laminates

C. Ramadas, Krishnan Balasubramaniam, Avinash Hood and C.V. Krishnamurthy

Rail Weld Inspection using Phased Array Ultrasonics

Probe

Girish.N.Namboodiri, Krishnan Balasubramaniam, T.Balasubramanian, Jerry James and Sriharsha

passage.. (Courtesy: QAD section of Indira Gandhi Centre for Atomic Research, Kalpakkam)

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.

Co-Editor Dr. BPC Rao

Published by Shri RJ Pardikar, General Secretary on behalf of Indian Society for Non Destructive Testing (ISNT)

Managing Editor Sri V Pari

Modules 60 & 61, Readymade Garment Complex, Guindy, Chennai 600032 Phone: (044) 2250 0412 Email: isntheadoffice@gmail.com and Printed at VRK Printing House Chennai vrkonline@gmail.com

bpcrao@igcar.gov.in

e-mail: scaanray@vsnl.com

Editorial Board Dr N N Kishore, Dr. CV Krishnamurthy, Dr. O. Prabhakar, Dr. MT Shyamsunder, Dr. B. Venkataraman, Dr. H. Wolf, Dr. K. Srinivas, Shri. P. Nanekar Shri 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.

Classifieds Scaanray Metallurgical Services (An ISO 9001-2000 Certified Company)

NDE Service Provider Process and Power Industry, Engineering and Fabrication Industries, Concrete Structures, Nuclear Industries, Stress Relieving Call M. Nakkeeran, Chief Operations, Lab: C-12, Industrial Estate, Mogappair (West), Chennai 600037 Phone 044-2625 0651 Email: scaanray@vsnl.com ; www.scaanray.com

Electro-Magfield Controls & Services & LG Inspection Services We manafucture : Magnetic Crack Detectors, Demagnetizers, Magnetic Particles & Accessories, Dye Penetrant Systems etc Super Stockist & Distributors: M/s Spectonics Corporation, USA for their complete NDT range of productrs, Black Lights, Intensity Meters, etc. Plot 165, SIDCO Industrial Estate, (Kattur) Thirumullaivoil, Vellanur Village, Ambattur Taluk Chennai 600062 Phone 044-6515 4664 Email: emcs@vsnl.net

Madras Metallurgical Services (P) Ltd Metallurgists & Engineers

Metallography Strength of Materials Non Destructive Testing Foundry Lab

Serving Industries & Educational Institutes for the past 35 years

24, Lalithapuram street, Royapettah, Chennai 600014 Ph: 044-28133093 / 28133903 Email: mmspl@vsnl.com

Transatlantic Systems

Betz Engineering & Technology Zone An ISO 9001 : 2008 Company 49, Vellalar Street, near Mount Rail Station, Chennai 600088 Mobile 98401 75179, Phone 044 65364123 Email: betzzone@vsnl.net / rg_ganesan@yahoo.com

International Training Division 21, Dharakeswari Nagar, Tambaram to Velachery Main Road, Sembakkam, Chennai 600073 www.betzinternational.com / www.welding-certification.com

KIDAO Laboratories NABL Accredited Laboratory carrying out Ultrasonic test, MPL and DP tests, Coating Thickness and Roughness test. We also do Chemical and Mechnical tests

A-3, Mogappair Indl. Area (East) JJ Nagar, Chennai 600037 Phone 044-26564255, 26563370 Email: kidaolab@giasmd01.vsnl.net.in; kidaolabs@vsnl.net www.kidaolabs.com

Dhvani R&D Solutions Pvt. Ltd 01J, First Floor, IITM Research Park, Kanagam Road, Taramani, Chennai 600113 India Phone : +91 44 6646 9880

Support for NDT Services NDT Equipments, Chemicals and Accessories Call DN Shankar, Manager 14, Kanniah Street, Anna Colony, Saligramam, Chennai 600093 Phone 044-26250651 Email: scaanray@vsnl.com

• Inspection Solutions • Software Products • Training • Services & Consultancy

CUPS, TAPS, CRISP, TASS SIMUT, SIMDR Guided Waves, PAUT, TOFD Advanced NDE, Signal Processing C-scans, On-line Monitoring

E-mail: info@dhvani-research.com

Southern Inspection Services NDT Training in all the following eleven Methods

Shri. K. Ravindran, Level III RT, UT, MT, PT, VT, LT, ET, IR, AE, NR and VA

www.dhvani-research.com

No.2, 2nd Floor, Govindaraji Naicker Complex, Janaki Nagar, Arcot Road, Valasaravakkam, Chennai 600 087 Tamil Nadu, India Phone : +91 44-2486 4332, 2486 8785, 4264 7537 E-mail: sisins@gmail.com and sisins@hotmail.com Website: www.sisndt.com/www.ndtsis.com/ wwwpdmsis.com

Indian Society for Non - Destructive Testing (Regd. Society: S. No. 49 of 1981) Module No. 60 & 61, Garment Complex, SIDCO Industrial Estate, Guindy, Chennai 600 032 Tele : 044-22500412, 044-42038175 E Mail: isntheadoffice@gmail.com , ncbisnt@gmail.com

NATIONAL CERTIFICATION BOARD ANNOUNCEMENT

ISNT – Level III Certification Programme January – February, 2013 Pune, India Dr. M.T. Shyamsunder Controller of Examinations National Certification Board, ISNT All payments shall be made through the means of a crossed Demand Draft drawn favouring “NCB - ISNT” and payable at “Chennai”. Cheques will not be accepted.

The last date for receipt of application along with payment is 21st December, 2012.

ISNT ANNUAL AWARDS ISNT Invites nominations/ applications for the National NDT Awards from Indian Nationals for their significant contributions and excellence in the field of NDT and the Best Chapter award from all the chapters of ISNT. An announcement to this effect is already circulated to eligible members / chapters. The various categories and awards are listed in every issue of the JNDE. The nominations / applications are to be sent in the prescribed form which can be downloaded from www.isnt.org.in and 15 copies of the same are to be sent to the following address by th 10 September, 2012.

Course Director and contact person at Pune: Shri Bhausaheb K. Pangare Course Director C/o. M/s. Quality NDT Services Plot No BGA 1/1,2,3, Bhosari General Block, MIDC, Bhosari, Pune-411026 Ph.: 020-27121843/27119490/8600100700 bkp_qndt@yahoo.in / qndt2712@bsnl.in / bhausaheb@qualityndt.org

Shri Dilip P. Takbhate Chairman, Awards Committee, Indian Society for Non Destructive Testing Module No.60 & 61, Third Floor, Readymade Garment Complex, SIDCO Industrial Estate, Guindy, Chennai – 600 032 isntheadoffice@gmail.com

Other Activities:

CHAPTER NEWS

EC Meeting held on 9th July 2012 at ISNT, Mumbai Office. EC Meeting held on 10th August 2012 at ISNT, Mumbai Office.

Chennai

AGM- 2012 on 8th September 2012 at Hotel Chakra, Sakinaka. Around 180 members had attended the Meeting and we also had a Lecture on Stress Management by Dr Shriniwas Kashalikar during AGM.

Courses & Exams Conducted : 1.

In-house on UT Level-II (ASNT) Kochi from 02.07.2012 to 10.07.2012.

RT Level-II (ISNT) course from 09.07.2012 to 22.07.2012.

UT Level-II (ASNT) course from 20.07.2012 to 29.07.2012.

RT Level-II (ASNT) course from 31.07.2012 to 08.08.2012.

MT & PT Level-II (ASNT) course from 03.08.2012 to 12.08.2012.

RT Level-II (ASNT) course from 17.08.2012 to 26.08.2012.

UT Level-II (ISNT) course from 03.09.2012 to 15.09.2012.

EC Meeting held on 28 th September 2012 at ISNT, Mumbai Office. 18 Life Members and 1 Associate Member were enrolled to ISNT, Mumbai Chapter during this period. 7.

Technical Talk: Three Technical Lectures cum events were held during the year.

EC Meeting held on 01.07.2012 EC Meeting held on 15.08.2012 AGM was celebrated on 14.07.2012 at ISNT, Conference Hall, Chennai. 2.

CE Marking by Shri. Hiremath- Hon.Jt. Secy of Pune Chapter.

ISNT NDE Stalwart Shri.K.Viswanathan on NDE in Indias Research Programme on 8.8.2012.

NDE Awareness Programme for the third year Engineering Students of Kolhapur Institute Of Technology (KIT) for 4 commonly used NDE methods viz. RT, UT, MT, PT in four Parallel sessions. Lecture on Career Opportunities in NDE was also delivered.

Other Activities:

Hyderabad Technical Talk: Dr. N. Kondal Rao Memorial Lecture “The pervasive importance of materials: Biological to Nuclear” was delivered by Padma Vibhushan Dr. R. Chidambaram, Principal Scientific Advisor to PM New Delhi on 14/07/2012. One day Work shop was held on HLT by Mr.Peter Nico Palenstyn, USA, which was jointly organised by ISNT HC & M/s Agilent Technologies at Hyderabad. Courses & Exams Conducted : Hyderabad Chapter conducted courses & Examinations on various methods like VT,PT,ET,UT for Level-I during August to October, 2012. VT Level-I conducted from 21-24 Aug-2012 LPT Level-I conducted from 27-30 Aug-2012 ET Level-I conducted from 11-17 Sept -2012 UT Level-I conducted from 25th Sep 01st Oct 2012. Other Activities: 2nd EC meeting was held on 25/08/2012

Delhi Core committee meeting was conducted on 25.08.2012 at Indian Coffee house connaught Place all about NDE 2012 and its promotion activities.

Pune

Kolkata Courses and Exams Conducted: 1. MPT Level II conducted from 15th 18th July, 2012 2. PT Level II conducted from 22nd 24th July, 2012 3. RT Level II conducted from 1st 7th August, 2012-11-12

Membership has marginally increased around 165 nos. 8.

Sriharikota Other Activities: New Executive Committee has been elected for the term 20122014.

Trivandrum Other Activities: Dr.V.R.RAVINDRAN NDT Vigyan Purashkar: ISNT Trivandrum chapter called for NDT V.R.Ravindran Purashkar in memory of DR. R.Ravindran for students of Academic Institutions in Kerala and Members of Trivandrum Chapter pursuing their students in any Institute of India. The Award is for Best Doctoral Thesis or Project Report or Technical Paper or Innovation in the area which directly or indirectly benefiting Non-Destructive Testing and Evaluation. One issue of Image, the technical bulletin of the chapter was released in June, 2012. AGM was conducted on 23 rd June, 2012 at Classic Avenue, Trivandrum: Dr. B. Venkatraman, Associate Director IGCAR delivered the MR Kurup memorial lecture. Annual Technical Meet: Sri. Anil Kesavan, NLPTA certified trainer delivered the Annual Technical Meet lecture on 23rd June 2012. The topic Why we behave the way we do. Seminar on He leak detection: A seminar on He leak detection and its techniques was organized by the chapter jointly with M/ s Agilent Technologies India Ltd. on 10 th Sep, 2012.

4. UT Level II conducted from 18th 24 th September, 2012. Other Activities Three Life Membership application were received during this period and the chapter has around 209 members. 5.

Kalpakkam

10. Trichy Technical Talk: Invited and joint lectures: ·

One day workshop in NDT for the Students of J J collage of Engg. Technology Trichy on 27 th September 2012.

Courses and Exams Conducted:

Courses & Exams:

Level I for LT & Level II Courses and Exams were conducted for UT, LT, VT, PT

Package program jointly with WRI and PSG College of tech, Level-II in RT, UT, MT & PT.

Mumbai

Certified Radiographer course RT-I in association with BARC, Mumbai.

One Year NDT Training Program for BHEL Employee wards in association with BHEL Educational Society, LevelII in RT,UT,MT & PT

Surface NDT methods Level II in PT & MT(09/07/12 to 16/07/12)

Courses and Exams Conducted: 1.

ECT Level- II examination was conducted at NDTS on 28th July 2012.

Welding Inspector examination was conducted on 2 ndSep, 2012 at ITT, Mahim.

General NDT Course for ONGC Engineers from 10 th September 2012 to 14th September 2012 at Hotel Atithi.

Journal of Non Destructive Testing & Evaluation

vol 11 issue 2 September 2012

Basics

illuminate the volume of the material. Upon interaction with an artefact, waves are scattered back and are then picked up either by the same transducer as in the pulse-echo mode in Figure 1(a) or by a receiving transducer in Figure 1(b). The total field f total in the medium is usually then considered as the sum of incident and scattered fields,

Modelling and simulation for Nondestructive Evaluation

f total = f incident + f scattered

Underlying this depiction are some basic abstractions we have made: that waves can travel in rays towards an artefact, that the material is homogenous enough not to disturb the rays in their path, the artefact is large enough to obstruct some of the rays causing them to reflect or scatter, and that a receiving mechanism lies in the path of such scattered rays.

Dr Prabhu Rajagopal Assistant Professor and Associate, Centre for Nondestructive Evaluation IIT Madras, Chennai 600036, T.N., India

Abstract This paper provides a rapid summary of the background, motivations and key features of modelling methods that underlie wave-based Nondestructive Evaluation (NDE) techniques. Different domains where modelling methods are used in the NDE process are first described in brief. This is followed by a quick description of different paradigms in the modelling of wave generation and scattering, together with their applications and limitations. The basis of various wellknown analytical, approximate and purely numerical procedures used for modelling of NDE phenomena are discussed. The article takes a descriptive rather than rigorous and mathematical approach, and aims to provide the reader with a ready summary with necessary reference material provided for further and advanced studies of the subject.

I. BACKGROUND Nondestructive Evaluation (NDE) methods make use of techniques and procedures that can provide insight into the integrity of structures and materials. Well-known NDE methods include simple ones such as dye penetrant testing, remote visual inspection as well as more advanced wave-based techniques such as ultrasonic testing, radiographic testing, eddy current testing and magnetic particle inspection. Often in order to obtain information regarding the volume of a structure, NDE procedures require the use of wave-based methods such as ultrasonic testing instead of surface preparation techniques such as dye-penetrants. In this article we will look at some basic concepts

(1)

regarding the modelling of waves in materials that underlie such wave-based NDE methods. The approach taken is to introduce various abstractions and methods, rather than a rigorous treatment (see for example, Rajagopal [1]).

II. INTRODUCTION Modelling essentially consists of an abstraction which helps us to understand, describe and make predictions of a physical phenomenon considered and as such, is almost instinctive to the way we think. Let us consider two simple inspection scenarios as shown in Figure 1, representing the puleecho and pitch-catch modes of inspection of a wave-based NDE method. Waves are launched from a transducer in both cases and

Understanding this process is essential to the success of the NDE method in practice, as we are looking for optimal positioning of the probing and receiving mechanism, a technique that is not disturbed by the material itself and is rather only sensitive to any defective regions, and some relationship of the defect features upon the measurements. In order to obtain this information we must also have insight into how the material responds to the transduction mechanism and the wave considered and how types of defects interact with the waves. Finally, a summation such as described in Eq.1 is possible only if the wave field can be assumed to behave linearly in the material considered and therefore we must know what regimes and excitation frequencies or types lead to linear or non-linear behaviour. Thus modelling, analysis and simulation are necessary for even a basic deployment and success of an NDE method. Accurate modelling often helps explain counterintuitive results, design experiments, and predict defect severity based on practical measurements.

Fig. 1 : Schematic showing (a) the pulse-echo and (b) pitch-catch modes of inspection of a wave-based NDE method.

vol 11 issue 2 September 2012

Journal of Non Destructive Testing & Evaluation

Basics III. MODELLING DOMAINS

wavelength for resolution of image artefacts [9,10].

Modelling is applied to several domains of the NDE process, where it serves different purposes as described below in this section.

III.4 Reliability: Quantification of errors in predictions is crucial to the usefulness of the results obtained by NDE methods and modelling plays a very important part of this. This is usually done through probability of detection (PoD) curves which plot the probability that a defect of a certain characteristic dimension is detected against increasing values of the dimension chosen [11]. Although much of PoD data is obtained by painstaking experimentation, modelling and simulation leading to Model-assisted or MA-PoD has become an important part of this process [12,13]. Modelling can provide guidance to explaining PoD curves, as well as in going about the PoD curve generation process through results for cases where experiments are difficult or time-consuming.

III.1 Wave generation and reception: This requires an understanding of the transduction mechanism used in the probes used for the NDE technique under consideration. For example, piezoelectricity is often used in ultrasonic transducers, and properties of piezoelectric crystals are modelled to gain insight into the excitation of particular (shear or longitudinal) modes and frequency characteristics (see for example, Auld [2]). The generating transducer is often considered the primary source for waves inside the material, whereas the receiving transducer is sensitive to waves scattered by artefacts in the material which act as secondary sources. The source for this terminology is the wellknown Huygens’ principle due to the Dutch Physicist Christiaan Huygens who first demonstrated this effect for light waves in a material that acts as a discontinuity for the wave can act as a secondary source: thus grain boundaries in a material often scatter waves causing the generally observed random noise in ultrasonic measurements. Cracks or voids act as collections of secondary sources distributed over a surface, whereas features such as material changes contribute secondary sources distributed across a volume. Recent advances in transduction such as the use of regular and conformable phased arrays require more sophisticated models to predict wave generation and reception [3,4]. III.2 Wave propagation and scattering : This involves a representation of how a material supports a wave and how defects in the material can interact with the waves. For example, we can often assume that metallic materials are homogenous and isotropic on a large scale, and on the other hand, materials such as composites have non-isotropic characteristics. III.3 Signal and image analysis A practical goal is to correlate experimental measurements with defect signatures. This usually follows from investigation of wave scattering by defects and may require extensive parametric studies involving different defect features (see for example, [5,6]). Imaging methods are also often be used to obtain practically useful maps of defect locations that can be read directly by operators [7,8]. The image space is a map of the actual space and obtaining the relation between these is the domain of imaging analysis. Imaging analysis can vary from simple methods to advanced procedures that may be used to resolve and distinguish artefacts mutually or from material features. Recent advances (termed super-resolution methods) are helping to break the traditional Rayleigh-limit barrier of half the operating

III.5. Remaining life prediction: Although usually not considered part of the domain of NDE, it is important part to obtain an estimate of active remaining life of a component diagnosed with defects, in order to decide if it must be replaced or can continue operation. Fracture mechanics is one of the main techniques used for modelling the growth of cracks in defective components. The predictions are based on a number of approaches that consider linear or non-linear behaviour for defects under applied loading conditions [14,15].

