Volume 11 issue 1 June 2012
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from the Chief Editor
This second issue in 2012 continues on the previous issue with the several features such as Horizon, Events & News, probe, etc. from the previous issues. The BASICS in this issue covers some of the fundamentals and application potential in the field of Guided Ultrasonic Waves for NDT. In addition, 4 technical papers are included in this issue. I would like to address a topic that is very dear to me. Education in NDE is a key component of the development of manpower that is necessary for the future of industries in India. The education opportunities in India are available in two forums i.e. through Level I, II, and III training and examination, and through formal degree programs in a few institutions. While the number of organizations offering Level I and Level II training program certified under the auspice of ISNT, PCN, or ASNT are many, the Level III programs are limited to a few programs, conducted by ISNT and a few reputed companies. The students enrolled in these programs usually possess a Bachelor’s degree in Engineering or Sciences, but do not have any formal exposure to NDE science or technology. Also, many of these short programs are preparatory in nature and are mostly oriented towards qualification in the respective exams. The imparting of the basics and the field experiences, that are necessary for undertaking inspection jobs in the respective areas of NDT, has been left to the concerned industries in which the individuals are employed. However, there is a significant gap here that must be filled, since the industries have other pressures and do not have the luxury to focus on training their employees, particularly in the basics. The other form of training is through the Master’s and PHD level programs in India which are offered by a select few institutions. These students do undergo formal training in the science and technology of NDE for at least 2-3 semesters. But, they are mostly not exposed to the industrial environment or its challenges. However, there are limited opportunities for employment, for the formally trained students, after their graduation with a Degree in the field of NDE and often have to accept jobs that have little or no relation with NDE, unless they too obtain a Level III certification. It is hence a situation of irony here that the “employable” are not formally trained and the formally trained ones are not “employable”. As a society, I hope we get an opportunity to debate on this in the near future. The NDE2012 will be held between 10-12 Dec 2012 in Sahibabad, NCR, New Delhi and we expect it to be a very grand success. It is hoped that all interested authors will submit their technical abstracts to the conference well in advance to paper.nde2012@gmail.com in order to avoid any disappointments. For more information, visit www.nde2012.org.
Dr. Krishnan B alasubramaniam Balasubramaniam Professor Centre for Non Destructive Evaluation IITMadras, Chennai balas@iitm.ac.in jndte.isnt@gmail.com URL: http://www.cnde-iitm.net/balas Journal of Non Destructive Testing & Evaluation
vol 11 issue 1 June 2012
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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 1 June 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 1 June 2012
Contents Chapter News
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The cover page shows the image of a result obtained using Scanning Eddy Current Themography on a graphite epoxy composite plate with impact damage defect. Graphite Epoxy composites are used in the aerospace industries for their improved performance due to their high stiffness to weight ratio and ability to engineer the performance of these material systems. The eddy current thermography uses an induction coils (that is on the left side of the image) that is scanned from right to left over the sample. The change in the temperature at the surface of the sample is observed using a thermal imaging (IR) camera in the video mode. Due to the conductivity of the graphite epoxy sample, the induction process intoduces heating in the material. At the locations of the impact damage in the sample, the electromagnetic field lines are disturbed and thus introducting local eddy currents at these locations. The additional eddy currents causes additional heating, also called as Joule heating, and can be observed using the thermal imaging camera. Hence, NDT for defects in such materials becomes feasible. Similarly, other conducting materials can also be inspected using this hybrid technique. Courtesy: Centre for Nondestructive Evaluation, Indian Institute of Technology Madras, Chennai, India
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Basics - Guided Ultrasonic Wave Techniques
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Horizon - Structural Health Monitoring (SHM)
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NDE Events
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NDE Patents
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NDE Puzzle Technical Papers Ultrasonic Evaluation of Glass-Epoxy Composites with Varied Void Content