IV. MODELLING TECHNIQUES The study of wave propagation in materials and scattering by artefacts is at the heart of many modelling procedures, and provides data for other modelling routines such as imaging and reliability analysis. Wave-based NDE methods make use of acoustic, ultrasonic or electromagnetic waves, as suited to different applications. Sound waves in air or non-viscous liquids and two-dimensional electromagnetic wave problems can be studied using a single scalar potential the wave equation,

with

(2a) General electromagnetic wave problems require a single → wave equation involving a vector potential Ψ :

(2b) which leads to three scalar equations,→one each in the three directions where Ψ is defined. The text by Jones (Clarendon Press, 1986) [16] provides an overview of methods in these areas, while the classical text by Morse

Journal of Non Destructive Testing & Evaluation

and Feshbach (McGraw-Hill, 1953) [17] sets an excellent rigorous basis of the various methods of analysis. A thorough review of exact and approximate methods in electromagnetics and acoustics can also be found in the book by Bowman et al. (NorthHolland, 1969) [18]. Here we only outline the methods in a summary manner. IV.1. Variable separation: The direct approaches to modelling of wave phenomena essentially aim to solve partial differential equations (see Eq.2) describing them, subject to some initial or boundary conditions: conditions occurring at material boundaries, or at defects. The problem of solving partial differential equations can be simplified to one of solving several ordinary differential equations, if solutions which are products of several functions each depending on only one variable can be found [19]. This forms the basis for the simplest modelling technique known as the wave function expansion method (also called the eigenfunction expansion or the variable separation method), which seeks variable-separable solutions to the wave equations as described in Eq. 2. If a defect is bounded by constant coordinate surfaces of a standard coordinate system in which the wave equations permit such separation, this method can be applied. The solutions are obtained as expansions or linear combinations of eigenfunctions of the resulting ordinary differential equations, which are often special functions with known properties. The unknown coefficients in the expansions are then evaluated from the system of linear equations resulting from applying appropriate continuity conditions at the boundaries of defects. However this kind of separation in variable is possible only for a limited number of coordinate systems: for the scalar wave equation, this happens only for 11 coordinate systems, enumerated in the book by Harker (p 67, Ch 3) [20]. Of these 11 systems, only 6 permit a similar decomposition for the vector wave equation, namely, the rectangular, circular, elliptic and parabolic cylindrical, spherical and the conical systems [21]. Under some special conditions when cylindrical symmetry is satisfied, more coordinate systems such as parabolic, oblate spheroidal and prolate spheroidal systems also join the list. This means that only defects whose contours can be captured by this small set of coordinate systems can be modelled by using the wavefunction expansion method. Even then, for geometries other than the cartesian, cylindrical or spherical, the analysis gets too rigorous to extract results for practical applications. IV.2 Integral representations and reciprocity: Physically, the scattering of elastic waves comes about from an obstacle to waves acting as a secondary source, as described by the Huygens Principle. Thus vol 11 issue 2 September 2012

the response of a system of defects to incident waves can be thought of as arising due to a set of secondary sources distributed on the surface of or in the volume of the defective region. The mathematical basis for this principle as applicable to acoustic (scalar) waves is given by Helmholtzs integral formulas in the steady state and Kirchhoffs generalization for arbitrary time-dependence [21]. Based on analogous results for vector and elastic waves, it is possible to derive an alternative integral representation for the scattering problem, which is more direct and intuitively closer to the physics (see Pao and Varatharajulu [22] and Gubernatis et al. [23] for example, for a discussion of elastic wave scattering). Reciprocity concepts relate loading-response pair configurations in a medium and as such form the basis of a number of related concepts in mechanics and electrostatics, such as the principle of virtual work. Reciprocity theorems in conjunction with the Greens function present another powerful and elegant route in arriving at integral relations for wave scattering problems. Tan [24, 25], Varatharajulu [26], Kino [27], Auld [28], Kino and Khuri-Yakub [29] and more recently, Achenbach (Cambridge University Press, 2003) [30] and Achenbach [31] provide a comprehensive overview and highlight the significance of reciprocity relations. The excellent book by de Hoop (Academic Press, 1995) [32] also provides extensive further reference in this regard. Both approaches lead to integral relations connecting the incident and the scattered waves, and can be solved either analytically or through integral equations that can be solved numerically. IV.3 Approximate analytical and numerical methods: The integral equation representation of the scattering problem provides a convenient starting point for a number of approximate analytical and purely numerical solution methods.(see Hackman (Academic Press, 1993) [33] for a review of some exact and approximate analytical work on elastic wave and acoustic scattering). Approximate methods are normally applicable to low- or high-frequency regimes of the waves considered. Here low and high are relative terms, describing the extent of defects of interest against the wavelength considered low frequencies being the domain where defects are small compared to the wavelength and high frequencies are where the wavelength is several times smaller than the defect dimension. IV.3.1. Low frequencies: In the limit of low frequency or long wavelength, a common approximation is to assume that the field and its gradient inside (volume defects) or on the surface (voids, cavities and cracks) of scatterers is the same as the response of that vol 11 issue 2 September 2012

Basics material region in the absence of defects. Versions of this (Born) approximation are used in many imaging algorithms. In the similar quasi-static approximation, fields and their gradients on or inside scatterers are approximated by those due to a static load equivalent to the low-frequency load applied. The Born approximation works best in backscattering while the quasi-static approximation is applicable in general in the long-wavelength limit [34-36]. IV.3.2 High frequencies: In the highfrequency limit, ray methods provide an excellent route to approximate solutions for scattering problems (The book by Achenbach et al.(Pitman Books, 1982) [37] for example, covers ray methods for elastic waves in detail). Ray methods are based on constructing high frequency series solutions to the governing wave equations (Eq. 2)

Fig. 2 : Illustration of illuminated and shadow regions as considered in the Kirchhoff approximation In the Kirchhoff approximation the surface of the obstacle is divided into illuminated and shadow regions (illustrated in Figure 2), denoted for example by S+ and S_ , and the total field is approximated as:

, where

(f total)+ = f incident + f incident + f reflected ; (f total)– = 0 (3)

ω is the circular frequency, h is a characteristic dimension of the defect and c is the wave speed, which would be valid

where the term (ftotal)+ can be called the ‘geometrical ray’ field

involving terms of the type

asymptotically as ωh/c→∝. In practice, they are known to give useful results even at wavelengths comparable to h and the results can be extended to the time-domain. Physically, such expansions have a simple geometric interpretation in terms of rays and the leading term is just what would be predicted by geometrical wave theory, while subsequent terms offer corrections to it. Simple geometrical ray theory involving reflection and refraction often yields accurate results close to the specular or near-specular directions this is the usual representation of wave phenomena in the NDE context, as illustrated in Figure 1. The standard geometrical ray (GR) theory can also be used directly to solve scattering problems when the obstacles considered do not have sharp edges. The GR field is quite accurate in describing certain problems such as backscattering from smoothly curved objects with a curvature larger than then incident wavelength. But when the edges of the object begin to have a strong influence leading to a sharply defined shadow, edge diffraction becomes important and standard ray theory becomes inadequate. Approximate solutions to the scattered field can then be obtained through the integral representation formulas, taking the prediction by ray methods as an estimate of the total scattered displacement on the obstacle. A simple way to improve the accuracy is to use the GR field as an approximation for the total scattered field on the obstacle in the representation integrals yielding the famous ‘Kirchhoff approximation’.

This way, the Kirchhoff approximation models the scattering behaviour as if at each element of the scattering surface, incident plane waves interact with unbounded interfaces having the same surface normal. Like the Born approximation with which it shares a number of features, the Kirchhoff approximation has been used widely in scattering and inverse problems Achenbach [38] Chapman [39, 40] Schmerr [41]. The last quoted work [41] contrasts the Born and the Kirchoff approximations and comments on their limitations. Schmerr et al., [42] presented a framework unifying these two approximate methods. The GR field has its limitations- it is discontinuous at the boundaries of shadows (defined by Snells law) and vanishes totally in the shadow region. In reality, energy is continuously radiated into the geometrical shadow of the obstacle by waves which travel around its surface (see Figure 2 above), causing diffraction. The Kirchhoff approximation recovers the first singly diffracted field, but even this becomes incorrect in the presence of sharp edges. The Geometrical Theory of Diffraction (GTD) [43], has been formulated to include the full effects of diffraction within the general framework of ray theory. Keller [44] first developed the method rigorously for scalar waves and the method was later extended to elastodynamics by Resende (2D case) and in general by Achenbach and co-workers [37]. The GTD presents a correction to the total scattered field of the form:

f total = f GR + f diffracted

(4)

Journal of Non Destructive Testing & Evaluation

Basics The term representing , the diffracted field, offers only an insignificant correction to the GR field in the backscatter and specular directions, but contributes strongly in the forward direction. itself is further constructed from components consisting of primary or the first edge diffraction and secondary or multiple diffraction:

(5) These diffracted fields are constructed analogously to the reflected fields of the geometrical ray method and the scattered amplitudes related to the incident ones through Diffraction coefficients. Appropriate canonical problems (for example, for crack problems, the canonical problem is that of elastic wave scattering from a semi-infinite straight-edge crack; for scattering from convex surfaces, the canonical problem is that from infinite cylinders see [45]) whose solutions are known are selected and the Diffraction coefficients are obtained by comparing the ray solution with them. In the usual ray manner, the geometry and the curvature of the wave front as well as the defect are then incorporated through the edge conditions, which lead to the generalized Snells Law for diffraction. IV.3.Intermediate frequencies: In the intermediate frequency regime, several numerical methods, including Finite Difference (FD) [46,47] Finite element (FEM) [48-50] and t-matrix (see [51] for an excellent description of the t-matrix method) [52-55] methods have been used. The Finite element method scores over the t-matrix method in that it can treat pulses in general and is not a single frequency method like the latter. Harumi and Uchida [56] provide a good review of various numerical studies, mainly the FEM. Numerical methods are very versatile in that they solve the scattering problem for any frequency regime and geometry and in this sense, can be seen more as experimental simulations than analytical solutions. A key issue though, is that they often dont provide generic results as they tend to be constructed for specific cases. However with ever advancing computer power and the wide availability of robust commercial packages in recent years, coupled with increasing complexity of practical inspection, numerical modelling has become widespread in recent years.

V. CONCLUSIONS This paper provided a summary of the motivations and key features of modelling methods that underlie wave-based Nondestructive Evaluation (NDE) techniques. Following a brief description of the domains where modelling methods are used, different paradigms in the modelling of

wave generation and scattering were presented. The basis of various well-known analytical, approximate and purely numerical procedures used for modelling of NDE phenomena were discussed, together with their applications and limitations. Reference material further and advanced studies of the subject are provided at the end of the article.

ACKNOWLEDGEMENT Part of the literature review featured in this paper was carried out when the author was a PhD student at Imperial College London, between 2003-2007.

REFERENCES 1. P. Rajagopal, Towards Higher Resolution Guided Wave Imaging: Scattering Studies, PhD Thesis, Mechanical Engineering Department, Imperial College London, 2008, London, UK. 2. Auld, B.A., Acoustic fields and waves in solids. Vols. 1 and 2. 1973, 1990, Florida: Robert E. Krieger Publishing Company. 3. Russell, J., Long, R. and Cawley, P. Development of a membrane coupled conformable phased array inspection capability, Review of Progress in Quantitative NDE, Vol 29, DO Thompson and DE Chimenti (eds), American Institute of Physics, pp. 831838, 2010. 4. Russell, J., Long, R. and Cawley, P. Development of a twin crystal membrane coupled conformable phased array for the inspection of austenitic welds, Review of Progress in Quantitative NDE, Vol 30, DO Thompson and DE Chimenti (eds), American Institute of Physics, pp. 811-818, 2011. 5. Alleyne, D.N. and P. Cawley, The quantitative measurement of Lamb wave interaction with defects, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 39(3): p. 381-397. 6. Ditri, J.J., Utilization of guided elastic waves for the characterization of circumferential cracks in hollow cylinders. Journal of the Acoustical Society of America, 1994. 96(6): p. 37693775. 7. Holmes, C, Drinkwater, BW & Wilcox, PD., Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive evaluation, NDT and E International, 38(8): p. 701-711, 2005 8. Davies J, Cawley P, The Application of Synthetic Focusing for Imaging CrackLike Defects in Pipelines Using Guided Waves, IEEE Transactions on ultrasonics, ferroelectric and frequency control, 2009, 56, : p. 759-771.

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9. M. Fleming, Far-field Super Resolution Imaging, PhD Thesis, Mechanical Engineering Department, Imperial College London, 2008, London, UK. 10. T Hutt and F Simonetti, Experimental observation of super-resolution imaging in highly attenuative media, Review of Progress in Quantitative Nondestructive Evaluation, eds D.O. Thomson and D.E. Chimenti, American Institute of Physics Conference Proceedings, 1430, pp. 739746, 2012. 11. W.D. Rummel, Probability of Detection As a Quantitative Measure of Nondestructive Testing End-To-End Process Capabilities, Materials Evaluation, â&#x20AC;&#x2DC;Back to Basicsâ&#x20AC;&#x2122; series, (1998). Available online at: http:// www.asnt.org/publications/materialseval/ basics/jan98basics/jan98basics.htm 12. S.N. Rajesh, L. Udpa and S.S. Udpa, Numerical Model Based Approach for Estimating Probability of Detection in NDE Applications, IEEE Transactions on Magnetics (1993), VOL. 29(2): p. 1857-60. 13. R.B. Thompson, A unified approach to model-assisted determination of probability of detection, Review of Progress in Quantitative Nondestructive Evaluation, (eds) D.O. Thomson and D.E. Chimenti, American Institute of Physics Conference Proceedings, 975: p. 16851692, 2008. 14. D. Broek, Elementary Engineering Fracture Mechanics, Kluwer Academic Publishers, Dordrecht, 1986. 15. T.L. Anderson, Fracture Mechanics Fundamentals and Applications, 3rd Edition, Taylor and Francis Group, 2005. 16. Jones, D.S., Acoustic and Electromagnetic Waves. 1986, Oxford: Oxford University Press. 17. Morse, P.M. and H. Feshbach, Methods of Theoretical Physics. 1953: McGrawHill Book Company. 18. Bowman, J.J., T.B.A. Senior, and P.L.E. Uslenghi, eds., Electromagnetic and acoustic scattering by simple shapes. 1969, Amsterdam: North-Holland Publishing Company. 19. Hopf, L., Introduction to the Differential Equations of Physics. 1949, New York: Dover Publications. 20. Harker, A.H., in Elastic waves in solids. With applications to Nondestructive Testing of Pipelines. 1988, IOP Publishing Ltd and British Gas plc: Bristol, England and Philadelphia, USA. 21. Martin, P.A. and G.R. Wickham, Diffraction of Elastic Waves by a PennyShaped Crack: Analytical and Numerical Results. Proceedings of the Royal vol 11 issue 2 September 2012

Basics Society of London. Series A, Mathematical and Physical Sciences, 1983. 390: p. 91-129.

22. Pao, Y.-H. and V. Varatharajulu, Huygens’ principle, radiation conditions, and integral formulas for the scattering of elastic waves. Journal of the Acoustical Society of America, 1976. 59(6): p. 13611371. 23. Gubernatis, J.E., E. Domany, and J.A. Krumhansl, Formal aspects of the theory of the scattering of ultrasound by flaws in elastic materials. Journal of Applied Physics, 1977. 48(7): p. 2804-2811. 24. Tan, T.H., Reciprocity relations for scattering of plane, elastic waves. Journal of the Acoustical Society of America, 1977. 61(4): p. 928-931. 25. Tan, T.H., Far-field radiation characteristics of elastic waves and the elastodynamic radiation condition. Applied Scientific Research, 1975. 31(5): p. 363 - 375. 26. Varatharajulu, V., Reciprocity relations and forward amplitude theorems for elastic waves. Journal of Mathematical Physics, 1977. 18(4): p. 537-543. 27. Kino, G.S., The application of reciprocity theory to scattering of acoustic waves by flaws. Journal of Applied Physics, 1978. 49(6): p. 3190-3199. 28. Auld, B.A., General electromechanical reciprocity relations applied to the calculation of elastic wave scattering coefficients. Wave Motion, 1979. 1: p. 3-10. 29. Kino, G.S., and Khuri-Yakub, B.T., Application of the Reciprocity theorem to Nondestructive evaluation. Research in Nondestructive Evaluation, 1992. 4: p. 193-204. 30. Achenbach, J.D., Reciprocity in Elastodynamics. 2003, Cambridge: Cambridge University Press. 31. Achenbach, J.D., Reciprocity and related topics in elastodynamics. Applied Mechanics Reviews, 2006. 59: p. 13-32. 32. de Hoop, A.T., Handbook of radiation and scattering of waves : acoustic waves in fluids, elastic waves in solids, electromagnetic waves. 1995, London: Academic Press. 33. Hackman, R.H., Acoustic scattering from elastic solids, in Physical Acoustics, A.D. Pierce and R.N. Thurston, Editors. Vol. XXII. 1993, Academic Press Inc: New York. 34. Domany, E., Krumhansl, J.A., and Teitel, S., Quasistatic approximation to the scattering of elastic waves by a circular vol 11 issue 2 September 2012

crack. Journal of Applied Physics, 1978. 49(5): p. 2599-2604. 35. Gubernatis, J.E., et al., The Born approximation in the theory of the scattering of elastic waves by flaws. Journal of Applied Physics, 1977. 48(7): p. 2812-2819. 36. Jain, D.L., and Kanwal, R.P., The Born approximation for the scattering theory of elastic waves by two-dimensional flaws. Journal of Applied Physics, 1982. 53(6): p. 4208-4217. 37. Achenbach, J.D., A.K. Gautesen, and H. Mcmaken, Ray methods for waves in elastic solids. 1982, Pitman Books Ltd: London. 38. Achenbach, J.D., et al., Diffraction of ultrasonic waves by penny-shaped cracks in metals: theory and experiment. Journal of the Acoustical Society of America, 1979. 66(6): p. 1848-1856. 39. Chapman, R.K. and J.M. Coffey, A theoretical model of ultrasonic examination of smooth flat cracks, in Review of Progress in Quantitative NDE, D.O. Thompson and D.E. Chimenti, Editors. Vol. 3. 1984, Plenum: New York. p. 151-162. 40. Chapman, R.K., Ultrasonic reflection from smooth flat cracks: Exact solution for the semi-infinite crack, in CEGB Report NW/SSD/RR/14/81. 1981, N.D.T Applications Centre, Scientific Services Department, Central Electricity Board, U.K., North Western Region. 41. Schmerr Jr, L.W., Song, Sung-Jin., and Sedov, Alexander, Ultrasonic flaw inverse sizing problems. Inverse Problems, 2002. 18: p. 1775-1793. 42. Schmerr Jr, L.W., A. Sedov, and C.-P. Chiou, A unified constrained inversion model for ultrasonic flaw sizing. Research in Nondestructive Evaluation, 1989. 1: p. 77-97. 43. Karal Jr, F.C., and Keller, Joseph B., Elastic wave propagation in homogeneous and inhomogeneous media. Journal of the Acoustical Society of America, 1959. 31(6): p. 694-705. 44. 108. Keller, J.B., and Karal Jr, Frank C., Geometrical theory of elastic surfacewave excitation and propagation. Journal of the Acoustical Society of America, 1964. 36(1): p. 32-40. 45. Podil’chuk, Y.N., Y.K. Rubtsov, and P.N. Soroka, Geometrical theory of diffraction in the scattering of harmonic elastic waves by smooth convex cavities. International Applied Mechanics, 1991. 27(2): p. 131140 (Translated from Prikladnaya Mekhanika, Vol. 27, No. 2, pp. 2635,

February, 1991). 46. Harker, A., Numerical modelling of the scattering of elastic waves in plates. Journal of Nondestructive Evaluation, 1984. 4(2): p. 89-106. 47. Harumi, K., Okada, Hiaso., Saito, Tetsuo., and Fujimori, Toshiaki., Numerical experiments of reflection of elastic waves by a crack or an elliptic cylinder. IEEE Ultrasonics Symposium, 1982: p. 1064-1069. 48. Datta, S.K., Fortunko, C.M., and King, R.B., Sizing of surface cracks in a plate using SH waves. IEEE Ultrasonics Symposium, 1981: p. 863-867. 49. Datta, S.K., A.H. Shah, and C.M. Fortunko, Diffraction of medium and long wavelength horizontally polarized shear waves by edge cracks. Journal of Applied Physics, 1982. 53(4): p. 2895 - 2903. 50. Abduljabbar, Z., S.K. Datta, and A.H. Shah, Diffraction of horizontally polarized shear waves by normal edge cracks in a plate. Journal of Applied Physics, 1983. 54(2): p. 461 - 472. 51. Pao, Y.-H., Mathematical theories of the diffraction of elastic waves, in Proceedings of the first international symposium on ultrasonic materials characterization held at NBS, Gaithersburg, Md. , June 7-9, 1978 (National Bureau of Standards special publication 596, ‘Ultrasonic materials characterization’), H. Berger and M. Linzer, Editors. 1980, National Bureau of Standards (USA). 52. Hackman, R.H., and Todoroff, Douglas G., An application of the spheroidalcoordinate-based transition matrix: The acoustic scattering from high aspect ratio solids. Journal of the Acoustical Society of America, 1985. 78(3): p. 1058-1071. 53. Varatharajulu, V., and Pao, Yih-Hsing., Scattering matrix for elastic waves. I. Theory. Journal of the Acoustical Society of America, 1976. 60(3): p. 556-566. 54. Visscher, W.M., A new way to calculate scattering of acoustic and elastic waves I. Theory illustrated for scalar waves. Journal of Applied Physics, 1980. 51(2): p. 825-834. 55. Visscher, W.M., A new way to calculate scattering of acoustic and elastic waves II. Applications to elastic waves scattered from voids and fixed rigid obstacles. Journal of Applied Physics, 1980. 51(2): p. 835-845. 56. Harumi, K., and Uchida, M., Computer simulation of ultrasonics and its applications. Journal of Nondestructive Evaluation, 1990. 9(2/3): p. 81-99.

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vol 11 issue 2 September 2012

Horizon being phase-sensitive, allowing flaw depth to be retrieved from the phase lag.

Sonic Infrared Imaging – Emerging NDE Tool

We shall look at three representative cases before we come to what causes this heating to take place. References given at the end provide additional material.

Dr. C V Krishnamurthy

Case 1: Metals

Centre for NDE and Department of Physics, IIT Madras Chennai 600036 Tamilnadu Email ID: cvkm@iitm.ac.in

Thermosonics or sonic IR is an emerging thermal nondestructive testing NDT method that is based on detecting the heat generated by defects in a material when a powerful ultrasonic excitation is applied it. The method has been effective in the detection of defects such as cracks and delaminations in metallic, ceramic, and polymer matrix composites. The major advantage of this technique is that large structures and structures with complex curvature can be evaluated rapidly and accurately – this advantage comes about from three features, namely (1) every material, irrespective of the shape, can be mechanically excited through acoustic/elastic vibrations and/ or waves, (2) vibrations and/or waves can interrogate large volumes, and (3) the heat generated can be imaged using infrared cameras that offer full field visualization. Although vibrothermal NDE techniques were pioneered in the early 1980s, due to lower sensitivity of the IR cameras, of the order of 0.2°C, available at that time, successful practical applications were reported only from the late 1990s. Considerable progress has since been made on various aspects, particularly in modelling and simulations. We also come across different names such as vibro-thermography, thermosonics, sonic IR, and thermoelastic stress analysis under which investigations are reported. In vibrothermography, the structure is mechanically excited with frequencies ranging from a few Hz to tens of kHz. In the low frequency range the structure is physically attached to a mechanical excitation source, such as a dynamic shaker. In the higher frequencies (kHz) range the structure is in contact with a piezo-shaker or an ultrasonic horn. In thermosonics/sonic IR, an ultrasonic horn, of the type used for plastic vol 11 issue 2 September 2012

welding, is usually used in contact mode with the excitation typically at tens of kHz. In this pulse sonic thermography, the pulse is applied for a short time (normally less than 1 s) to produce localized heating at the defect, which can be revealed by a high speed infrared camera through monitoring its effect on the surface temperature distribution. In this way very quick defect detection can be achieved but a relatively high ultrasonic excitation power, that could potentially damage the inspected part, is usually needed to heat the defect. A variant is the ultrasound lock-in thermography, where the high frequency ultrasonic wave used to excite the part is amplitude modulated at a very low frequency (a few tens of millihertz) and the recorded sequence of images is processed in the frequency domain, producing two images: an amplitude image and a phase image. This thermal imaging technique has the advantage of

Sonic IR imaging technique used was a short sound pulse (50 – 200 ms) of high frequency (typically 20–40 kHz ultrasonic welding generator (power ~ 1 kW)). It was applied on the surface of the object under inspection through a gun containing a piezoelectric transducer that coupled to the specimen through the 1.3-cm dia tip of a steel horn. This singlepulsed technique required only a few tens of milliseconds to acquire a sequence of images of the entire time evolution of the heating process. The 20 kHz thermoelastic heating and cooling variations associated with the sonic pulse were averaged out over the integration time (~ 1 ms) of the IR camera. Figure 1 is an example of how a small (0.7 mm long) tight crack in an Aluminum plate sample shows up in the thermal image. The crack was initiated from a saw cut by fatiguing the sample in a cyclic-loaded mechanical tensile testing machine. The single pulse technique is distinct from thermoelastic temperature variations

Fig. 1 : (left panel) Optical micrograph of a fatigue crack in a 3-mm-thick aluminium alloy bar. The bottom edge of the saw cut used to initiate the crack is seen (black) just above the top of the crack, which is approximately 0.7 mm long and closed. (right panel) Selection of four frames from a sequence of sonic IR images (3 – 5 µm) of the crack. The top left image was taken prior to turning on the sonic excitation, the top right immediately following the excitation pulse, and the bottom left and right images taken at two later times during the 50 ms sonic excitation.[from Favro et al., Rev. Sci. Instrum., 71 (2000) 2418-2421] Journal of Non Destructive Testing & Evaluation

HORIZON that are synchronous with loading cycles as it averages over such time scales leaving only irreversible processes to be imaged. It is faster than lock-in thermography since the latter utilizes very low frequency (a few tens of milliHertz) sinusoidal amplitude modulation of the acoustic source, coupled with video lock-in IR imaging at that very low frequency requiring several minutes to produce processed lock-in images. Case 2: Graphite/epoxy composite structures The material used in this study consisted of 5208 epoxy reinforced with woven ([0/90]4s) IM7 carbon fiber. The thickness and fiber volume fraction of each panel are nominally 3.0 and 0.55 mm, respectively. The specimens are 25 mm wide and 216 mm long. The notched samples have a hole of 6.2 mm diameter at the geometric center.

Fig. 2 : Schematic diagram of sonic-infrared imaging technique. [from Mian et al., Composites Science and Technology 64 (2004) 657â&#x20AC;&#x201C;666].

Figure 2 shows the schematic of the experimental set-up. A 20 kHz ultrasonic welding gun (max. Power 1 kW) is used to insonify the sample with a 200 ms pulse of sound. The tip of the welding horn is pressed against the sample through an intermediate coupling material (leather). Figure 3 shows the sonic-IR images for a notched sample and an un-notched sample at various load cycles. It is seen from the figures that damage was present before the samples were subjected to fatigue loadings. However, additional damage occurs near the circular notch as the samples were subjected to cyclic loading. The damage mechanisms are predominantly delamination and distributed matrix cracks. It is observed that the damage areas increased with the fatigue load cycles as expected. In this case, the initial damage due to machining the samples from a large composite plate appears to have initiated further damage. A finite element analysis of a notched sample was carried out and was found to validate the assumption that the mating surfaces at the fatigue damage do slide during the application of the sound pulse. Case 3: Cast Iron Structures Figure 4 shows two batches of several defective (size and location unknown)

Fig. 3 : (Left panel) Sonic-IR images for an un-notched sample at (a) 0 cycle, (b) 3000 cycles, and (c) 13, 000 cycles.(Right panel). Thermosonics images for a notched sample (a) at 0 cycle, (b) 3000 cycles, (c) at 13, 000 cycles. [from Mian et al., Composites Science and Technology 64 (2004) 657â&#x20AC;&#x201C;666].

and sound turbocharger housing parts, made from spheroidal graphite cast iron (EN-GJS-400). For validation purpose, each sample was later examined by 3D computed tomography (CT). By amplitude modulating the high frequency ultrasonic excitation with a low frequency lock-in signal (typically between 0.01 and 1 Hz), a periodical temperature pattern was generated at the componentâ&#x20AC;&#x2122;s surface, and recorded by a high-resolution infrared camera over several modulation periods. The surface temperature pattern was then

Journal of Non Destructive Testing & Evaluation

reconstructed using four equidistant data points for each modulation cycle and the resulting infrared images sequence processed in the frequency domain by means of a Fourier transformation tuned to the frequency of amplitude modulation. This process led to the computation of a magnitude and a phase image that are used to present the relevant information about subsurface flaws. The results shown in Figure 5, 6 and 7 suggest that a high power excitation is not necessary to heat the flaw. Because vol 11 issue 2 September 2012

HORIZON

Fig. 4 : (left panel) Cast iron turbocharger housing parts: (a) type I and (b) type II. (middle panel) Schematic illustration of ultrasound activated lock-in vibrothermography. (right panel) Experimental setup for concurrent measurement of the heating response and the vibration response of cast iron components by means of infrared thermography and laser Doppler vibrometry. [from Montanini, Freni, and Rossi, Rev. Sci. Instrum. 83, 094902 (2012)]

Fig. 5 : Vibration profiles measured by laser Doppler vibrometry on sound and defective samples.Thermo-acoustic spectrum and its derivative obtained by exciting a defective part with an ultrasonic sweep in the 15–25 kHz range.

Fig 6 : Flow chart of the phase image processing algorithm used for defects sizing. [from Montanini, Freni, and Rossi Rev. Sci. Instrum. 83, 094902 (2012)]

of unavoidable losses due to the imperfect coupling between sonotrode and sample, not all the nominal power is actually transferred to the part. Typically, the effective acoustic power injected into the part, especially when components with complex geometry are considered, is only a small fraction (between 0.5% and 10%) of the nominal one. More importantly, the nonlinearity of the vol 11 issue 2 September 2012

coupling between the test specimen and the acoustic horn normally causes the excitation of harmonics and subharmonics of the driving frequency, resulting in a broadband excitation that enhances the thermosonic signal. It is believed that rather than the ultrasound intensity, it is this “hammering” effect that actually produces resonance of the part and heats the flawed region.

MECHANISMS, ISSUES AND CHALLENGES A widely held view of the process of heat generation is the following: Sound waves at low frequencies propagate with appreciable amplitude over distances much longer than a wavelength. For typical complex-shaped

Journal of Non Destructive Testing & Evaluation

HORIZON

One of the challenges in all these methods is in producing a consistent contact between the source (say, horn) and the sample to obtain a repeatable excitation so as to detect the damage of interest consistently. Several materials such as Teflon, Aluminum, Copper, and even materials like card stock and leather have been used to produce a repeatable contact with some degree of success. A related issue is to determine the location of clamping points on the sample, the pressure applied at the clamps, and the pressure applied at the horn tip as these have been reported to affect results. Fig. 7 : Ultrasound activated vibrothermographic NDT phase angle measurements on cast iron turbo housing parts (batch II): (a) gray level phasegram of sample IIa at 0.2 Hz modulation (defective, front view); (b) gray level phasegram of sample IIa at 0.1 Hz modulation (defective, front view); (c) gray level phasegram of sample IIa at 0.05 Hz modulation (defective, front view); (d) gray level phasegram of sample IIb at 0.05 Hz modulation (sound, front view); (e) gray level phasegram of sample IIa at 0.5 Hz modulation (sound, back view); and (f) x-ray radiography showing the buried multi-segmented porosity flaw. All tests were performed with fc = 22 019 Hz, P = 264 W (12%), number of preheating/ heating cycles: 0/1, and PTFE coupling.[from Montanini, Freni, and Rossi Rev. Sci. Instrum. 83, 094902 (2012)]

industrial parts reflections from various boundaries of the specimen introduce repeated conversions among the vibrational modes, leading to a very complicated pattern of sound within the specimen. This sound field completely insonifies the regions under inspection during the time that the excitation pulse is applied. If a subsurface interface is present, say a fatigue crack in a metal, or a delamination in a composite structure, the opposing surfaces at the interface will be caused to move by the various sound modes present there. The complexity of the sound is such that relative motion of these surfaces will ordinarily have components both in the plane of the crack and normal to it. Thus, the surfaces will ‘‘rub’’ and ‘‘slap’’ against one another, with a concomitant local dissipation of mechanical energy. This energy dissipation causes a temperature rise, which propagates in the material through thermal diffusion. This dissipation is monitored through its effect on the surface temperature distribution. The resolution of the resulting images depends on the depth of the dissipative source as well as on the time at which the imaging is carried out. It is worth noting that traditional synthetic defects such as electro-

discharge-machining (EDM) notches and flat-bottom holes do not generate vibrothermographic indications because they have no rubbing surfaces. Theories of frictional heating have inspired simulations, but little quantitative experimental data has been available to validate those theories, in part because of the large amount of experimental variability. For metals, there is still substantial debate over the relative significance of friction versus other mechanisms such as plastic deformation. Some numerical simulations suggest that plastic deformation is significant and others suggest it is not. It has been argued that frictional rubbing between crack faces is not responsible for heat generation or energy dissipation, but that crack heating is entirely due to interactions in the elastoplastic region of a crack. Despite numerous finite element simulations and theoretical explanations, no definitive experimental validation of either theory has been presented and no single theory has been universally accepted to date to explain the source—or sources—of heat generation in vibrothermography. Given the nature of excitation, it is possible that a combination of two or more mechanisms may occur.

Journal of Non Destructive Testing & Evaluation

Further, when the transducer is coupled to a sample through a nonlinear coupling material, the vibration in the sample shows more a much more complicated form, as shown in Figure 8b measured using a laser Doppler vibrometer. The vibrometer beam is reflected from the surface of the sample in the vicinity of the defect, and the surface velocity of the sample in the direction parallel to the laser beam is recorded in a computer. A recorded waveform of an uncoupled ultrasonic gun, in this case one designed to produce nominal 40kHz vibration, is shown in Figure 8a. The difference between these two systems (one being the isolated transducer, the other being the combined system of transducer/coupling material/ specimen) can be better seen through the corresponding spectra of the waveform. Figure 8c, the Fourier Transform of the waveform in Figure 8a, shows a single 40 kHz response, indicating that horn is producing this pure frequency only. However, Figure 8d, which is a Fourier Transform of the waveform in Figure 8b, shows many harmonics of the 40 kHz fundamental. Thus, the vibration at the site of the crack may be quite different from what one might have expected from the known output of the horn. Thus the nonlinear, high amplitude excitation could lead to crack opening or closure making the inspection not entirely nondestructive. To obtain a detailed understanding of the sonic heating effect, one must take these complications into account. We look at a few controlled studies aimed at resolving some of these issues.

vol 11 issue 2 September 2012

HORIZON

Fig. 8 : (Top left) Waveform due to a 40 kHz horn not coupled to any sample. Note that the amplitude is constant after it reaches a steady state. The source was set for 800 milliseconds. (Top right) Waveform when the transducer is coupled to a target through a compliant coupling material. The vibration in the sample shows a more complicated form. The source was turned on for 200 milliseconds. (Bottom left) Fourier Transform of the waveform shown in the top left panel indicates the single source frequency. (Bottom right) Fourier Transform of the waveform shown in the top right panel shows the harmonic frequencies of the fundamental one. [from Han X., Favro L., and Thompson R.L.,, CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti (2003) 500-504]

EXAMPLE #1: HEAT GENERATION DUE TO FRICTION The left panel in Figure 9 shows IR crack heating and microscopy images for a crack in a titanium (Ti 6-4) sample. The crack was grown to a length of 8.0 mm in three-point bending with an R-ratio (min/ max stress) of 0.5 and maximum stress of 772MPa. Regions of heat generation (in vibro-thermography) correlate exactly to regions where fracture surface damage was observed when the crack was broken open and observed. If frictional rubbing occurs, it can cause modifications (damage) to rubbing asperities on surfaces such as fretting, plastic deformation, melting, etc. The left panel in Figure 9 shows a correlation between the heat generated from a crack and the damaged regions on the crack faces caused by intense frictional rubbing. EXAMPLE #2: HEAT GENERATION DUE TO PLASTIC DEFORMATION Planar cracks were grown in 2024 aluminum eccentrically loaded single vol 11 issue 2 September 2012

edge tension (ESET) samples. An annealing heat treatment was used to relieve closure stresses along the cracks. These cracked samples were then loaded into the mounting apparatus and vibrated using an ultrasonic welder that generated high vibrational amplitudes at a frequency of 20 kHz. The images on the top panel of the middle panel in Figure 9 shows the heat generation of a propagating crack. Image processing was used to isolate regions of heat generation in the image shown at the bottom. The lower image shows some heat generation along the crack and two distinct regions of significant heat generation (indicated by white arrows) near the crack tip. There are two primary regions of heat generation due to branching in the crack as it grew, the left heating region correlates to a branch in the crack that did not continue to grow while the right branch continued to grow, indicated by the heating region on the center-right side of the image, and indicated by the arrow on the right side of the figure. Heat generation was found to occur more than 0.5 mm past the crack tip where plastic deformations were taking place, giving strong evidence of plasticity-induced heat generation. Despite the likelihood of some heat near the crack tip due to friction during the portion of the vibration cycle where the crack faces were in contact, the majority

Fig. 9 : Testing heat generation mechanisms in metals and composites [from J. Renshaw et al., NDT&E International 44 (2011) 736â&#x20AC;&#x201C;739] (Left panel) Correlation of frictional heat generation to crack face fretting in Titanium (Ti 6-4), (top,left) raw heating data after 1.0s of excitation;(top,center) processed heating at a isolating regions of heat generation; (top,right) fretting observed on crack faces; and (bottom) a schematic of the crack and sample geometry. (Middle panel-top) Raw infrared (IR) heating data compared to (bottom) processed IR data isolating regions of heat generation along the crack and at the crack tips. (Right panel) The CFRP sample (bottom) containing an array of drilled holes, (left) schematic of a CFRP sample containing drilled holes with arrows showing the direction of applied stress, and (right) observed infrared heating in the bar due to viscoelasticity and the stress concentration at the holes after 1.0s of excitation. Journal of Non Destructive Testing & Evaluation

HORIZON of the heat generated in this experiment appears to have been the result of plastic deformations past the crack tip. In both these examples, the vibrational stresses used were small in comparison to the fatigue limit of the material (i.e. less than 40% of the materials fatigue limit) to avoid causing additional damage to the specimen, such as crack growth which has been observed at high vibrational stress levels . After testing was completed on each sample, the cracks were observed and their lengths were carefully measured to check whether crack growth had occurred or not. After a surface examination, the cracks were broken open to observe the rubbing crack faces using optical and scanning electron microscopy to check for friction-induced damage to the rubbing surfaces. EXAMPLE #3: HEAT GENERATION DUE TO VISCOELASTICITY Heat generation has been observed at artificial delaminations in composites when vibrated at certain resonant frequencies. Simulations seem to indicate that viscoelastic heating could be a dominant mechanism in polymer composites. However it is possible that additional viscoelastic heating may be present in the vicinity of a defect due to the high stress concentrations around such defects when they are vibrated intensely, as in vibrothermographic studies. The images on the right panel of Figure 9 show viscoelastic heating in a carbon fiber reinforced composite (CFRP) measured using an infrared camera. Holes were drilled in the bar shown at the bottom of figure to generate stress concentrations primarily above and below the holes when vibrated in the direction indicated in the image. Applied vibrational stresses were kept low to avoid plastic deformation. When the sample containing the drilled holes was vibrated, the stress concentrations around the holes generated heat above the baseline heating of the excited resonance in a pattern consistent with the expected stress concentration around a stressed hole, as shown in the IR heating image in the figure. EXAMPLE #4: ROLE OF VIBRATION STRESS ON CRACK SIZE

substantial promise for finding cracks in metals, to date no quantitative relation has been shown between vibrothermographic heating and key characteristics of the crack such as length or morphology. Because the vibration is usually applied at frequencies on the same order as natural resonances, vibrothermographic crack detection performance is a strong function of both the overall shape of the specimen and the location of the crack within the pattern of resonances. Quantitative studies to evaluate the relationship between vibrothermographic crack heating, crack size, and vibrational stress have been carried in a series of tests on 63 specimens each of Ti-6-4 titanium and Inconel 718 at three different sites with different equipments. In these experiments the specimens were tuned to resonate in their third order flexural (bending) mode at approximately 20 kHz. The tuned specimens naturally vibrate near the excitation frequency, minimizing the nonlinear hammering effect and the presence of other frequencies. Since the resonant mode shape is known it is possible to calculate the motion everywhere in the specimen from the motion at a single point, such as can be measured with a laser vibrometer. The tuned specimens and resonant vibrations mean that the experiments described here are far more controlled than in the typical vibro-thermography experiment. Motion is, for the most part, at a single frequency and a single resonant mode. From the mode shape, frequency, and motion measured at a single point the vibrational stresses at the crack can be calculated so as to directly relate those

stresses to observed heating. Figure 10 shows the schematic of the experimental set up. A vibration source located off-center excites motion in the specimen from the front (crack side). The specimen is mounted using rubber pins. A laser vibrometer records motion directly behind the crack, while an infrared camera images crack heating. As heating depends on both crack length and stress level, a useful way to evaluate detectability was based on crack detectability surface - crack heating as a function of length(h) and stress (s). Such a surface fit based on a simple power law ΔT = c 1η c2 σ c3 (neglecting the camera noise) is shown in Figure 11 (see a Video in the cited reference). The surface is colored red beyond 50mK, well above the detectability threshold of a modern IR camera and gray elsewhere. If the crack length and stress are within the red region then the crack should be detectable. Surface fits give c2=2.8 ± 0.2, 870:2, c3=1:73 ± 0.06 for titanium and c2 = 1.9 ± 0.2, c3 = 1.49 ± 0.05 for Inconel. These experiments indicate that crack heating increases both with crack length and dynamic vibrational stress level. Case 4: Non-contact excitation The basic principle of this new technique shown on the left in Figure 12 is the excitation of a material with high amplitude acoustic waves without contact between the specimen and excitation source, and measuring the change in the temperature caused by the interaction of acoustic waves with

Fig. 10 : (left) Experimental configuration (right) Specimen geometry[from S.D. Holland et al., NDT&E International 44 (2011) 775–782]

While vibrothermography has shown Journal of Non Destructive Testing & Evaluation

vol 11 issue 2 September 2012

the material. The middle panel of Figure 12 shows the calibration of the horn displacement with input power indicating the linear response unlike what is found in the usual vibrothermography (see Figure 8). When acoustic wave pulse encounters a boundary with a material a portion of the energy is reflected from the boundary and another portion is transmitted into the material. As the transmitted portion propagates through the material a portion is absorbed by the material and a portion continues to propagate until it reaches another boundary at which point the process begins again. This process continues until the total pulse energy is reflected, absorbed, or transmitted by the material. The length of the acoustic horn has been designed to be equal to a quarter wavelength, with the tip being an antinode for maximum longitudinal displacement. The combination of the tip being an anti-node and the tip moving freely in air with minimum

HORIZON resistance appear to be reasons that there was no observable change in temperature at the horn tip during the excitation at all input power levels and durations tested. The right panel of Figure 12 shows the time-temperature profiles indicating that heat generation appears to persist beyond the duration of the acoustic excitation. An example of the application of NCATS on a defective aluminium wheel component is shown in Figure 13. The heat generation in the region shown in Figure 13 is considered to be a combination of internal friction in aluminum alloy (providing the background) and frictional heating due to the rubbing of the crack faces with excessive heat due to crack face rubbing highlighting the presence of the crack. The combination of internal friction mechanisms, its relaxation behavior, and the thermal gradient in the sample thickness impact the maximum temperature and time required to attain

Fig. 11 : Graphical animation of the crack detectability surface for titanium: Crack heating as a function of crack length and dynamic vibrational stress. Red data points come from ISU, yellow from PW, and green from GE. The semitransparent surface fit is colored gray below the threshold of easy detectability (50 mK) and red above the threshold. Supplementary material related to this article can be found online at doi:10.1016/j.ndteint.2011.07.006. [from S.D. Holland et al., NDT&E International 44 (2011) 775–782]

maximum temperature on the opposite side of the sample. Although cracks can be detected and identified using NCATS, it would be necessary to scan the entire structure point by point. In summary, the primary sources of heat generation in a vibrating crack include frictional rubbing, plastic deformations, and viscoelasticity in and around defects depending on the material, type of defect, and the applied vibrational stress level. Frictional rubbing occurs, especially in cracks, and is evidenced by alterations or damage to rubbing crack faces. Plastic deformations in the plastic zone of a propagating crack may also occur at high vibrational stress levels and will generate heat in addition to friction, especially in regions beyond the crack tip. Viscoelastic heating can also occur depending on the material and is related to the vibrated material’s properties and level of applied vibrational stress. Viscoelastic heating is increased in regions of stress concentration and does not require a rubbing interface to generate heat. Improved understanding of the sources of heat generation in vibrothermography will help to design robust testing procedures to implement vibrothermography in industry as well as improve issues with experimental repeatability. It appears that multiple heatgenerating sources must be taken into account for successful implementation of vibrothermography as a nondestructive evaluation technique.