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Shubhendu Verma, Shashwat Anand,C R L Murthy and R M V G K Rao
Low Heat Flux Transient Thermography for Defect Detection in Thick Composite Structures
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K Srinivas, T Murugesh and J Lahiri
Detection of fine defects in steam generator tubes of 220 MWe Indian PHWRs using eddy current array probes
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H.M. Bapat, Manojit Bandyopadhyay, R.K. Puri and Manjit Singh
Automatic Defect Recognition (ADR) System for Real Time Radioscopy (RTR) of Straight Tube Butt (STB) Welds
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Deepesh.V, R.J. Pardikar, K.Karthik, A. Sricharan S. Chakravarthy and K. Balasubramaniam
Probe
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Chief Editor Prof. Krishnan Balasubramaniam e-mail: balas@iitm.ac.in The Journal is for private circulation to members only. All rights reserved throughout the world. Reproduction in any manner is prohibited. Views expressed in the Journal are those of the authors' alone. Published by Shri RJ Pardikar, General Secretary on behalf of Indian Society for Non Destructive Testing (ISNT) Modules 60 & 61, Readymade Garment Complex, Guindy, Chennai 600032 Phone: (044) 2250 0412 Email: isntheadoffice@gmail.com and Printed at VRK Printing House Chennai vrkonline@gmail.com
Co-Editor Dr. BPC Rao bpcrao@igcar.gov.in
Managing Editor Sri V Pari e-mail: scaanray@vsnl.com
Topical Editors Dr D K Bhattacharya Electromagnetic Methods
Dr T Jayakumar, Ultrasonic & Acoustic Emission Methods
Sri P Kalyanasundaram Advanced NDE Methods
Sri K Viswanathan Radiation Methods
Editorial Board Dr N N Kishore, Sri Ramesh B Parikh, Dr M V M S Rao, Dr J Lahri, Dr K R Y Simha, Sri K Sreenivasa Rao, Sri S Vaidyanathan, Dr K Rajagopal, Sri G Ramachandran, Sri B Ram Prakash
Advisory Panel Prof P Rama Rao, Dr Baldev Raj, Dr K N Raju, Sri K Balaramamoorthy, Sri V R Deenadayalu, Prof S Ramaseshan, Sri A Sreenivasulu, Lt Gen Dr V J Sundaram, Prof N Venkatraman
Objectives The Journal of Non-Destructive Testing & Evaluation is published quarterly by the Indian Society for Non-Destructive Testing for promoting NDT Science and Technology. The objective of the Journal is to provide a forum for dissemination of knowledge in NDE and related fields. Papers will be accepted on the basis of their contribution to the growth of NDE Science and Technology.
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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
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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
vol 11 issue 1 June 2012
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
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Journal of Non Destructive Testing & Evaluation
vol 11 issue 1 June 2012
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vol 11 issue 1 June 2012
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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 Indian Society for Non-Destructive Testing 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. Following are the details of the 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
Journal of Non Destructive Testing & Evaluation
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 10th September, 2012. 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
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vol 11 issue 1 June 2012
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CHAPTER NEWS
KALPAKKAM UT Level-II course was conducted in June, 2012. LT Level –I & II courses was conducted in June, 2012.
CHENNAI
SRIHARIKOTA
MT & PT Level-II (ASNT) course was conducted from 18.05.2012 to 27.05.2012. RT Level-II (ASNT) course was conducted from 08.06.2012 to 17.06.2012. UT Level-II (ASNT) course from 22.06.2012 to 01.07.2012. UT Level-II (ASNT) course from 02.07.2012 to 10.07.2012. Workshop on “CAREER PROSPECTUS IN ENGINEERING SECTORS THRO’ NDT” held at AATRAL ARANGAM, IES, ANNA UNIVERSITY CAMPUS on 28.03.2012 & 29.03.2012 in association with Society of Mechanical Engineers, Anna University. ISNT DAY was celebrated on 21.04.2012. Members with their family participated. Mr. R. Sundar was the Chairman and Mr. R.G. Ganesan was the Convener. Dr. S. Suresh, General Manger of BHEL was the Chief Guest. EC Meeting held on 29.04.2012EC Meeting held on 03.06.2012.
A Technical Talk on ‘Advances in NDE’ by CV. Krishnamurthy, CNDE, IIT Chennai on 05.06.12
DELHI Core Committee meeting held on 27th April to discuss and finalize Venue/ official visit from head office.5th Executive body meeting held on 4th may 2012 at Indian coffee house Conaught place.Chapter Chairman explained the discussion held at Durban /Final discussion regarding NDE2012.6th Executive Committee meeting held at Hotel Majestic East of Kailash in the presence of high official team (Shri.P.Kalyasundaram President-ISNT, Shri V. Pari-President -Elect ISNT, Shri DJ Varde, Vice President -ISNT ) .Finalization of Venue/Presentation for NDE2012 seminar and other issues related to NDE 2012 were discussed in detail.