REFERENCES 1. L.D. Favro, Xiaoyan Han, Zhong Ouyang, Gang Sun, Hua Sui, and R.

Fig. 12 : (left) Schematic of the non-contact acousto-thermal signature (NCATS) experimental setup. (middle) Displacement of the horn with increasing input power. (right) NCATS time–temperature signatures at different input powers to the horn. Distance between horn and sample: 300 ìm. Duration of excitation: 250 ms. Sample: Ti-6Al-4V [from Sathish et al. Rev. Sci. Instrum. 83, 095103 (2012)] vol 11 issue 2 September 2012

Journal of Non Destructive Testing & Evaluation

HORIZON

generation in vibrothermography, NDT&E International 44 (2011) 736– 739

Fig. 13 : (left) Position of the IR camera, acoustic horn, and the wheel component to image the crack. (right) NCATS imaging of a crack in aluminum wheel component [from Sathish et al. Rev. Sci. Instrum. 83, 095103 (2012)]

L. Thomas, Infrared imaging of defects heated by a sonic pulse, Rev. Sci. Instrum. 71, (2000) 2418 2. Ahsan Mian, Xiaoyan Han, Sarwar Islam, Golam Newaz, Fatigue damage detection in graphite/epoxy composites using sonic infrared imaging technique, Composites Science and Technology 64 (2004) 657–666 3. B. Hosten, C. Bacon, and C. Biateau, Finite element modeling of the temperature rise due to the propagation of ultrasonic waves in viscoelastic materials and experimental validation, J. Acoust. Soc. Am. 124, (2008) 3491–3496

4. Marco Morbidini and Peter Cawley, The detectability of cracks using sonic IR, J. Appl. Phys. 105, (2009) 093530 5. Xiaoyan Han, L.D. Favro and R.L.Thomas, Sonic IR Imaging of delaminations and disbonds in composites, J. Phys. D: Appl. Phys. 44 (2011) 034013 6. Stephen D. Holland, Thermographic signal reconstruction for vibrothermography, Infrared Physics & Technology 54 (2011) 503–511 7. Jeremy Renshaw, John C.Chen, Stephen D. Holland, R. Bruce Thompson, The sources of heat

8. Stephen D.Holland, Christopher Uhl, Zhong Ouyang, TomBantel, Ming Li, William Q. Meeker, John Lively, Lisa Brasche, David Eisenmann, Quantifying the vibrothermographic effect, NDT&E International 44 (2011) 775–782 9. R. Montanini, F. Freni, and G. L. Rossi, Quantitative evaluation of hidden defects in cast iron components using ultrasound activated lock-in vibrothermography, Rev. Sci. Instrum. 83, (2012) 094902 10.Shamachary Sathish, John T. Welter, Kumar V. Jata, Norman Schehl, and Thomas Boehnlein, Development of nondestructive non-contact acoustothermal evaluation technique for damage detection in materials, Rev. Sci. Instrum. 83, (2012) 095103

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 National NDT Award for Young NDT Scientist / Engineer

ISNT - Lifetime Achievement Award

ISNT

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 Journal of Non Destructive Testing & Evaluation

vol 11 issue 2 September 2012

NDE events We hope that this new feature added to the journal during the last year 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 December 2012

February 2013

National Seminar on NDE 2012 December 10- 12, 2012 ; New Delhi, India http://www.nde2012.org

PNAA’s 2013 Aerospace Conference February 12-14, 2013 ; Lynnwood, WA, USA http://www.pnaa.net/events/annual-conference/82events/192-2013-aerospace-conference

The 2nd International Workshop and Congress on eMaintenance December 12- 14, 2012 ; Lulea, Sweden http://www.emaintenance2012.org/index.html January 2013 2013 API Inspection Summit & Expo January 7-10, 2013 ; Galveston Island Convention Center Galveston, Texas, USA http://www.api.org/inspectionsummit The Annual Reliability and Maintainability Symposium 2013 January 28-31, 2013 ; Rosen Shingle Creek Resort and Golf Club Orlando, FL USA http://www.ndthub.com/ndt-events/the-annualreliability-and-maintainability-symposium-2013

First International Symposium on Optical Coherence Tomography for Non-Destructive Testing February 13-14, 2013 ; Linz, Austria http://www.ndthub.com/ndt-events/first-internationalsymposium-on-optical-coherence-tomography-for-nondestructive-testing 8th International Conference on Advances in Metrology – AdMet-2013. February 21-23, 2013 ; National Physical Laboratory, New Delhi, India. http://www.admetindia.org

INFRARED THERMOGRAPHY SURVEY Lowest Rates for Thermography Job with highest quality Thermography charges per day is Rs.12,000 for Maximum 100 points and 8 Hours Job Thermography will be done & analyzed by the qualified experts Contact: Mr. K. RAVINDRAN SOUTHERN INSPECTION SERVICES, #.2, IInd FLOOR, GOVINDARAJI NAICKER COMPLEX, JANAKI NAGAR, ARCOT ROAD, VALASARAVAKKAM, CHENNAI 600 087. TAMILNADU, INDIA TEL: +91 44 - 24864332 / 24868785 / 42647537 Email ID: sisins@hotmail.com Website: sisndt.com, www.ndtsis.com, www.pdmsis.com

vol 11 issue 2 September 2012

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 last few issues 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 Continuing our endeavor to provide you updates on NDE and Inspection related patents, listed below are a few recent patents from a variety of different areas related to Nondestructive Evaluation and 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/

associated with vessels and/or buildings, using nondestructive evaluation (NDE). Characterizing the dielectric properties of materials associated with a target can be executed by directing primary

UNITED STATES PATENT 8,270,556 September 18, 2012

UNITED STATES PATENT 7,521,926 April 21, 2009

Apparatus for forming stress corrosion cracks

Method for testing a component in a

from the target, analyzing the

non-destructive manner and for

secondary microwave energy signals,

Abstract

producing a gas turbine blade

and characterizing dielectric properties

An apparatus for forming stress

Abstract

microwave energy from a source towards a target, receiving secondary microwave energy signals returned

of materials associated with the target

corrosion cracks comprises a heating unit which includes a conductive member and a heating coil disposed adjacent to the conductive member to generate steam pressure in the tube specimen, an end holding unit, and a control unit for controlling the heating unit and the end holding unit. The stress corrosion cracks occurring in the equipment of nuclear power plants or apparatus industries during operation can be directly formed in a tube specimen using steam pressure under

based on analysis of the secondary

The invention relates to a method for the nondestructive testing of a component, in which corrosion regions close to the surface, composed of oxidized carbides or sulfided base material close to the surface, are determined by means of an eddy current measurement. This allows the blades or vanes to be ground down and/or separated out in particular prior to a complex process of cleaning and coating the gas turbine blade or vane.

conditions similar to those of the actual

Inventors: Beck; Thomas (Panketal,

environment of nuclear power plants,

DE), Reiche; Ralph (Berlin, DE),

thus increasing accuracy for analysis

Wilkenhoner; Rolf (Kleinmachnow,

of properties of stress corrosion cracks

DE)

which are in actuality generated, thereby improving reliability of nuclear power plants or apparatus industries and effectively assuring nondestructive testing capability, resulting in very useful industrial applicability. Inventors: Lee; Bo Young (Goyang, KR), Kim; Jae Seong (Goyang, KR), Hwang; Woong Ki (Seoul, KR) Assignee:

Industry-University

Cooperation Foundation Hankuk Aviation University (Gyeonggi-do, KR) vol 11 issue 2 September 2012

signals.

system

for

non-

destructively characterizing a target materialâ&#x20AC;&#x2122;s dielectric properties can includes a microwave energy source, a waveguide for directing microwave energy towards a target, a receiver for receiving microwave signals reflected off targets, an analyzer for assessing the difference between incident and reflected microwave signals to determine the presence of corrosion within a target or a targeted area, and an indicator for providing results of analysis. Inventors:

Bray;

Alan

(Spicewood, TX), Schmidt; Gary R. (Austin, TX), Cuevas; Alfonso (Austin, TX), Dube; Victor (McDade, TX)

Assignee: Siemens Aktiengesellschaft (Munich, DE)

UNITED STATES PATENT 6,373,245 April 16, 2002

UNITED STATES PATENT 6,674,292 January 6, 2004 Microwave corrosion detection systems and methods

Method for inspecting electric resistance welds using magnetostrictive sensors

Abstract

Corrosion, mold and moisture can be

A method and apparatus is shown for

detected under outer layer of structures,

such

surfaces

implementing magnetostrictive sensor techniques for the nondestructive

Journal of Non Destructive Testing & Evaluation

NDE PATENTS

evaluation of plate type structures

adhesive tape corrosion sensor which

Assignee: Dacco Sci, Inc. (Columbia,

such as walls, vessels, enclosures, and

is utilized under actual field or

MD)

the like. The system includes

laboratory conditions in detecting

magnetostrictive sensors specifically

coating and substrate degradation

designed for application in conjunction

using Electrochemical Impedance

with plate type structures or pipes that

Spectroscopy (EIS) of coated or

generate guided waves in the plates

uncoated metal structures has been

or pipes which travel therethrough in

developed. The invention allows for

a direction parallel to the surface of

broad applicability, flexibility in

the plate or pipe. Similarly structured

utilizing the sensor in various

sensors are positioned to detect the

environments without structural

guided waves (both incident and

compromise and the ability to inspect

reflected) and generate signals

and evaluate corrosion of the actual

Abstract

representative of the characteristics

structure, regardless of the size,

of the guided waves detected that are

shape, composition, or orientation of

A method and apparatus is shown for

reflected from anomalies in the

the structure. The electrodes may be

structure such as corrosion pits and

removed once a measurement is

cracks. The sensor structure is

made or remain in the original fixed

longitudinal in nature and generates

position

a guided wave having a wavefront

measurements may be made with the

parallel to the longitudinal axis of the

same electrode. The nondestructive

sensor, and which propagates in a

sensor apparatus is comprised of a

direction perpendicular to the

pressure-sensitive adhesive tape that

longitudinal axis of the sensor. The

consists of a conductive film or foil and

generated guided waves propagate in

conductive adhesive overlapping

the plate within the path of the

another pressure-sensitive adhesive

propagating wave. The reflected

tape that consists of a conductive film

waves from these abnormalities are

or foil and non-conductive adhesive.

detected using a magnetostrictive

The conductive tape serves as the

sensor. Shear horizontal waves may

sensing element or device. The non-

also be created by rotating the

conductive tape serves as the lead

magnetic bias 90.degree. and used for

between the sensing element and the

similar inspection techniques. Pipes,

point

which act as curved plates, may also

alternative configuration, the tape with

be inspected as well as electric

the conductive adhesive may be used

resistance welds therein. In addition,

alone, acting as both sensor electrodes

steel sheet butt welds may be

and the lead to the point of

inspected with this technique.

measurement. The metal structure or

Inventors:

Kwun; Hegeon (San

Antonio, TX), Kim; Sang Young (San Antonio, TX)

that

subsequent

measurement.

other substrate being sensed or evaluated for degradation serves as the working electrode. This two electrode sensing device is responsive

Assignee: Southwest Research

to water uptake, incubation, and

Institute (San Antonio, TX)

corrosion by measuring differences in impedance spectra. The invention can

UNITED STATES PATENT 6,328,878 December 11, 2001

readily detect, quantify and monitor

Adhesive tape sensor for detecting and evaluating coating and substrate degradation utilizing electrochemical processes

visual indication of corrosion appears,

Abstract A portable and nondestructive vol 11 issue 2 September 2012

coating and metal degradation from its earliest stages, well before any under both laboratory and field conditions. Inventors:

Method and apparatus for nondestructive inspection of plate type ferromagnetic structures using magnetostrictive techniques

implementing magnetostrictive sensor techniques for the nondestructive evaluation of plate type structures such as walls, vessels, enclosures, and the like. The system includes magnetostrictive sensors specifically designed for application in conjunction with plate type structures or pipes that generate guided waves in the plates or pipes which travel threrethrough in a direction parallel to the surface of the plate or pipe. Similarly structured sensors are positioned to detect the guided waves (both incident and reflected) and generate signals representative of the characteristics of the guided waves detected that are reflected from anomalies in the structure such as corrosion pits and cracks. The sensor structure is longitudinal in nature and generates a guided wave having a wavefront parallel to the longitudinal axis of the sensor, and which propagates in a direction perpendicular to the longitudinal axis of the sensor. The generated guided waves propagate in the plate within the path of the propagating wave. The reflected waves from these abnormalities are detected using a magnetostrictive sensor. Shear horizontal waves may also be created by rotating the magnetic bias 90.degree. and used for similar inspection techniques. Pipes, which act as curved plates, may also be inspected as well as electric

Davis;

Guy

(Baltimore, MD), Dacres; Chester M. (Columbia, MD), Krebs; Lorrie A. (Baltimore, MD)

UNITED STATES PATENT 6,294,912 September 25, 2001

resistance welds therein. Inventors:

Kwun; Hegeon (San

Antonio, TX)

Journal of Non Destructive Testing & Evaluation

NDE PATENTS Assignee: Southwest Research Institute (San Antonio, TX)

UNITED STATES PATENT 5,854,492 December 29, 1998 Superconducting quantum interference device fluxmeter and nondestructive inspection apparatus Abstract

Abstract A

Inventors: Polly; Daniel R. (Oxnard,

nondestructive

method

and

CA)

apparatus for optical detection and

Assignee: The United States of

monitoring corrosion in structures

America as represented by the

normally inaccessible to light and

Secretary of the Navy (Washington,

observation. An optical fiber coated

DC)

with a corrosion sensitive compound is embedded in the structure. Tapped Bragg gratings of different Bragg

UNITED STATES PATENT 4,826,650

periods are spaced along the fiber and refract

narrow

bandwidth

A nondestructive inspection apparatus having a SQUID is made with compact configuration and is capable of detecting a metallic or non-metallic metal for defects, corrosion, and the like, by forming the SQUID and a magnetic field applying coil on the same substrate. The SQUID comprises two Josephson junctions, a washer coil connected to the Josephson junctions to form a superconducting loop, shunt resistors, a damping resistor, and a feedback modulation coil, all of which are formed from a superconducting thin film on a supporting substrate. A magnetic field applying coil is formed on the same supporting substrate with a superconducting thin film or a normal conducting metal thin film. The magnetic field applying coil, which generally has plural turns around the SQUID, applies a dc or ac magnetic field to a sample. The change in magnetic field caused by a defect in the sample is detected by the washer coil, and the position and size of the defect may thus be determined. Since the magnetic field applying coil is integrated on the same substrate as that on which the SQUID is formed, the apparatus may be made compact.

component of a broad beam light

Inventors: Chinone; Kazuo (Chiba, JP), Morooka; Toshimitsu (Chiba, JP), Nakayama; Satoshi (Chiba, JP), Odawara; Akikazu (Chiba, JP)

of electrochemical current flow. The

Assignee: Seiko Instruments Inc. (JP)

the probe potential with respect to a

pulse transmitted through the fiber. Due to corrosion, the refracted components are reflected by the compound and their amplitudes are

May 2, 1989 Ultrasonic examination of reactor pressure vessel top guide Abstract

detected and displayed for each

In a boiling water reactor, an

narrow bandwidth.

apparatus and process for ultrasound

Inventors:

Perez; Ignacio M.

(Yardley, PA), Agarwala; Vinod S. (Warminster, PA)

inspection of the top guide is disclosed. The top guide constitutes a lattice of stainless steel bars overlying the core plate and being assembled

Assignee: The United States of

at confronting grooves with the lattice

America as represented by the

mounted at the side edges to the

Secretary of the Navy (Washington,

reactor pressure vessel. This lattice

DC)

braces the upper ends of the vertically supported fuel assemblies in their requisite orientation and spaced apart

UNITED STATES PATENT 4,927,503

relation to enable among other things the required spatial interval to be

May 22, 1990

maintained for control rod moderation

Method for assessment of corrosion activity in reinforced concrete

of the reaction. Because of the proximity of the top guide to the fuel assemblies, the individual bars making up the lattice need to be checked for

Abstract

cracking, especially that cracking

The probe is a nondestructive testing device for locating and measuring corrosion activity in reinforced concrete structures by direct detection device consists of a surface probe valved

present

alternative

measurement paths when measuring remote reference electrode, allowing the measurement of IR drops

UNITED STATES PATENT 5,646,400 July 8, 1997

associated

with

corrosion

Corrosion detecting and monitoring method and apparatus

internal corrosion (the primary cause

reinforcement â&#x20AC;&#x153;rebarâ&#x20AC;?. By grid surveys of concrete structures, areas suffering of marine concrete deterioration) can be located and the level of corrosion activity determined.

Journal of Non Destructive Testing & Evaluation

produced by irradiation assisted stress crack corrosion. With a defined cell in the lattice emptied of its contained and adjoining fuel assemblies, there is disclosed an ultrasound test for cracking. A sound transducer on a first special frame sweeps horizontally across the top of a bar interrogating the bar with vertical ultrasound waves for detecting horizontal cracks. Similarly, a sound transducer on a second

special

frame

sweeps

vertically across the side of a bar interrogating the bar with angularly incident horizontal ultrasound waves for

detecting

vertical

cracks.

Nondestructive testing of the lattice assembly occurs without required disassembly. vol 11 issue 2 September 2012

NDE PATENTS Inventors:

Richardson; David L.

channels to remove heat from the fuel

reusable channels and an undesirable

(Los Gatos, CA), Clark; Jack P. (San

elements. A typical fuel assembly of

expenditure of time and money. Thus

Jose, CA), Patterson; Peter M.

this type is shown, for example, by B.

there is a need for remotely operable,

(Livermore, CA), Perry; Richard W.

A. Smith et al. in U.S. Pat. No.

nondestructive corrosion measuring

(San Jose, CA)

3,689,358. An example of a fuel

equipment for determining whether or

element or rod is shown in U.S. Pat.

not a radiated component is fit for

No. 3,378,458. Additional information

further service. Fuel assembly

on nuclear power reactors may be

channels are normally formed of a

found, for example, in “Nuclear Power

zirconium alloy made up of two U-

Engineering”, M. M. El-Wakil, McGraw-

shaped members welded together.

Hill Book Company, Inc., 1962. While

They are usually factory processed by

the various reactor components are

autoclaving (exposure to high

thoroughly factory tested before being

temperature steam) to form a thin,

placed in the reactor, there is a

tight protective oxide surface film of

Abstract

continuing need for in-service

deep gray or black color. In service

inspection equipment which can

oxide corrosion usually occurs at local

Remotely manipulatable probe and

rapidly and conveniently verify the

areas, expecially at portions which

apparatus for positioning a corrosion

integrity of or detect any anomalies

have been exposed to highest

thickness sensing transducer over

in such components at the reactor site,

temperatures and neutron flux

selected areas of the surface of a

particularly after such components

density, and develops as clusters of

radioactive component submerged in

have been subjected to reactor

pin point spots or nodules of corrosion

a pool of water for radiation shielding.

service and have, therefore, become

which are light grey or white in color

BACKGROUND In known types of

radioactive.

radioactive

and which thus give the local area a

nuclear power reactors, for example

condition of used components

“salt and pepper” appearance. As

as used in the Dresden Nuclear Power

requires remotely operable equipment

such corrosion progresses, the

Station near Chicago, Ill., the reactor

which can examine such components

nodules

core comprises a plurality of spaced

under water to protect the test

eventually coalesce to form a

fuel assemblies arranged in an array

equipment operators from radiation.

continuous oxide corrosion film or

capable of self-sustained nuclear

A particular need is inspection

sheet over the local area. Continued

fission reaction. The core is contained

equipment which can provide a

corrosion results in a thickening of the

in a pressure vessel wherein it is

nondestructive examination and

oxide film and eventual spalling, that

submerged in a working fluid, such as

quantitative indication of corrosion

is, a flaking off of the oxide particles.

light water, which serves both as

formation, such as oxide formation,

Under present procedures, the

coolant and as a neutron moderator.

on such reactor components. It is

channel is removed from service

Each fuel assembly comprises a

particularly desirable to provide

before spalling is expected to occur

removable tubular flow channel,

corrosion measurement of removable

to avoid contamination of the coolant

typically of approximately square

reactor components which potentially

with the oxide particles. Measurement

cross section, surrounding an array of

have a relatively long service life such

of thickness of the corrosion film can

elongated, cladded fuel elements or

as fuel assembly flow channels. For

be used to preduct the onset of

rods containing suitable fuel material,

example, as mentioned above, each

spalling. Measurement of corrosion

such as uranium or plutonium oxide,

fuel assembly is surrounded by a

thickness can also be used to indicate

supported between upper and lower

removable tubular flow channel. While

the effectiveness of heat treatment

tie plates. The fuel assemblies are

the normal service life of a fuel

and other processes used to provide

supported in spaced array in the

assembly in the reactor core is in the

improved corrosion resistance. It is

pressure vessel between an upper

order of four years, the flow channel

also desirable to examine other local

core grid and a lower core support

can be removed and reused on a

areas of the channel such as weld

plate. The lower tie plate of each fuel

replacement fuel assembly in the

seams, for indications of corrosion.

assembly is formed with a nose piece

absence of excessive corrosion or

Therefore it is an object of the

which fits in a socket in the core

other defects. Previous methods of

invention

support plate for communication with

determining the extent of channel

nondestructively measure formation

a pressurized coolant supply chamber.

corrosion involved the cutting up of a

The nose piece is formed with

channel and the shipping of samples

component. It is another object of the

openings

Assignee: General Electric Company (San Jose, CA)

UNITED STATES PATENT 4,145,251 March 20, 1979 Corrosion measuring apparatus for radioactive components

through

which

Such

expand

corrosion

area

remotely on

and

radioactive

the

of corroded portions to a laboratory

invention to provide a corrosion

pressurized coolant flows upward

for examination. This approach

thickness sensing means which readily

through the fuel assembly flow

resulted in destruction of potentially

and remotely can be positioned over

Journal of Non Destructive Testing & Evaluation

vol 11 issue 2 September 2012

Journal of Non Destructive Testing & Evaluation

NDE PATENTS a selected area of a radioactive

body portion transmits light and

component.

refracts light at its conical surface, the

commercially available which uses an

Equipment

operator can, in effect, see through

eddy-current technique for indicating

and beneath the probe to position the

the distance between a transducer and

transducer over the desired local area

a conductive surface. The transducer

of the component being examined. In

includes a coil which is energized by

the illustrated embodiment, the body

a high frequency current. Magnetic

is surrounded by a ring of metal of

flux from the coil produces eddy

sufficient weight to provide a desired

currents in the conductive surface.

force of the resiliently mounted

Thus the power or energy supplied by

transducer against the surface under

the coil to produce the eddy currents

examination.

is also proportional to the distance between the transducer and the conductive surface. This displacement

on a metal. It is another object of the invention to utilize an eddy-current technique to remotely measure

UNITED STATES PATENT 7,822,273 October 26, 2010 Method and apparatus for automatic corrosion detection via video capture

of radioactive components. SUMMARY

during the inspection of tanks carrying

These and other objects of the

caustic material this device has been

invention

created which will allow a tank to be

transducer

containing

probe,

inspected remotely. A camera is used

suspended at the end of a manually

to take an image of the tank surface

manipulatable pole, which can be

and software is incorporated into the

visually positioned over selected areas

method that will analyze the surface

intermediate a layer of metal and an

radiant thermal energy pulses impinging on the external surface of phase angle difference between a waveform representing the energy pulses and a waveform representing the undulating temperature response of the paint surface to the energy pulses. Use of the system for detecting corrosion of a military or other aircraft for corrosion inspection and correction is

contemplated.

detection

from

corrosion presence are achieved with respect

Inventors:

characteristics. This data is complied

Heights, OH)

shielding water. The probe comprises

in an easily readable for the operator.

material and having the general shape of a frustum of a cone, the transducer being resiliently supported in a central bore of this body portion. Since the

Inventors:

Arcaini;

Gianni

other

corrosion

arrangements.

of the tank for certain corrosion

a body portion formed of a transparent

measurement

variations and earlier detection of

component

radioactive

Enhanced

independence of the corrosion

submerged to a suitable depth in

similar located

in order to avoid stripping the aircraft

Abstract In order to prevent human injury

achieved

products

the coated metal and responds to the

thickness of corrosion on the surface

are

corrosion

oxidation

employs a continuing stream of

circuitry and converted to a calibrated

thickness of a nonconductive coating

detecting

protective material. The system

(San Jose, CA)

device can be used to measure the

A pulsed heat energized system for

Jose, CA)

detected by suitable electronic

conductive surface. Thus such a

Abstract

Inventors: Qurnell; Frank D. (San

Assignee: General Electric Company

between the transducer and the

Corrosion detection by differential thermography

overlying layer of paint or other metal

dependent variation in power is

display or recording of the distance

UNITED STATES PATENT H2,127 October 4, 2005

Byrd; Larry W. (Huber

Assignee: The United States of America as represented by the

(Jacksonville, FL), Arpa; Aydin

Secretary

(Jacksonville, FL), Ruan; Yanhua

(Washington, DC)

the

Air

Force

(Jacksonville, FL), Kuchi; Prem (Jacksonville, FL)

OBITUARY

Dr. Rameswar Das doyen of NDT Society of India / ISNT passed away on July 14th, 2012. Dr. Das was born in Dhaka and did his Post Graduation in Chemistry before completing his Doctorate in BioChemistry from Indian Institute of Science, Bangalore. He worked for Oriental Chemical Works at Kolkata and he was instrumental in development of various sectors like Railways, Defense, Aviation and Nuclear etc. Dr. Das is a founder Member of many Professional Societies including ISNT and contributed immensely for the growth of ISNT. He has also received many Awards in his career. His passing away is a great loss to ISNT. Our heartfelt condolences to the bereaved members of his family for the irreparable loss. May his soul rest in peace. ISNT Colleagues Journal of Non Destructive Testing & Evaluation

vol 11 issue 2 September 2012

ndt puzzle Conceptualized & Created by Dr. M.T. Shyamsunder, GE Global Research, Bangalore

We hope you enjoyed solving the “Wordsearch Puzzle” which was published in the last issue. We are still receiving entries from the readers and will be announcing the WINNERS and also publish the solution in the next issue.

WORDSEARCH PUZZLE MAGNETIC PARTICLE TESTING

In this issue, we have another Wordsearch puzzle to continue stimulating your brain cells! We hope you will find this section interesting, educative and fun filled. Please send your feedback, comments and suggestions on this section to mandayam.shyamsunder@gmail.com Introduction The “Word Search Puzzle”, contains more than thirty (30) words related to Magnetic Particle Testing. These include techniques, terminologies, phenomenon, famous people, etc. These words are hidden in the puzzle and may be present horizontally, vertically, diagonally in a forward or reverse manner but always in a straight line. Instructions -

All you have to do is identify these words and mark them on the puzzle with a black pen Preferably you may take a photocopy of the Puzzle sheet and mark your answers on that (see the marked example) Once completed please scan your answered puzzle sheet as a PDF file and email the scanned sheet to jndte.isnt@gmail.com with your name, organization, contact number and email address

Rules & Regulations -

Only one submission per person is allowed The marked answers should be legible and clear without any scratching or overwriting The decision of the Editor-in-Chief, Journal of NDT &E is final and binding in all matters

The correct answers and the names of the prize winners will be published in the next issue vol 11 issue 2 September 2012

NAME

: ___________________________

ORGANIZATION : __________________________ PHONE

: ___________________________

EMAIL ID

: ___________________________

Journal of Non Destructive Testing & Evaluation

NCB – ISNT ANNOUNCES ISNT LEVEL – III CERTIFICATION PROGRAMME AT PUNE (7TH JANUARY to 17TH FEBRUARY, 2013) The Announcement for the ISNT LevelIII Certification programme has been displayed in the web “www.isnt.org.in” with complete details along with application form. Interested participants may avail this opportunity for better prospects. Last date for receipt of application form along with payment is December 21, 2012

Technical Paper

Quality Assessment of Composite Adhesively Bonded joints by Non-linear Ultrasonic Method R L Vijayakumar1, M R Bhat and CRL Murthy Department of Aerospace Engineering, Indian Institute of Science, Bangalore-560012, INDIA Email: vkumar.aero@gmail.com

ABSTRACT Quality assessment of adhesive joints is an involved and challenging non destructive evaluation problem, both during their production and life cycle. The load carrying capacity of an adhesive joint is governed by the properties of the adhesive layer itself as well as the interface characteristics. Therefore an important task of NDE is the development of techniques to characterize the bond quality or even to measure the bond strength of the joints. Since an adhesive layer can be treated as a soft interface between two components, it is expected that nonlinear effects arise from the propagation of an ultrasonic pulse. Binding forces are nonlinear and cause a nonlinear modulation of ultrasonic waves transmitted through the bonded region. As a consequence, the generated higher harmonics of an insonified monochromatic wave will yield information about the adhesive bonds. In case of resonance the amplitude of strain in a soft interface layer is strongly increased and therefore, the layer considerably contributes to the amplitude of the second harmonic. The non linear behavior of such a layer and its influence on nonlinear parameter (Î˛) was studied experimentally. Nonlinear ultrasound inspection was carried out on adhesive joints with Carbon Fiber Reinforced Plastic (CFRP) adherends, the quality of the epoxy adhesive layer was varied by adding different amounts of poly vinyl alcohol. It was found that the non linear parameter (Î˛) increased with increasing amount of degradation, giving an indication of degradation. Keywords: composites, adhesive joints, nonlinear ultrasound, bond quality.

1. INTRODUCTION Adhesive bonding plays an important role in the assembly of aerospace and automobile structures and has augmented or replaced the conventional joining methods like welds and rivets. Though there are many advantages of adhesive bonding like reduced stress concentration, uniform stress distribution, improved fatigue strength etc, they do have some disadvantages as well such as susceptibility to degradation when exposed to moisture and temperature. The quality of the joint may degrade over a period of time leading to catastrophic failure of the structure. This emphasizes the role of non destructive inspection methods in keeping the structures in good health. Ultrasonic wave based methods have been widely used in the field of non-destructive testing of adhesive joints [1, 2]. However, most of these conventional ultrasonic methods are aimed at detection , location and evaluation of gross defects like delaminations and voids by way of studying the reflection and transmission characteristics of the waves at the boundaries of the discontinuities. They are but less sensitive to evenly distributed micro-cracks, pores or degradation. Moreover, general degradation in strength may be found in apparently flawless joints. Therefore a NDE method that can reflect material degradation in an adhesively bonded joint at any stage of service becomes more important. Ultrasonic wave based techniques with a non-conventional approach can be a powerful tool for non destructive inspection where the characteristics of its propagation are directly related to the properties of material. It is known that material failure is usually preceded by some kind of nonlinear mechanical behavior before Vol. 11, Issue 2 September 2012

significant material damage occurs [3, 4]. If ultrasonic waves are made to propagate through the material at this stage a strong non linear effect may be generated due to the non linear properties of that material. Thus, it could be expected that the degree of material degradation can be evaluated by measuring the ultrasonic wave parameters that are affected by this non-linearity. The classical theory of nonlinear wave propagation in solids has been discussed and presented by Truell and Hikata [5,6]. Most of the research on nonlinear phenomenon has followed along these lines. The nonlinearity of ultrasonic wave means that the second or higher order frequency component exists besides the fundamental component when the wave propagates through a degraded medium. The magnitude of these higher order components is related to properties of material as well as the wave amplitude and propagation distance. Therefore, the magnitude of the higher order component will appear differently in normal and degraded material, when amplitude of the wave and propagation distance is maintained the same. This tendency is known from previous research carried out on metals for fatigue damage evaluation [7, 8]. In the last few years the NDE community has turned its attention to investigating the possibility of using non linear acoustic techniques to measure properties of adhesive joints [9, 10]. A detailed theoretical description on non linear propagation of waves in layered media has been given by Hirsekorn and Brekhovskikh [11, 12]. The basic parameter that has to be evaluated in this context is the nonlinear parameter (Î˛). There are two approaches to obtain this parameter. First is from studying the second harmonic Journal of Non destructive Testing & Evaluation

38 generation and the second from acousto-elastic effect [9]. The latter requires application of stress and measurement of very small change in sound velocity in a thin adhesive layer and therefore does not seem to be very practical. Thus, the method of second harmonic generation is preferred by most of the researchers. The objective of this study is to utilize the second harmonic method in characterizing CFRP-Epoxy adhesive joints with varied bond quality and to show the correlation between nonlinearity and adhesive degradation with the help of experimental results. The mechanism of second harmonic generation during propagation of ultrasonic wave through the degraded joint is firstly shown on the basis of nonlinear elasticity. RITEC high power nonlinear ultrasonic system was used to generate and measure the second harmonic frequency component. Series of experiments were carried out in order to obtain a correlation between the amplitude of the second harmonic frequency and the adhesive degradation. Sets of specimens with different degrees of degradation were prepared and tested. Experimental results showed that nonlinear acoustic effect can be used as an effective tool for the evaluation of degradation of adhesive joints.

2. MATERIALS AND SAMPLES Single lap shear joints were prepared as per ASTM D 5868 standard using carbon fiber reinforced plastic (CFRP) material as substrates and a two part epoxy adhesive; Araldite AV138M / Hardener HV 998. The CFRP adherend was fabricated using 14 layers of unidirectional carbon prepreg CP150ns with each layer having a thickness of 0.18mm. All the layers were stacked in the 0° direction and cured in an autoclave according to the curing cycle (60°C for the first 30 minutes, 125°C for 90 minutes and 7bar external pressure with vacuum) suggested by the prepreg manufacturer. The composite laminate thus obtained had a thickness of 2.5mm. It was subjected to non-contact, water immersion ultrasound scanning using a 5 MHz focused transducer to ensure that the substrate material was free from gross defects and anomalies. The laminate was then cut to the required size of 101.6mm x 25mm, surface preparation in the region to be bonded was carried out according to ASTM D 2093 standard. An area of 25mm X 25mm was bonded using the two part epoxy system in which the resin and hardener were mixed in the ratio of 100:40 as per the manufacturer’s recommendation and cured for 24 hours at room temperature. A uniform

Technical Paper

bond-line thickness of 0.76mm was maintained using a mold specially designed and fabricated for the purpose. The dimensions of the resulting composite lap shear joints are shown in Fig. 1. Five different sets of samples having six samples in each (total of 30 samples) were prepared; the quality of the epoxy resin was degraded adding different amount of poly vinyl alcohol (PVA). While the healthy samples (H) were free of PVA, others were denoted as P10 (with 10% PVA by total weight of the epoxy-hardener mix), P20 (20% PVA), P30 (30% PVA) and P40 (40% PVA).

3. NONLINEAR ACOUSTIC EFFECT IN MATERIALS The nonlinear behavior of materials can be explained using the nonlinear version of Hooke’s law as shown in Eq. (1) [7, 8] σ = Eε(1+βε+ . . . )

(1)

Where ‘E’ is Young’s Modulus and ‘β’is a higher order elastic coefficient commonly known as nonlinear parameter. This relationship has been approved experimentally for metallic materials by some researchers [6]. The contribution of material degradation towards higher order harmonics is very small and hence all higher order terms except the second, can be neglected. In order to explain the generation of higher order harmonic waves, consider the case where a single frequency ultrasonic longitudinal wave is incident on one side of a bar with degradation and received on the other side. If ‘A1’ is the amplitude of the initial sound pressure and ‘ω’ is the angular frequency and ‘k’ is the wave number; the incident longitudinal wave can be expressed mathematically as [13] uo = A1cos(kx–ωt)

(2)

where ‘uo’ is the initial displacement of the excited wave. If the effect of attenuation is neglected, then the equation of motion for longitudinal planar wave in a material can be represented as

(3)

Where ‘ρ’ is the density of the medium, ‘u’ is the displacement, ‘x’ is the propagation distance of the sound waves in the medium, ‘σ’ is the stress and ‘t’ is the time. Using Eq. (1) and (3) and the relationship between strains and displacements, one can obtain the nonlinear wave equation for displacement ‘u’

(4)

Fig. 1 : Single Lap shear joint Journal of Non destructive Testing & Evaluation

In order to obtain a solution, let us assume the displacement ‘u’ to be Vol. 11, Issue 2 September 2012

Technical Paper

u = uo+u1

(5)

Where ‘uo’ is the initial excited wave and ‘u1’ is the first order solution. If ‘uo’ is the single frequency sinusoidal waveform then the solution to the second order will be [14] u

= uo+u′

β = A1cos(kx–ωt) – — A12k2xs in 2(kx–ωt) 8

(6)

The second term in Eq. (6) represents the second harmonic frequency component, as a result we can explain how second order harmonic component occur through the propagation process [15, 16]. The magnitude of the second order harmonic component (A2) depends on ‘β’ which represents the nonlinear elastic characteristics of the material and is closely related to the degradation of the material. Therefore, a measure of the magnitude of ‘β’ can be utilized to evaluate the change of the material’s properties or degradation.