HYDERABAD Extra-ordinary General body meeting was held on 26th April 2012 and office bearers were elected. Distinguished NDTian of the decade award was presented to Sri JR Joshi, Dy. Project Director- DRDL 23rd April 2012.Our life Member Sri. MNV Viswanath has bagged Best presentation award during 18th WCNDT (16 - 20 April 2012) held at Durban, South Africa.
MUMBAI Conducted NDT Level- III Refresher Courses on PT, MT, RT, BASIC & UT from 2nd April 2012 to 22nd April 2012 at Hotel Jewel of Chembur. Conducted Welding Inspector examination at ITT, Mahim on 13th May 2012. EC meeting was held on 30th March, 2012 at ISNT, Mumbai office. APCNDT 2013 committee Meeting was held on 18th June 2012.
JAMSHEDPUR Materials Evaluation by Magnetic Techniques Speaker : Dr. Amitava Mitra Chief Scientist & Group Leader : NDE & Magnetic Materials. Materials Science & Technology Division National Metallurgy Laboratory Jamshedpur.
Journal of Non Destructive Testing & Evaluation
AGM was conducted on 05.06.12.New Executive Committee has been elected for the year 2012-2014. TRICHY Package programme for PSG college students Level-II in PT, MT, RT, and UT (28/02/12 to 25/05/12).Radiographers LevelI, BARC course (23/04/12 to 11/05/12.One year package programme for BHEL employees wards under progress. Conducted chapter EC meeting on14/05/12.
TRIVANDRUM ISNT Level II certification course on Ultrasonic Testing ISNT Level II certification course in Ultrasonic testing was organized by the chapter from 7th May 2012 to 11th May 2012. Thirty three participants attended the course from industries, educational institutions and government organizations. Sri P. S. Veeraraghavan Director , VSSC inaugurated the course. A technical talk on ‘Advances in Industrial Radiography’ by Sri S.C. Sood, MD, M/s CIT was organized by the chapter in association with M/s Kalva Engineers Pvt Ltd on 16th May 2012 at Hotel Horizon Trivandrum.MR Kurup memorial lecture and Annual technical meetDr. B. Venkatraman, Associate Director, IGCAR delivered the MR Kurup memorial lecture on 23rd June 2012 at Hotel Classic Avenue Trivandum. This was followed by ATM lecture and was delivered by Sr. Anil Kesavan INLPTA certified trainer. Three executive meeting were conducted during March, April and May 2012.A visit to Titanium sponge Plant, Kerala Minerals and Metals Ltd, Chavara was organized by the chapter on 21st April 2012. A Two-day lecture and demo on NDT was organized by ISNT Thiruvananthapuram Chapter to the instructors and staff members of Government ITI, Attingal, Thiruvananthapuram on 3rd and 4th of March 2012.Two issues of IMAGE, the quarterly technical bulletin of the chapter, was released for the first and second quarters of 2012AGM was held on 23rd June 2012 at Hotel Classic avenue. Hon. Secretary presented the report of activities of 2011-2012 and Hon Treasurer presented the audited account of 2011-2012.
OBITUARY Mr. O.P Singhania, a founder member since inception of ISNT passed away on 13th March, 2011. He had a long association with ISNT and has contributed immensely for the benefit of ISNTs’ growth. It’s a great loss to ISNT and our heartfelt condolences to the bereaved members of his family for the irreparable loss. May his soul rest in peace. ED
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Basics
BASICS OF GUIDED WAVES
Guided Ultrasonic Wave Techniques Prof. Krishnan Balasubramaniam Professor of Mechanical Engineering and Head of Centre for Nondestructive Evaluation Department of Mechanical Engineering Indian Institute of Technology, Madras, Chennai 600 036 Email: balas@iitm.ac.in
ABSTRACT In this paper, a review of the current status, on the use of guided wave modes, and their interaction with cracks and corrosion damage in pipe-like structures will be discussed. Applications of guided ultrasonic wave modes have been developed for inspection of corrosion damage in pipelines at chemical plants, flow accelerated corrosion damage (wall thinning) in feed-water piping, and circumferential stress corrosion cracks in PWR steam generator tubes. [1-25] It has been demonstrated that this inspection technique can be employed on a variety of piping geometries (diameters from 1 in. to 3 ft, and wall thickness from 0.1 to 6 in.) and a propagation distance of 100 meters or more is sometimes feasible. The guided waves can be classified into Long Range, Medium Range and Short Range modes. Intuitively it must be noted that, the longer the distance (range) of propagation (inspection), the lower will be the frequency used and consequently the lower will be the resolution of discrete defects. This technique can also be used in the inspection of inaccessible or buried regions of pipes and tubes. Also, guided ultrasonic waves can be used for other quantitative NDE applications such as a. Measurement of elastic moduli as a function of temperature, [26] b. Structural health monitoring of components and structures using in-situ sensors, [27-28] c. Fiber orientations in reinforced composites, [29] d. Adhesive weakness detection in bonded structures, [30] e. Inspection of solar panel Si wafers for cracks,[31] f. Measurement of gradual wall thinning in shells, pipes and tubes, g. Measurement of stresses (residual) [32] h. Measurement of fluid properties such as temperature, density, viscosity, degree of curing/crosslinking, level of fluids, flow front of resins, etc., both at room and at elevated temperatures up to 2000 C. [33,34] i. Recently, air coupled ultrasonic methods have become viable for low impedance materials such as fiber reinforced composites, plastics, and even thin metal structures. A new phenomenon called as “Turning Modes� has been shown to have the ability to detect delaminations in composite structures in regions that cannot be inspected normally.