4. EXPERIMENTAL DETAILS A schematic diagram of an experimental setup to measure the magnitude of second harmonic component in the received ultrasonic waves is shown in Fig. 2. Experiments were carried out using an ultrasonic pulser-reciever RITEC RPR-4000 instrument which adopts the heterodyne signal detection technique in order to reduce additive electrical noise effectively. In through transmission technique while a 1MHz contact PZT transducer was used as a pulser, a 2MHz contact transducer was used as a receiver to pick up the second harmonic. Both the transducers were coupled to the CFRP adherend surface using glycerin as couplant and a spring clip was used to hold both the transducers in position at a constant contact pressure. An input signal with varying amplitudes was given to the pulser. In order to avoid overlap of the echoes within the adherends, the wave was restricted to 6 cycles. On the other hand, the burst was long enough to ensure interference effects in the adhesive layer. The resulting signal from the 2MHz receiver was amplified using a preamplifier (60dB),

Fig. 3 : Received time domain signals for a healthy sample and degraded P40 sample

while an internal band pass filter in the range 800 kHz to 2.5MHz was used to ensure that the noise is minimized. Figure 3 shows the received signal which was then transmitted to the digital oscilloscope where the signal was digitized for further signal processing. The amplitudes of the first (A1) and second harmonic components (A2) were recorded at different excitation voltages ranging from 100 to 400 volts, by means of Fast Fourier transformation (FFT). Though in such a set up the nonlinear effects may have contributions from many components such as delay line of the transmitting and receiving transducers, couplant film, adherend, and adhesive layer itself, in the present work, considering all the other parameters unaltered, and only the material properties of the adhesive being degraded using PVA, any change in the nonlinear parameter can be attributed to adhesive layer degradation. An analytical description in first order can be achieved by using a volume model approach that takes the interference in the adhesive layer partially into account [9], since A1 >> A2, only interference of the fundamental wave is considered this implies

(7)

Where, ‘λ’ denotes the wavelength and ‘x’ a distance equal to thickness of the adhesive layer. It is clear from the above equation that ‘βá[A 2 /(A 1 ) 2 ]’, hence a higher magnitude of A1 contributes significantly towards the magnitude of second harmonic. However, a high magnitude fundamental harmonic can be obtained in the condition of resonance which occurs if Eq. (8) is satisfied for n = 0,1,2… (8)

Fig. 2 : Experimental arrangement for nonlinear ultrasonic inspection of adhesive joints Vol. 11, Issue 2 September 2012

It needs to be noted that in the case of resonance the increase of A1 amounts to a factor of about 3.5. As A2 is squarely proportional to A1, its value can be high. Therefore, it appears to be promising to observe the nonlinear properties of an adhesive layer by studying the second Journal of Non destructive Testing & Evaluation

Technical Paper

Fig. 5 : Variation of the nonlinearity parameter (β) with PVA percentage Fig. 4 : Variation of the second harmonic amplitude (A2) with fundamental harmonic (A1)2 at different levels of input voltage.

harmonic. The appearance of ‘λ’ in Eq. (7) implies that the second harmonic must also depend on the sound velocity. However, since the influence of wavelength is seen even in numerator i.e., on A1, a simple prediction of its influence on second harmonic is not possible. Figure 4 shows the variation of the magnitude of the second harmonic (A2 ) with fundamental harmonic (A1) 2, at different input voltage values. As shown, the amplitude of A2 increases with an increase in the magnitude of input, since at higher voltages A1 becomes high and contributes significantly towards A2. It can also be seen that the slope of the variation also increases with increased PVA percentage, indicating the influence of degradation on the propagated ultrasonic wave and higher harmonics. The slopes of these curves give a measure of nonlinearity parameter (β) which was computed and plotted against PVA percentage as shown in Fig. 5, it can be observed that the nonlinearity parameter (β) increases with increase in PVA percentage indicating degradation. It should be noted that the nonlinear parameter for severely degraded (P40) sample increases by a factor of 7 as compared to a healthy sample. The samples were loaded in a testing machine till failure to determine its strength. Figure 6 shows the variation of ‘β’ with the average shear strength of the CFRP adhesive joint samples, as shown the nonlinear parameter decreases with increasing strength. It is reported that [9] the influence of the nonlinearity at the interface of the joint due to strong intermolecular forces of attraction is negligibly small towards changes in β, hence any significant changes in β can be attributed to loss of stiffness in the bond line due to adhesive degradation. The results obtained clearly shows the influence of degradation on nonlinear modulation of the input ultrasonic waves. The nonlinear parameter (β) increases with increase in degradation. Higher nonlinear parameter implies lower strength. It should however be remembered that there are a number of factors which influences the nonlinear parameter (β), including joint properties like bond-line Journal of Non destructive Testing & Evaluation

Fig. 6 : Variation of the nonlinear parameter (β) with Average bond strength of the CFRP joints

thickness, density of adhesive, type of adhesive used and its properties, attenuation in the medium adherends etc, ‘β’ is also influenced by type of transducer used, electronic noise, couplant, pressure applied and so forth. The results obtained in these studies though are for CFRP adhesive joints prepared, it can be expected to be applicable to other types of polymer composites as well. Despite an internal band pass filter used in the RITEC RPR-4000 pulser- receiver, the received signal appeared to be quite noisy. However, since all the other parameters remained fairly constant except for the adhesive property, changes in ‘β’ could be measured and attributed to the degradation of the adhesive joint.

5. CONCLUSION Experimental investigations were carried out using nonlinear ultrasonics approach to study degradation in the adhesive layer of a CFRP-epoxy-CFRP adhesive lap joint. The results obtained show very interesting and encouraging correlation between nonlinear parameter and the degradation in the adhesive joint. The acoustic pulse gets nonlinearly modulated due to degradation in the adhesive, leading to an increase in the nonlinear parameter (β) with increased degradation. These results can give a quantitative assessment of the health of a bonded joint. Though the results obtained are for the CFRP-epoxy-CFRP adhesive lap joint specimen, the approach should be applicable to other types of bonded joints. However, since non linear parameter ‘β’ in a realistic situation may also depend upon Vol. 11, Issue 2 September 2012

Technical Paper

a lot of other parameters like adhesive properties, geometry, transducers used, noise etc., these factors can influence ‘β’ in a real structure and the contribution of each of these parameters on nonlinear modulation of a propagating ultrasonic wave needs to be understood.

REFERENCES 1. R.D. Adams, B.W. Drinkwater, Nondestructive testing of adhesively-bonded joints, NDT&E international 30, (1997), 9398. 2. Shuo yang, Lan Gu, R. F Gibson. Nondestructive detection of weak joints in adhesively bonded composite structures, Composite structures, 51, (2001) 63-71. 3. A. Sutin, Nonlinear acoustic nondestructive testing of cracks, 14th International Symposium on Novel Aromatic Compounds (ISNA), (1996), 328-334. 4. J.K. Na, J.H. Cantrell, W.T. Yost, Linear and nonlinear ultra sonic properties of fatigued 410Cb stainless steel, in: D.O. Thompson, D.E. Chimenti (Eds), Review of progress in QNDE, 15, 1996, pp.1347-1356. 5. R. Truell, C. Elbaum, B.B. Chick, Ultrasonic Methods in Solid State Physics, Academic Press, New York, (1969), pp. 38-63 6. A. Hikata, B.B. Chick, C. Elbaum, Dislocation contribution to the second harmonic generation of ultrasonic waves, Journal of Applied physics, 36 (1) (1965) 229-338. 7. K.Y.Jhang, K.C.Kim, Evaluation of material degradation using nonlinear acoustic effect, Ultrasonics, 37 (1999) 39–44.

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41 8. R.K.Oruganti, R.Sivaramanivas, T.N.Karthik. Quantification of fatigue damage accumulation using non-linear ultrasound measurements. International Journal of Fatigue, 29 (2007) 20322039. 9. M.Rothenfusser, M.Mayr, J.Baumann. Acoustic nonlinearities in adhesive joints Ultrasonics 38 (2000) 322-326. 10. P.P.Delsanto, S.Hirsekorn, V.Agostini, R.Loparco, A.Koka. Modeling the propagation of ultrasonic waves in the interface region between two bonded elements, Ultrasonics, 40 (2002) 605–610. 11. S. Hirsekorn. Nonlinear transfer of ultrasound by adhesive joints - a theoretical description, Ultrasonics 39 (2001) 57-68. 12] L.M. Brekhovskikh, in: Harcourt, Brace, Jovanovich (Eds.),Waves in Layered Media, 2nd edition, Academic Press, 1980. 13. J.A. TenCate, K.E. Van Den Abble, Laboratory study of linear and nonlinear elastic pulse propagation in sandstone, Journal of Acoustical Society of America, 100 (1996) 1383-1391. 14. I.E. Shkolnik, T.M. Cameron, Nonlinear acoustic methods for strength testing of materials, International Symposium on Novel Aromatic Compounds (ISNA), (1996) 316-327. 15. A. Hikata, B.B. Chick, C. Elbaum, Effect of dislocations on finite amplitude ultrasonic waves in aluminum, Applied Physics l3 (11) (1963) 195-203. 16. W.T. Yost, J.H. Cantrell Jr., M.A. Breazeale, Ultrasonic nonlinearity parameters and third-order elastic constants of copper between 300 and 3 K, Journal of Applied Physics 52 (1) (1981) 126-132.

Journal of Non destructive Testing & Evaluation

Technical Paper

Solution to the third eddy current benchmark problem of WFNDE centers S. Thirunavukkarasu, B. Purna Chandra Rao, S. Shuaib Ahmed and T. Jayakumar Nondestructive Evaluation Division, Metallurgy and Materials Group Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, TN, India E-mail: bpcrao@igcar.gov.in

ABSTRACT This paper presents solution to third eddy current benchmark problem by World Federation of NDE (WFNDE) Centers. CIVA modeling software has been used for predicting the impedance changes for a pancake type eddy current coil in presence of a rectangular slot in an Inconel-600 tube, as defined in the problem. Quantitative comparison of the model predictions has been made with the experimental data provided by WFNDE centers. A very good agreement has been observed at lower frequencies. For higher frequencies, transfer function based approach has been followed to account for non-linear effects associated with cable and contacts that have not been modeled. This approach has been successfully validated. Keywords: Eddy current, CIVA software, Inconel-600, tube testing, benchmark problem

1. INTRODUCTION The World Federation of Nondestructive Evaluation Centers (WFNDEC) was founded in July 1998 with an objective to improve NDE technology and its uniform application on a worldwide scale [1]. The founding members of WFNDEC include leading NDE research centers in Argentina, Belarus, Brazil, China, India, Republic of Korea, Russia, South Africa, Ukraine, and the United States with a permanent secretariat at Centre for NDE, Iowa State University, Ames, USA. Indira Gandhi Centre for Atomic Research (IGCAR) is a founding member with active participation in all the WFNDEC activities. Towards benchmarking of various NDE techniques, the WFNDEC regularly defines benchmark problems in ultrasonics, magnetic flux leakage and eddy current techniques, to the NDE community for solving by numerical or analytical means. The WFNDEC has announced in February 2012, a new benchmark problem in eddy current technique and this is the third in the series [1]. The first eddy current benchmark problem was purely based on axisymmetric model (coil and defect configurations) and involves the prediction of impedance changes of a bobbin differential coil due to groove type defects and support plates [2]. The second benchmark problem had an axisymmetric tube coil condition with 3D localized flaws. Both finite element and semi numerical approaches yielded accurate prediction of the second benchmark problem [3,4]. The third problem is completely free from axi-symmetry. The objective of the problem is to predict the impedance changes in a pancake type eddy current coil due to a rectangular slot in an Inconel-600 tube at five different frequencies and validation of the predictions using controlled experimental measurement data provided by the WFNDEC. The third benchmark problem is challenging. The authors used CIVA software for solving this benchmark Journal of Non destructive Testing & Evaluation

problem. This paper presents the definition of the problem, solution method employed, model predictions and comparison of the model predictions with experimental data.

2. DEFINITION OF THE EDDY CURRENT BENCHMARK PROBLEM The third eddy current benchmark problem has been defined to predict the impedance changes of a pancake type coil at different axial positions due to the presence of a rectangular slot (simulated crack) in an Inconel-600 (nonferromagnetic) tube [1]. The impedance changes are to be predicted at excitation frequencies of 25 kHz, 50 kHz, 100 kHz, 150, kHz and 200 kHz. The cross sectional view illustrating the benchmark problem is shown in Figure 1.

Fig. 1 : Cross sectional view defining the third eddy current benchmark problem. Vol. 11, Issue 2 September 2012

Technical Paper

Table I : Dimensions and material properties defined by WFNDEC Geometry Tube

Material Inconel-600

Electrical conductivity, MS/m 0.84

Tube dimensions, mm OD - 18.990 Thickness -1.175

Other details Nil

Coil

Copper

59.80

OD - 7.836 ID - 3.058 Height -1.044 (x2-x1 as shown in Fig. 1)

Lift-off (λ) - 1.1mm Number of turns 305

Defect

Air filled rectangular

0.00

Length-12.20 Width-0.085 Depth-1.175

85 ìm slot through thickness slot

The slot is through wall thickness with small opening of 0.085 mm, simulating a crack. The tube, coil, slot dimensions and the test conditions to be used by CIVA model are given in Table I. WFNDEC has organized for performing controlled experiments and provided impedance change data for model validation purpose. The slot was fabricated by electrical discharge machining and the impedance change measurements were made using an impedance analyzer. Experimental data was acquired with the coil placed symmetrically over the slot and moved incrementally in the axial direction in steps of 1.0 mm. The impedance changes were measured at 25 kHz, 50 kHz, 100 kHz, 150, kHz and 200 kHz and this data was shared with the authors on request.

3. MODELING OF THE BENCHMARK PROBLEM USING CIVA SOFTWARE As there is no axi-symmetry in the problem geometry with respect to the coil axis due to curved surface of the tube, 2D axi-symmetric modeling is not possible. 3D Finite element and boundary element techniques are, in general, computationally intensive [5, 6]. In this context, use of semi-analytical approach such as the one used in CIVA is advantageous. CIVA is benchmarked software for numerical simulation of ultrasonic, eddy current and radiography techniques. The eddy current module of CIVA software is based on semi-analytical methods using dyadic Greens function approach [4, 7, 8]. In this approach, the interaction between defect and electric field generated by the probe is described with an integral equation given below.

–—

ee Ω containing the defect. The dyad GΩ links the fictitious current density to the electric field it creates inside Ω. The contrast function f(r) in equation (1) is defined by

(2)

where r is the position coordinates, σ 0 is the tube conductivity and σ(r) is the flaw conductivity. CIVA eddy current module has been extensively validated through a series of experiments [8]. The CIVA software version 9.2 was used for solving the benchmark problem. This version of the software is not capable of solving 3D coils inside a tube, rather it solves for equivalent plate geometry ignoring the curvature. However, the influence of curvature effects is minimal for the pancake type localized probe. Figure 2 shows the 3D geometry of the benchmark eddy problem with absolute pancake type coil and rectangular slot. In order to predict the impedance changes at different axial positions, the coil was moved from -24 mm to +24 mm along the axial direction at a scan pitch of 1 mm with the defect center at the origin as carried out in the experiments. An optimized mesh based on trial and error and memory limitation was chosen for solving the problem. The optimized mesh had 15 elements along the length, 10 elements along the thickness and 20 elements along the width of the slot.

(1)

where JΩ is the unknown fictitious current density defined in the volume Ω containing the defect and depends on the total electric field. This equation is derived from the Maxwell’s equations and is solved numerically using the method of moments. The solved current density is used for calculating the probe response or signal from a defect. The term J0 in (1) is an excitation term that depends on the total primary electric field E0(r) by the probe in the region Vol. 11, Issue 2 September 2012

Fig. 2 : CIVA model geometry of the eddy current benchmark problem. Journal of Non destructive Testing & Evaluation

Technical Paper

Fig. 3 : Model predicted and experimental impedance changes of the pancake coil due to the rectangular slot at different frequencies.

3. RESULTS Figure 3 shows the model predicted impedance changes for the rectangular slot when the coil is scanned in the axial direction. Figure 3 also shows the experimental data given by the WFNDE centers. A difference in the amplitude and phase angle of the model predictions was observed with the experimental measurements and this difference is more prominent at 150 and 200 kHz. There exists a huge difference when the coil is exactly placed over the middle of the slot. This is attributed to skin effect, lift-off and other nonlinear effects associated with cable as well as contacts [10]. In order to quantitatively assess the closeness of the model predictions with the experimental measurements, the sum of Euclidian distances (D) of individual impedance points was used. The computed D-values for different frequencies are given in Table II. The difference between model predictions and experimental measurements increases monotonically with increase in the excitation frequency. This increase suggests that the difference is predominantly due to the nonlinear effects associated with cable impedance at higher frequencies in obtaining the experimental data. These unknown effects influence the experimental measurements, which results in the difference between the model predictions and experimental measurements. Hence, it is appropriate to take into account the effect due to the above said variables using transfer function approach [9,10]. The transfer function T takes the form given below: ΔZ E = T×ΔZ

(3)

where ΔZE is the experimentally measured impedance change and ΔZM is the CIVA model predicted impedance change of the pancake coil. In the present case, ΔZE and ΔZM are 49 x 5 matrices with complex impedance data (rows representing scan positions and columns representing frequency). Hence, the transfer function T should essentially be a 49 x 49 matrix. In order to compute the transfer function, matrix inversion method has been used as shown below: Journal of Non destructive Testing & Evaluation

(4)

Thus, to compute T, it is necessary to find the inverse of the rectangular matrix ΔZM. In order to find the inverse of ΔZM, singular value decomposition (SVD) has been used [11]. In linear algebra, SVD is used for factorization of real or complex matrices. Formally, the SVD of an m×n real or complex matrix M is a factorization of the form M=UΣV*, where U is an m×m real or complex unitary matrix, Σ is an m×n rectangular diagonal matrix with nonnegative real numbers on the diagonal, and V* (the conjugate transpose of V) is an n×n real or complex unitary matrix. The diagonal entries Σ are known as the singular values of M. Computing the inverse of M is easy, as the inverse of unitary matrices U and V is the conjugate transpose of the matrices themselves and the inverse of the diagonal matrix Σ is, nothing but, the reciprocal of the diagonal elements. Hence, M-1=VΣ-1U*

(5)

Figure 4 shows the surface plot of the absolute value of complex transfer function, T computed by matrix inversion method using SVD. The shape of the transfer function is similar to the sinc function. The model predicted impedance changes were corrected by multiplying this transfer function matrix to get impedance changes data equivalent to the experimental data. Figure 5 shows the corrected model predictions and the experimental impedance changes. As can be seen, model predictions are in good agreement with the experimental measurements. The Euclidian distance metric (D) after accounting for the transfer function characteristics for different frequencies are also shown in Table II. As can be noted, the magnitude of D’s is small and tending to zero. This indicates that the corrected model Vol. 11, Issue 2 September 2012

Technical Paper

Fig. 4 : Surface plot of the absolute value of the complex transfer function computed by matrix inverse method.

Fig. 6 : Results of the leave-one out cross-validation strategy for 150 kHz and 200 kHz impedance data not being used for computing the transfer function.

predictions through the established transfer function are exactly similar to the experimental measurements. It must be noted here that T is specific to a probe and it must be determined for each and every probe. In order to further study the transfer function approach, a leave-one out cross-validation strategy has been adopted. In this, one frequency data was not used to compute the transfer function matrix while model validation was performed for all the selected frequencies using the transfer function matrix. Figure 6 shows the result of the leave-one out cross-validation strategy when 150 kHz data 200 kHz data were not used for computing the transfer function. As can be observed, a good agreement between experimental and model predictions exists even when the data from one of the frequencies was not used for computing T. The computed Euclidian distance metric in both the cases was found to be 7.3904 and 13.3177 respectively for 150 kHz and 200 kHz data, which is reasonably good. Table II : Computed Euclidian distance for different frequencies. Frequency, kHz

Euclidian distance (D) D after transfer for model predicted data function correction

8.8717

0.0449x 10-4

15.3214

0.0876x 10-4

100

26.5983

0.1524x 10-4

150

36.5687

0.2007x 10-4

200

45.8545

0.2380x 10-4

4. CONCLUSION

Fig. 5 : Model predicted signals after transfer function correction and experimental impedance changes of the pancake coil at different frequencies. Vol. 11, Issue 2 September 2012

CIVA modeling software was used for solving the third eddy current benchmark problem, towards predicting the impedance changes of a pancake type eddy current coil at different axial positions due to the presence of a rectangular slot (simulated crack) in an Inconel 600 tube. The transfer Journal of Non destructive Testing & Evaluation

46 function approach has been adopted to account for the variables that are not considered in the model. The transfer function was computed by matrix inverse method using Singular Value Decomposition. The model predictions after correction by transfer function are in good agreement with the experimental measurements (obtained from the WFNDE centers).

ACKNOWLEDGEMENTS Authors thank Shri S.C. Chetal, Director, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam for his encouragement and support. Authors also thank Shri S. Mahadevan, Scientific Officer, NDE Division, IGCAR, for many useful technical discussions during the course of this work. Shri S. Shuaib Ahmed is thankful to IGCAR for providing the research fellowship.

REFERENCES 1. h t t p : / / w w w. w f n d e c . o r g / b e n c h m a r k p r o b l e m s _ f i l e s / 2012%20EC%20Benchmark%20Announcement.pdf. 2. Y. Li, Z. Zhang, Y. Sun, L. Udpa and S. Udpa, “Numerical Simulation Results for the Eddy Current Benchmark Problem”, Proceedings of the conference on Rev. of Prog in QNDE, edited by D. O. Thompson and D. E. Chimenti, Plenum, New York, Vol. 21, pp. 1902-1908, 2002. 3. Tian, Y., Li, Y., Udpa L. and Udpa, S., “Simulation of the World Federation’s Second Eddy Current Benchmark Problem”, Proceedings of the conference on Rev of Prog in QNDE, Edited by D. O. Thompson and D. E. Chimenti, Plenum, New York, Vol. 22, pp. 1816-1823, 2003.

Journal of Non destructive Testing & Evaluation

Technical Paper 4. G. Pichenot, C. Reboud, R. Raillon, and S. Mahaut, “Results of 2007 benchmark obtained with CIVA at CEA: Prediction of ECT inspection over tubes with 2D and 3D flaws”, Proceedings of the conference on Rev of Prog in QNDE, Edited by D. O. Thompson and D. E. Chimenti, Plenum, New York, Vol. 27, pp. 1775-1782, 2008. 5. Nathan Ida, “Numerical modeling for electromagnetic nondestructive evaluation”, Chapman & Hall Publisher. 6. S. Thirunavukkarasu, B.P.C. Rao, S. Mahadevan, T. Jayakumar, Baldev Raj, Z. Zeng, L. Udpa and S.S. Udpa, “Finite element modeling for detection of localized defects using remote field eddy current technique”, Journal of Research in Nondestructive Evaluation (Taylor and Francis), Vol.20, No:3, pp. 145-258, 2009. 7. C. Reboud, D. Prémel, G. Pichenot, D. Lesselier and B. Bisiaux, “Development and validation of a 3D model dedicated to eddy current nondestructive testing of tubes by encircling probes”, International Journal of Applied Electromagnetics and Mechanics, Vol. 25, pp. 313-317, 2007. 8. C. Reboud, G. Pichenot, D. Prémel and R. Raillon, “2008 Benchmark Results: modeling with CIVA of 3D flaws responses in planar and cylindrical work pieces””, Proc. of the conference on Rev of Prog in QNDE, Edited by D. O. Thompson and D. E. Chimenti, Vol. 28, 1915-1921, 2009. 9. B.P.C. Rao and N. Nakagawa, “Validation of boundary element model for eddy current NDE”, Proc. of the workshop on electromagnetic NDE (eNDE-2004), 2004. 10. B.P.C. Rao and N. Nakagawa, “A study of boundary element eddy current model validation”, AIP Conference proceedings, Rev of progress in QNDE-2005, Edited by D.O. Thompson and D.E. Chimenti., Vol. 25, 2005. 11. William H. Press, Saul A. Teukolsky, William T. Vetterling, Brian P. Flannery, “Section 2.6, Numerical Recipes: The Art of Scientific Computing (3rd ed.)”, New York: Cambridge University Press, ISBN 978-0-521-88068-8.

Vol. 11, Issue 2 September 2012

Technical Paper

Characteristics of turning Lamb modes in composite sub-laminates C. Ramadas2, Krishnan Balasubramaniam1, Avinash Hood2 and C.V. Krishnamurthy3 1

Center for Non-destructive Evaluation, Indian Institute of Technology Madras, Chennai-600 036, INDIA 2 Composites Research Center, R & D E (E), Dighi, Pune-411 015, INDIA 3 Department of Physics, Indian Institute of Technology Madras, Chennai-600 036, INDIA

ABSTRACT When the fundamental anti-symmetric Lamb mode (Ao) propagating in a sub-laminate encounters a delamination edge, reflection and transmission into the main laminate and sub-laminate takes place. The Lamb mode propagating from one sub-laminate to the other is termed as ‘Turning Lamb Mode’. Numerical and experimental studies, employing air-coupled transducers, were carried out to understand the variations in reflection and transmission factors of Lamb modes in the sub-laminates of glass/epoxy quasi-isotropic laminates. The power reflection and transmission coefficients of the reflected and turning Lamb modes were also estimated for various interfaces of delaminations in three laminates - unidirectional, cross-ply and quasi-isotropic. The variations in transmission factor and power transmission coefficient, with respect to the thickness ratio, of ‘Turning Lamb Mode’ were observed to be dissimilar. Keywords: Turning Lamb mode, Power transmission coefficient, Power reflection coefficient, Air-coupled transducers, GFRP composites.

1. INTRODUCTION Delamination is one of the damages that debilitate a composite structure’s performance in flexural and compression loading. In recent times, many researchers [1] explored the use of ultrasonic Lamb waves [2] for Structural Health Monitoring (SHM) and Non-destructive Evaluation/Testing (NDE/T) of laminated composite structures. The main complexity involved with Lamb waves, when employed for NDE and SHM applications, is analysis and interpretation of received Lamb wave signal. This is because, during the interaction of Lamb waves with delamination type defects, Lamb waves exhibit characteristics like mode conversion, amplitude reduction, reflection and transmission etc. Some studies were carried out on the interaction phenomenon between Lamb modes and various defects. Žukauskas and KaŽys [3] investigated both numerically and experimentally, the interaction of ultrasonic wave with a delamination type defect in GLARE3-3/2 composite sample in the through transmission mode. It was shown that in a defective zone additional Ao modes were generated. Guo and Cawley [4] studied the interaction of Lamb waves with delaminations in a cross-ply composite laminate. Investigations were carried out on delaminations using the fundamental symmetric Lamb mode (So-) in a pulse-echo configuration. Delaminations were introduced at various interfaces between the plies. When So mode encounters a delamination in its propagation path, a reflected wave was shown to be generated. It was found that, when shear stress is zero at that interface, no wave reflection could be observed. Karthikeyan et al. [5] and Ramadas et al. [6] studied, both numerically and experimentally, the interaction of primary anti-symmetric Lamb mode with symmetric delaminations, by using air-coupled ultrasonic transducers. Symmetrical delaminations were investigated Vol. 11, Issue 2 September 2012

in pitch-catch mode. It was found that when Ao mode interacts with a symmetric delamination, it generates a new mode, S-o-, which is confined only to the sub-laminates i.e. within the delamination region. This mode cannot be detected in the main laminate, although a mode converted Ao mode (when the So mode interacts with the exit portion of the delamination) can be used to infer the presence of the So mode in the delamination. Zhou and Yuan [7] analytically studied the reflection and transmission of flexural modes in an isotropic beam containing a semi-infinite axial crack. It was observed that the power of the reflection and transmission coefficients depend on both the frequency and the position of the crack across the beam thickness. Finally the results were verified using conventional finite element method (FEM). Yuan et al. [8] carried out analytical studies on flexural wave reflection and transmission from main beam to sub-beams in uni-directional (UD) composite beams having a semiinfinite delamination of open and closed nature. It was found that the portion of reflected and transmitted power depends strongly on the frequency of incident wave and position of delamination across beam thickness. The results were verified using FEM. Wang and Rose [9] investigated wave propagation in isotropic beams containing closed semi-infinite delamination. A good understanding of the interaction phenomenon between Lamb modes and defects helps in efficient implementation of damage detection techniques. Numerical simulations give a better picture, on interaction phenomenon, than experiments. Delamination in a composite laminate divides the main laminate, locally, into two sub-laminates. When a Lamb mode is incident at the edge of a delamination, it reflects back and also transmits from one sub-laminate to the other as ‘Turning Lamb Mode’ (TLM) [10-11]. Ramadas et. al. [10] estimated the Journal of Non destructive Testing & Evaluation

48 transmission factors, based on amplitudes, of turning Lamb modes using Hilbert Transform. The main focus of this work is to study the variations in power reflection coefficient of Lamb mode in sub-laminates and power transmission coefficients of TLM with respect to thickness ratio, which is defined as the ratio of thickness of sub-laminate in which Lamb mode is excited, to the main laminate thickness. Numerical simulations were carried out on three different laminates, unidirectional (UD), cross-ply (CP) and quasi-isotropic (QI). Transmission and reflection factors, based on wavelet transform, of Lamb modes were estimated for QI laminate and compared with experimentally obtained values. It was observed that the nature of variations in transmission and reflection factors with respect to thickness ratio were same. Subsequently, power transmission and reflection coefficients of Lamb modes, when interface of delamination is at various locations across the laminate thickness, in all three laminates – UD, CP and QI, were estimated. There was found to be a new trend in the nature of variations in power transmission coefficients, with respect to thickness ratio, of TLMs. The organization of this paper is as follows. Section two deals with numerical simulations carried out on QI. Section three illustrates the experimental work carried out on QI laminates. Estimates of power transmission and reflection coefficients are brought out in Section four. Results and discussion is presented in Section five. The paper concludes in Section six with a recapitulation of the important findings.

2. NUMERICALLY SIMULATED TRANSMISSION AND REFLECTION FACTORS Fig 1(a) shows the specifications of the model used for simulations. Numerical modeling was carried out using finite element code, ANSYS, on QI glass/epoxy (GFRP) laminate having stacking sequence of [0/±45/90]s. Table-1 lists the mechanical properties of GFRP material. Since the thickness of each lamina was 0.33 mm, the total thickness of each laminate was 2.64 mm. Damping was not considered in numerical simulations. Excitation pulse modulated by Hanning window had five cycles with 200 kHz as central frequency of excitation. Lamb mode employed in the investigation was the fundamental antisymmetric mode (Ao). This mode was excited by giving the displacement pattern, obtained from DISPERSE [12], across the sub-laminate thickness. The type of element used for FE modeling was a solid element. There are three translatory degrees of freedom (DoF) at each node. Delamination was modeled by demerging the nodes at that location. The type of delamination considered in this work is ‘semi-infinite’ because the delamination started from one of the edges of the plate and extended up to half the plate length (150 mm), and had only one edge as shown in Fig 1(a). The length of delamination was selected in Journal of Non destructive Testing & Evaluation

Technical Paper

such a manner that the arrival time of reflected wave groups is much higher than those from the delamination edge. The wave groups captured in this work were propagating towards and/or from the delamination edge. Since each laminate contains a total number of eight plies, stacked symmetrically with mid-plane, it is possible to have a delamination at any one of the four interfaces across the thickness. Table 1 : Material properties Material

E11(GPa) E22(GPa) u13

Glass/Epoxy 44.68

6.90

0.280

u23 G13(GPa) rkg/m3 0.355

2.54

1990

The following delineates the numerical simulation carried out when the interface of delamination introduced was between the plies [90] and [-45]. The transmitter (T) and receiver (R1) were on the top sub-laminate while the receiver (R2) was placed over the bottom sub-laminate as shown in Fig 1(a). Receivers R1 and R2 capture the A o mode generated by the transmitter, reflection from the delamination edge and the TLM, respectively. A-scan captured at receiver R1 is shown in Fig 1(b). The first wave group was the incident Lamb mode and the second one is the reflection of the first from the edge of delamination. Over each wave group, wavelet transform was carried out. The mother wavelet chosen was Morlet. Figs 1(b) and 1(d) show the envelope of the wavelet coefficients and time-frequency representation (using wavelet transform), respectively of the wave groups, the incident and reflected. Fig 1(c) shows the TLM captured by the receiver R2. The envelope of wavelet coefficients was fitted over the TLM. Time-frequency representation is shown in Fig 1(e). From time-frequency representation it was found that the central frequency was approximately 200 kHz and the arrival times of wave groups closely matched with those of the Ao modes propagating in the sub-laminates. The reflection factor of the reflected wave group is defined as the ratio of the peak of wavelet coefficients of the reflected wave group (Fig 1(b)) to the incident (Fig 1(b)). Similarly, the transmission factor of the TLM is defined as the ratio of the peak of wavelet coefficients of the TLM (Fig 1(d)) to the incident (Fig 1(b)). Delamination was introduced between each interface of the plies and in each case, the reflection and transmission factors were computed. These factors were plotted with respect to the ‘thickness ratio’, which is defined as the ratio of the sub-laminate thickness to the main laminate thickness. From numerical simulations it was found that, the reflection factor was found to decrease with increase in the thickness ratio , whereas the transmission factor increased with increase in the thickness ratio as shown in Figs 4(a) and 4(b).

3. EXPERIMENTAL WORK Experiments were carried out on GFRP quasi-isotropic laminate having a ply sequence of [0/+45/-45/90]s. Vol. 11, Issue 2 September 2012

Technical Paper

Fig. 1 : (a) Model used for numerical simulations, (b) and (c) A-scans obtained at receivers R1 and R2 respectively, (d) and (e) wavelet transforms of A-scans shown in (b) and (c) respectively.

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Since the thickness of each ply was 0.33 mm, the total thickness of the laminate was 2.64 mm. Four specimens of each size 300 ´ 200 mm2 were fabricated using the Resin Film Infusion (RFI) technique. Delamination was introduced in between the plies (a) [0] and [+45] (b) [+45] and [-45] (c) [-45] and [90] and (d) and [90] and [90] in the first, second, third and fourth specimen, respectively. Size of delamination introduced in each specimen was 150 ´ 100 mm2 as shown in Figure 2. Air-coupled ultrasonic transducers with a central frequency of 200 kHz, provided by Ultran Group, USA, were used for carrying out experiments. Pitch-catch arrangement was employed. Fig. 2 : Quasi-isotropic laminate with semi-infinite delamination.

The angles of air-coupled probes were adjusted to generate and receive Ao modes in the sub-laminates. The distances of separation between the probes and their locations over the sub-laminates are shown in Fig 3. Ultrasonic probes were oriented in three different configurations to capture the incident, reflected and turning Lamb modes. Figs 3(a), 3(b) and 3(c) show the timefrequency representation of the incident, reflected and the TLM respectively. The same procedure depicted in numerical simulations was followed here for computation of the transmission and reflection factors. Figs 4(a) and 4(b) show the variations in the reflection and transmission factors with respect to the thickness ratio.

4. POWER REFLECTION AND TRANSMISSION COEFFICIENTS The power reflection and transmission coefficients were estimated through numerical simulations, assuming plane strain condition, on an assortment of three symmetric laminates – UD, CP and QI having stacking sequence of [04]s, [0/90/90/0]s and [0/+45/-45/90]s respectively. The power associated with the reflected and transmitted Ao wave groups was estimated through numerical simulations. The following expression [1] gives the time averaged power flow, <P>, across any cross-section.

(1)

where, the stresses sxx, txz and displacements u and w are functions of x, y and t (time). From equation (1) it is clear that the integration has to be performed across the laminate thickness in the time interval, to. Power transmission coefficient is defined as ratio of power transmitted to the incident power. Similarly, power reflection coefficient is defined as ratio of power reflected to the incident power.

Fig 3 : (a) Incident wave group at delamination edge (b) reflected wave group from delamination edge (c) turning wave group. Journal of Non destructive Testing & Evaluation

To begin with, the semi-infinite delamination was modeled between the first lamina and the second lamina in UD laminate. The top and bottom sub-laminates were [0] and [07] respectively. Ao mode was excited over [0] sublaminate by giving appropriate displacement profile taken Vol. 11, Issue 2 September 2012

Technical Paper

Fig. 4 : Variation of (a) reflection factor and (b) transmission factor with respect to the thickness ratio.

from DISPERSE. There were three receivers, one each over the top sub-laminate, bottom sub-laminate and main laminate. The receiver at the top sub-laminate captured the incident and reflected Lamb modes from the delamination edge. The receivers at the bottom sub-laminate and the main laminate captured the TLM and the transmitted Lamb mode respectively. The average power associated with each wave group was calculated using equation (1). The power reflection and transmission coefficients were estimated as defined above. Now, the delamination was introduced between the second and third laminae. Again, the procedure described above was followed to calculate the power associated with reflected and transmitted Lamb modes. This process was repeated for all seven interfaces of delaminations in each laminate â&#x20AC;&#x201C; UD, CP and QI. Figures 5 (a), (b) and (c) show the variation in power reflection and transmission coefficients of the reflected and transmitted Ao Lamb modes. Vol. 11, Issue 2 September 2012

Fig. 5 : Variation of power (a) reflection coefficients, (b) transmission coefficients of TLM and (c) transmission coefficients of Lamb mode transmitted into the main laminate.

5. RESULTS AND DISCUSSION Numerical simulations carried out on GFRP QI laminate showed that the reflection and transmission factors defined based on the wavelet transform were found to decrease and increase respectively with respect to thickness ratio. Journal of Non destructive Testing & Evaluation

52 Experiments were carried out, employing air-coupled ultrasonic probes, on GFRP QI laminates containing semiinfinite delamination as shown in Fig 2. The reflection and transmission factors were estimated in each laminate. Figs 4(a) and 4(b) show variation in reflection and transmission factors of the reflected Lamb mode and the TLM with respect to thickness ratio. The nature of variation in reflection and transmission factors in numerical simulations and experiments was identical. The mismatch between the numerically simulated and experimentally obtained reflection and transmission factors was attributed mainly to attenuation. In numerical modeling, attenuation was not taken into account. However, this endeavor revealed that the trend in variation of reflection and transmission factors with the thickness ratio in numerical modeling and experiments is same. The power associated with the incident, transmitted and reflected Lamb modes was calculated using equation (1) on a variety of GFRP laminates - UD, CP and QI containing semi-infinite delaminations. Fig 5(a) shows the variation in power reflection coefficient of Lamb mode reflected at the edge of delamination. The power reflection coefficient decreased with increase in the thickness ratio. When the thickness ratio was 0.125, the power transmission coefficient was around 0.75 in all three laminates â&#x20AC;&#x201C; UD, CP and QI. With subsequent increase in the thickness ratio from 0.125 to 0.875, the power transmission coefficient decreased from 0.75 to 0.015 (approx). An interesting phenomenon was noticed in case of the TLM. The power associated with the TLM was found to increase when the interface of delamination was moved from the top lamina to the fifth lamina. When the interface of delamination was in between the fifth and sixth laminae (thickness corresponds to 0.625), the power transmission coefficient of the TLM was high as shown in Fig 5(b). The power transmission coefficient was found to decrease when the interface of delamination was moved further down. Initially, the variation in power transmission coefficient with respect to thickness ratio (from 0.125 to 0.375) was very high. When the thickness ratio was further increased from 0.375 to 0.625, variation in power transmission coefficient was low as shown in Fig 5(b). For the thickness ratio from 0.625 to 0.875, variation in power transmission coefficient with respect to thickness ratio was again very high as seen in Fig 5(b). Among all three laminates, the maximum value of power transmission coefficient was 0.35 in QI laminate, followed by UD and CP laminates, at the thickness ratio, 0.625. When Ao was incident at the edge of delamination, it transmitted into the main laminate through the edge of delamination. Fig 5(c) shows the power associated with the Lamb modes transmitted into the main laminate. The power transmission coefficient of the Lamb mode transmitted into the main laminate was found to increase with increase in the thickness ratio. The power transmission coefficients vary from 0.05 to 0.8 (approx) for the thickness ratio variations from 0.125 to 0.875, respectively as shown in Fig 5(c). Journal of Non destructive Testing & Evaluation

Technical Paper

The trend in variation in power transmission and reflection coefficients in all three laminates â&#x20AC;&#x201C; UD, CP and QI for all seven interfaces of delamination is observed to be the same. The reflection factor (based on wavelet coefficients) and the power reflection coefficient (based on power) of Lamb modes exhibit a decreasing trend with increase in thickness ratio as shown in Figs 4(a) and 5(a), respectively. The transmission factor (based on wavelet coefficients) and the power transmission factor (based on power) of the TLM exhibit dissimilar behavior with increase in the thickness ratio as shown in Figs 4(b) and 5(b) respectively. The transmission factor increases with increase in the thickness ratio, whereas the power transmission coefficient reaches the maximum at thickness ratio 0.625, then starts decreasing with increase in the thickness ratio. At a given thickness ratio, the sum of the power reflection coefficient and power transmission coefficients is not equal to unity because of the following reason. When Ao mode is incident at a delamination edge, in addition to reflection and transmission, it also generates a new mode, So, which also propagates along with the reflected and transmitted Ao modes. Since there is some power associated with So mode as well, the total power carried by the reflected and transmitted Ao modes is not equal to unity. This study has revealed the fact that the trend of variations in power transmission coefficients of the TLM with respect to the thickness ratio is completely different from that of the transmission factor.

6. CONCLUSIONS Numerical and experimental studies carried out on the transmission and reflection characteristics of Lamb modes in the sub-laminates revealed that amplitudes of the Turning Lamb Mode and reflected Lamb mode increase and decrease respectively with increase in the thickness ratio. Variation in power reflection coefficient with respect to the thickness was also found to follow a similar trend as that of the reflection factor. There was an increase in amplitude of the Turning Lamb Modes with every increase in thickness ratio, whereas the power was found to rise to maximum when the thickness ratio was 0.625, and then showed a decreasing trend with increase in the thickness ratio.

ACKNOWLEDGEMENTS The authors acknowledge the help rendered by Dr. Rahul Harshe and Mr. Vinod Durai Swami for fabrication of laminate by RFI process, all from R&DE (E), Pune. Help rendered by Mr. Janardhan Padiyar from CNDE, IITM in experiments is acknowledged. The authors gratefully acknowledge the critical comments given by Prof Peter Cawley, Imperial College, London.

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REFERENCES 1. Raghavan A and Cesnik, C. E. S (2007), Review of guided wave structural health monitoring. Shock Vib. Dig. 39(2), 91-114.

53 7. Zhou Li and Yuan Wanchun (2008), Power reflection and transmission in beam structures containing a semi-infinite crack, Acta Mechanica Solida Sinica, 21(2), 177-188.

2. Rose, J.L. (1999), Ultrasonic Waves in Solid Media. Cambridge University Press, Cambridge.

8. Yuan Wan-Chun, Zhou Li and Yuan Fuh-Gwo (2008), Wave reflection and transmission in composite beams containing semiinfinite delamination, J. of Sound and Vibration, 313, 676-695.

3. Žukauskas E and KaŽys R (2007) Investigation of the delamination type defects parameters in multilayered GLARE33 / 2 composite material using air – coupled ultrasonic technique. Ultragarsas (Ultrasound), 62, 44 – 48

9. Wang C H and Rose J L (2003), Wave reflection and transmission in beams containing delamination and inhomogeneity, J. of Sound and Vibration, 264, 851-872.

4. Guo N and Cawley P (1993), The interaction of Lamb waves with delaminations in composite laminates, J. Acoust. Soc. Am. 94, 2240-46. 5. P. Karthikeyan, C. Ramadas, M. C. Bhardwaj and Krishnan Balasubramaniam (2009), Non-Contact Ultrasound Based Guided Lamb Waves for Composite Structure Inspection: Some Interesting Observations AIP Conference Proceedings, Rev. of Prog. QNDE (Ed. D. Thompson and D.E. Chimenti) Vol. 28 1096, 928. 6. C. Ramadas, Krishnan Balasubramaniam, M. Joshi and C.V. Krishnamurthy (2009), Interaction of primary anti-symmetric Lamb mode with symmetric delaminations: Numerical and experimental studied. Smart Mater. and Struct., 18(8), 085011.

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10. C. Ramadas, Krishnan Balasubramaniam, M. Joshi and C. V. Krishnamurthy (2011) Numerical and experimental studies on propagation of Ao mode in a composite plate containing a semiinfinite delamination: Observation of turning modes. Composite Structures, Vol. 93(7), 1929-1938 11. C. Ramadas, Krishnan Balasubramaniam, M Joshi and C V Krishnamurthy (2011) Reflection and transmission of Lamb waves in sub-laminates, Proceedings of 16th International Conference on Composite Structures, June 2011, Porto, Portugal 12. DISPERSE Software version 2.0.16b (2003), Imperial College, London, UK

Journal of Non destructive Testing & Evaluation

Technical Paper

Rail Weld Inspection Using Phased Array Ultrasonics Girish.N.Namboodiri1, Krishnan Balasubramaniam2, T.Balasubramanian1, Jerry James2, Sriharsha2 1

National Institute of Technology, Tiruchirappalli 2 Indian Institute of Technology, Madras

ABSTRACT Inspection of rail welds has always been a challenge to the Railways. The conventional ultrasonic methods which are employed now for the detection of defects are not found to be good enough for defects that exist in different parts of the weld. Phased Array Ultrasonics which performs sectorial scanning could be used effectively for detection of defects in rail welds. The analysis of phased array images basically concentrates on defects that are volumetric in rail. The feasibility studies conducted in parts of rail other than welds were promising. Defect indications were seen very much separately from its surroundings. Accurate positioning of the defect is possible. Close lying defects can be seen separately which assures better resolution. Linear normal scans were very much suitable to detect cracks of complex geometries, as it gives a specific indication pattern each time when it is present. Keywords: Phased Array, Sectorial Scan, Rail Weld, Cracks PACS: 43.38.Hz, 81.70.-q, 43.35.Zc, 81.70.Cv

INTRODUCTION Most commonly used methods of welding rails in India are alumino thermic welding and flash butt welding.Alumino thermic welds are used to weld rails in areas where there is a lack of power source. Basically alumino thermic reactions which are exothermic chemical reaction are made use in this process of welding. Strength of this weld just comes to about 56% of the parent rail. It is having a high failure rate compared to flash butt weld and the quality of the weld is also poor. Thermite welds are more prone to corrosion also. A wide variety of defects also occur in these types of welds. Porosity, blowholes, cracks; slag inclusions are a few of them. These defects may arise at any region of the weld but mostly the defects which arise from bottom of the weld are found to be more dangerous. On the other hand flash butt welds are good quality welds with strength almost equivalent to that of parent rails. Their failure rate is very less compared to alumino thermic welds. Very few defects namely lack of fusion and oxide inclusions arise in them. Requirement of a power source and a unique mobile flash butt welding machine limits its use in remote areas where alumino thermic welds are mostly preferred. Greg Garcia [1] carried out his research which consisted of 2 phases as a part of the program to determine how phased array ultrasonic technology can be applied towards the inspection of rail in service. The phased array research effort was performed in conjunction with RD Tech/Olympus NDT, a manufacturer and supplier of ultrasonic and eddy current phased array systems. A phased array approach for rail flaw detection and sizing performed by Transportation Technology Centre Inc. under Federal Railroad Administration sponsorship has been focused on sizing transverse defects located in the railhead. The Phased array process evaluated during this research effort uses an Journal of Non destructive Testing & Evaluation

electronic scanning method of transmitting and receiving ultrasonic energy from various locations of the railhead. A detailed study on the defects that arise at various positions in the rail was done [2]. Total cost of all rail failures was also estimated and found that controlling rail failure could reduce it to a great extent. A novel damage detection technique based on wave propagation of rails was proposed in the year 2006 which was concentrated on the identification of structural surface damage on rail structures [3]. At present, conventional ultrasonic probes are used in rail weld inspection. A detailed study has been conducted to check the probability of detection of this technique. A periodic testing of complete weld by hand probing of weld head/web and bottom flange is done using 0° 2 MHz, 70° 2 MHz, 45°2 MHz and 70° 2 MHz probes. Out of these, the 0° 2 MHz normal probe scanning is aimed at detection of defects like porosity, blowholes etc. in thermite welds and for the detection of oxide inclusions as in the case of flash butt welds. The phased array linear scan at 0° using a 2 MHz probe can perform the same objective in a much better way. Apart from detection of the defects mentioned above, it could also detect the cracks if present inside the weld. The image pattern generated by the crack helps in easy characterisation and analysis. The 70° 2 MHz head scan is intended to pick up blowholes, lack of fusion, slag inclusion etc. in the head region of the weld. Phased array sectorial scan using 2 MHz probe at an angular range of 10° to 60° can very well detect these defects in the head region. The set of angles generated helps to reduce the chance of missing any defects which could arise in between this range. A probing is also performed from the sides of the rail using a 2 MHz probe at an angular range of 10° to 60° for the detection of transverse defects which could arise inside the weld head region. Vol. 11, Issue 2 September 2012

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Detection of defects that arise from the foot of rail weld is a matter of deep concern. Presently a 45° 2 MHz conventional probe is used for the detection of these defects. A particular type of crack which occurs in the form of a half moon is a potentially harmful defect which needs to be detected as soon as possible after its origin. The sectorial scan using the 2 MHz probe at the angular range of 10° to 60° serves this purpose to a great extent. Presence of bolt holes near the weld region affects the conventional scanning method in this case. However the lower angles used in phased array scan helps to overcome this difficulty. Inspection of the flange region is performed with a 70° 2 MHz conventional probe at present. A phased array sectorial scan using 5 MHz probe at an angular range of 60° to 80° is suitable for replacing the above scan plan. The higher frequency ensures better sensitivity also in addition to the advantage of the variable angular range. They can detect defects in the flange region at their earlier stages itself. Since each indication from a phased array sectorial scan gives the exact location of the reflecting surface, it becomes easy to distinguish between a signal from a defect and those coming from the sharp corners at the bottom of welds.

involved in the scanning, with only a set of elements activated at an instant. Scanning is done at a constant angle and along the total aperture length. In sectorial scan, the beam is swept through a particular range of angles for a particular focal depth. The beam steering taking place during sectorial scan maps the components at appropriate angles optimizes the probability of detection of defects [5]. Electronic focusing optimizes the beam shape and size at expected defect location and also can improve the Signal to Noise ratio significantly.

EXPERIMENTAL SETUP A manual inspection of the rail weld using phased array ultrasonic probes was done. A portable phased array equipment was used for the inspection. The experimental setup is shown in the Fig. 1.

THEORY Phased array probes consist of a set of piezoelectric elements arranged in an array. When excited, the elements produces ultrasonic waves, which interacts with each other constructively or destructively leading to an increase or decrease in the resultant wave energy respectively. By varying the time at which these elements are excited, it is possible to use these effects to both steer and focus the resulting combined wavefront. This is the basic principle behind Phased array testing. Software called Focal Law Calculator is used to establish suitable specific delay times for firing each group of elements, so that the required beam shape could be generated through wave interactions. It also takes into account the probe and wedge characteristics as well as geometry and acoustical properties of the material while establishing the focal laws [4]. Electronic linear scan and sectorial scan are special features phased array has over conventional UT. In electronic linear scan, the same focal law and delay are multiplexed across a group of active elements and the entire elements are

Fig. 1 : Experimental setup.

Scan plans prepared were aimed at achieving a maximum coverage area of the weld region. Based upon the probe position, range, gain and angular range used for inspection, 5 groups were set up for the complete inspection of the weld. The details of these groups are mentioned in the Table 1. The setup for each group was saved accordingly in the equipment. At the time of inspection, a shifting from one group to another has to be done and the data corresponding to each scan has to be acquired. The analysis of the results was done after a set of welds are inspected and based upon the rejection criteria and sensitivity settings given so

Table 1 : Groups for inspection with defined values to the variables. Group

Thickness (mm)

Wave Type

Angle range (degrees)

Elements excited

Range (mm)

Gain (dB)

Velocity (m/s)

172

Longitudinal

10-60

1-16

200

5890

172

Longitudinal

10-60

1-16

110

5890

Longitudinal

10-60

1-16

5890

4(a)

Shear

60-80

48-64

3240

4(b)

Shear

60-80

48-64

3240

172

Longitudinal

All

200

5890

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56 that, welds could be characterised as a) immediately replaceable b) to be kept under observation or c) good quality weld. For the inspection of the flange region of the weld, the size of the probe should be as small as possible. Two calibration samples, one for thermite weld and other for flash butt weld, were prepared with artificial defects made at different positions inside the weld. Side drilled holes and flat bottom holes were drilled at various locations in these samples to replicate the common defects which arise in these welds. Three side drilled holes (SDH) were drilled at depths of 70 mm, 85 mm and 100 mm respectively on the thermite weld sample. A 3 mm diameter hole was drilled from the top surface to replicate the transverse defect in the head region of the weld. In the flange region, a 3 mm diameter SDH was drilled at a depth of 10 mm from flange surface. A flat bottom hole of diameter 3 mm was also drilled at the bottom of the slanting portion of the flange region. In the case of flash butt weld sample, the holes in the web region were put at the region of intersection of the weld and parent rail since lack of fusion is the most common type of defect here. Calibrations were performed on these samples using the artificial defects made at various depths. Separate groups were made for the two types of weldments because of their varying nature. For thermite welds, an additional 10 mm thickness is considered in the scan setup while scanning from the top because of the layer of reinforcement metal coming at the bottom. In the foot region, inspection was performed in 2

Technical Paper

stages a) in the region of flange where thickness is nearly constant at 10 mm b) in the slanting region of the flange region where the thickness was varying slightly. In the latter, while setting the thickness in the equipment, an average value was set. The flange region scanning had to be done with a lot of care as most of the critical defects originate from this region. These 2 stages of scanning in the foot region can provide a nice coverage of this region.

RESULTS The first group of inspection basically targets the defects in the foot of the welds. In thermite welds, cracks grow in the shape of half-moon from the foot. This defect has to be identified as soon as it is formed. To replicate this type of defect, a 3 mm diameter flat bottom hole was made in the foot to a depth of 10 mm. When scanned from the top surface at the angular range of 10째 to 60째, a very nice indication of the defect was obtained at an angle of 17째. The true depth (DA) value and the distance from probe surface to the top surface of defect (PA) helped in pinpointing the defect. A clear indication of the flat bottom hole (FBH) was obtained and was clearly seen in the sectorial scan shown in Fig. 2(b). Similar results were obtained from the FBH drilled in the flash butt welds also. Scanning needs to be performed from both sides of the weld. The S scan image of weld region without any defect is shown in Fig. 2(a).

Fig. 2 : a) Defect free image b) Indication from FBH of 10 mm depth from the bottom surface.

Fig. 3 : a) Defect free image b) Indications from holes at 70 mm, 85 mm and 100 mm depths. Journal of Non destructive Testing & Evaluation

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The second group is almost the same as the first group but with a reduced range of 110 mm to visualize the region near the head in a much better way so as to reduce the chance of missing any defect. Three 3 mm diameter side drilled holes were made in the web region of the weld at different depths which were seen clearly and separately in the sectorial image shown in Figure 3(b).This scanning needs to be performed from both sides of the weld for ensuring better coverage area. Figure 3(a) shows the no defect image. Based upon the values of true depth, forward position of the reflector with respect to tip of the wedge and sound path length, the position of these indications were checked and confirmed that the indications were from these defects itself. The peak of the indication from 3mm diameter hole at 70 mm was set to 60 % by varying the gain. The gain was around 35 dB which will be set as constant for further scanning of thermite welds. The three holes can be seen separately which assures good resolution. The third group scanning was done with probe kept at the sides of the rail head. The intention of this scan was to pick up the transverse defects which could arise inside the head region. A 3 mm diameter hole was drilled from the top surface of the rail head. When scanned from the sides, an indication was obtained from the exact position of the defect. A sectorial scan was taken from the side of the rail at an angle range of 10° to 60°. The indication in this case

was coming from a depth of 55 mm as shown in the Figure 4(b). A no defect image is also shown in Figure 4(a). Scanning was repeated from the other side of the weld also so as to completely cover the weld head region. In the fourth group, a 5 MHz probe was used to inspect the foot region of the rail weld. The surface in this part of rail is very rough and rusty. Hence cleaning of this surface is very much important so as to provide a smooth surface for the probe to be placed for inspection. Ultrasonic gel or grease needs to be used as couplant which should form a good coupling between the probe and surface. To ensure complete coverage of the foot region, an angle range of 60° to 80° was selected. Probe was kept and S scan was taken. The side drilled hole at 10 mm depth was seen very much clearly. Shear wave was used for inspection as they possess higher energy than longitudinal waves and was giving better indications from the defect regions. The same probe was used to catch the flat bottom hole of 5mm depth drilled at the bottom of the slanted portion in the foot of the rail. A sectorial scan at an angular range of 60° to 80° was taken as shown in Figure 5(b). A very nice indication was got at a depth of 9.68 mm from the top of the flange region. The image with no defects is shown in Figure 5(a). The fifth group of scanning is was a normal linear 0° phased array scan with a 2 MHz probe. The purpose of

Fig. 4 : a) Defect free image b) Indication from hole at 56 mm depth.

Fig. 5 : a) Defect free image b) Indication from a hole of depth 10 mm. Vol. 11, Issue 2 September 2012

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Fig. 6 : a) Defect free image b) Indication from side drilled holes at depths 70 mm and 90 mm respectively.

Fig. 7 : Horizontal crack in the web region of weld.

this scan was to detect the horizontal cracks which arise inside the weld region. The cracks under linear phased array scan produce a particular scan pattern which helps in its identification. The holes drilled at different depths in the rail weld were clearly seen when the scanning was performed from the top flat surface. The probe was placed at the top flat surface. In Figure 6(a), an image taken in a no defect weld region is shown where the back wall echo could be seen clearly. In Figure 6(b), the indication from holes drilled at depths of 70 mm and 90 mm can be seen clearly. Horizontal cracks present inside the weld were found to generate an image pattern similar to the orientation of the crack. By taking into consideration the regions of higher amplitudes in the indications, an idea of the extent to which the crack has propagated can be obtained. Any horizontal cracks lying in the head or web region can easily be detected by the linear scanning performed using phased array probe. Experiments conducted on samples with cracks at known locations provided satisfactory results. Image patterns almost replicated the cracks present in the sample. The advantage which these patterns offer to the inspectors in field is huge. Chance of missing a potentially harmful defect like a crack in weld will be decreased to a great extent thus ensuring better safety. An indication from a Journal of Non destructive Testing & Evaluation

horizontal crack in the web region of the rail weld, which has propagated from a nearby bolt hole is shown in Fig. 7. The regions marked in the figure were showing a decrease in the depths from 80 mm to 60 mm. These 5 groups of scanning ensure a complete coverage area of the weld. The setups corresponding to each group of scanning could be preset and focal laws could be calibrated before moving for an inspection. The gain values are set by considering the reflection from known defects at known depths. The data acquired corresponding to each group could be saved and analysis may be done at a later stage by taking into consideration of the rejection criteria.

CONCLUSION This paper presents a method of inspection of rail welds using phased array ultrasonics which proves to be much efficient than the existing methods. Based on the experimental study conducted, following conclusions were made. z

Location of the defects in rail welds could be exactly determined using the scanning performed. This helps in confirming whether the indication is coming from the defect inside the rail weld itself or from some sharp corners of the weld. Vol. 11, Issue 2 September 2012

Technical Paper z

Almost complete coverage of the rail weld region is ensured with the help of the different scan positions proposed.

2. D. F. Cannon, K. O. Edel, S. L. Grassie, K. Sawley, “Rail defects: an overview”, Fatigue Fract Engng Mater Struct, 26 (2006), pp. 865-887.

Analysis of the data acquired is quite easy which helps in arriving at a conclusion easily. Based upon a suitable rejection criteria prepared, decisions could be taken quickly for the rejection or acceptance of the inspected rail weld.

3. G. Zumpano and M. Meo, “A new damage detection technique based on wave propagation for rails”, International Journal of Solids and Structures 43 (2006), pp.1023–1046. 4. “Phased Array Testing: Basic Theory for Industrial applications”, Olympus NDT, First edition, November 2010.

The wide range of angles used in the sectorial scans helps in reducing the chance of missing a defect.

5. R/D Tech, (2004), Introduction to Phased Array Ultrasonic Technology Applications, R/D Tech Guideline, 1st ed., R/D Tech Inc., Quebec, Canada.

Cracks can be detected much easily using this technique as cracks produce a specific pattern which helps in its easy identification.

6. Azar L, Y. Shi, and S.C. Wooh, “Beam Focusing Behaviour of Linear Phased Arrays”, NDT and E International, 33 (2000), pp. 189-198.

Presence of bolt holes near the welds will not affect the scanning in anyway because of the lower angles employed in sectorial scan.

“Manual for ultrasonic testing of rails and welds”, Research designs & standards organization, Ministry of Railways, Government of India, Revised-2006, pp. 95-97.

REFERENCES

8. U. Zerbst, R. Lundén, K. O. Edel, and R. A. Smith, “Introduction to the damage tolerance behaviour of railway rails – a review”, Engineering Fracture Mechanics 76 (2009), pp.2563–2601.

1. Greg Garcia, TTCI, Jinchi Zhang, Olympus NDT, “Application of Ultrasonic Phased Arrays for Rail Flaw Inspection”, Federal Rail Road Administration, July 2006.

9. Y. Fan, S. Dixon, R. S. Edwards, and X. Jian, “Ultrasonic surface wave propagation and interaction with surface defects on rail track head”, NDT&E International 40 (2007), pp. 471–477.

Vol. 11, Issue 2 September 2012

Journal of Non destructive Testing & Evaluation

PROBE Karuppan , a poor stone breaker had to get up at 4.00 Am every day, 365 days an year, walk 5 km to reach his work spot and break stones to earn a living. His immense faith in God and his prayers worked in his favour and so when one day out of sheer frustration when he wished to be a rich man, God granted the wish and he became a rich man. He was enjoying his new status and was regarded highly by those surrounding him. One day he saw the King passing by and all the citizens were venerating the king. Now Karuppan desired to be a king so that he will be treated like the King and the wish was granted by God. Days passed as he was enjoying the newly acquired status till it became unbearably hot one day. Karuppan thought that if he became the Sun, then he can be the master of the earth. His wish was granted and he became the Sun. He was shining all over the earth and was generally benevolent. The season changed and rainy season came, during which clouds formed and were obstructing the Sun’s rays reaching the earth. Karuppan then wished that the clouds are more powerful than the Sun and wished to be the clouds. Lo and behold he became the clouds and was travelling all over the earth until he was stopped by a tall mountain. Karuppan was now sure that the mountains are the mightiest of all and desired to be a mountain. As in the past his wish was granted and he became a mighty granite mountain. The granite mountain attracted the humans and they started cutting the mountain into pieces. Karuppan could not bear the pain and was pretty sure that the mightiest of all creations is a stone breaker and wished to become the stone breaker. I recall this story, which I heard when I was young, because some of you are wondering “why an article like PROBE in a technical journal?” Karuppan was actually longing for freedom and liberation. Every one of us towards the end of our innings asks the question “why was I born? Whatever we do in life we end up with that question. The question is when that question shall be asked. The answer will be the sooner the better, so that we have no regrets. Let us get back to the origin of the universe. Black Hole -Big Bang – Galaxies - Life – Animals – Human Beings. This means that there was nothing and energy in the beginning. The energy was given mass by God particle and the evolution took place. Therefore the basis for existence is energy. The basic energy manifests itself in different forms, meaning that there is no difference between us and the materials we use. We are a miniature universe. Materials do not behave differently from us. They also get stressed, fatigued, age, react to the presence of other materials (corrode) like human beings. They also exhibit properties like toughness, elasticity, hardness, yield etc. If we burn our bodies and analyse we end up with 12 chemicals. The study of NDT is the study of finding out the truth. Spirituality also is about finding out the truth. Both proclaim “Sathyemeve Jeyathe” (Truth Prevails)”.

Ram.