[35-38] j. Recently, a new phenomena called as the Higher Order Modes Cluster (HOMC) has been shown to have some unique properties that allows for the detection and sizing of corrosion in hidden regions of pipes and storage tank annular plate.[39,40]
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The ultrasonic guided waves, unlike bulk wave modes like longitudinal and transverse, are a manifestation of geometrical confinement of acoustical waves by one or more boundaries. [1,2] In many instances, these waves travel long distances, depending on the frequency and mode characteristics of the wave, and follow the contour of the structure in which they are propagating. Usually, these waves not only propagate along the length of the structure but also cover the entire thickness (for plates) and circumference (in the case of cylinders and rods). The use of guided wave modes is potentially a very attractive solution to the problem of inspecting the embedded portions of structures because they can be excited at one point on the structure propagated over considerable distances, and received at a remote point on the structure, in a pitch-catch mode, as schematically illustrated in Figure 1 for an elbow pipe. The received signal contains information about the integrity of the material that lies between the transmitting and receiving transducers. Alternate approaches, where the receiving and transmitter are co-located, similar to a pulse-echo method is also possible. Since there are several types of guided waves, there are many ways to classify them. The first classification can be based on the type of structure in which it is generated. These include (a) Plate waves, (b) Cylindrical waves, (c) Rod waves, etc., depending upon the type of structure. The waves mode characteristics for each of the above wave type is distinctly different, but can be theoretically predicted if the material properties are well known. The second method of classification of the wave mode is based on the nature of the particle vibration with respect to the direction of wave propagation (like in the case of bulk
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Basics waves). In this type of classification, the types include (a) Extensional or Longitudinal waves, (b) Shearhorizontal waves, (c) Flexural waves, and (d) Torsional waves [3-5]. Here, the first two types are similar to the Longitudinal and Shear wave vibration. The Flexural waves are modes where the structure flexes in a wave like pattern and the Torsion waves exists when the particle motion is circumferential in nature while the wave moves along the structure. Also, the wave modes can also be broadly classified into symmetric and anti-symmetric modes based on the type of symmetry of the displacement profile exhibited by the wave during propagation. This classification is based on whether the out-of-plane displacement in a structure is symmetric about the neutral axis of the bounded structure ie. if the two outer particles simultaneously move away from the center axis, then it is a symmetric mode and if they move together, then it is anti-symmetric. This is well illustrated in Figure 2. Finally, for a give type of guided wave, there are many orders of modes that can exist. The modes have mode shapes are analogous to vibration modes in a beam. These modes are numbered numerically with zero representing the basic fundamental modes and the higher order modes representing more complex behavior. Thus, while defining a guided wave mode, a complete description will require the specification of all of the above classifications. For instance, a cylindrically guided, flexural, antisymmetric, fundamental mode would represent a guided wave that is traveling along the length of a cylindrical structure, that has a fundamental flexural type particle vibration direction, that is not symmetric about the axis. The cylindrically guided wave modes are
by using one as a reference mode and the other as a sensing mode, defect and/or material characterisation becomes feasible.
Fig. 1 : A schematic comparison between bulk wave inspection and guided waves using surface mounted transducers in plates and pipes. The greyed region shows the coverage of inspection.
often represented as L(n,m)Longitudinal, F(n,m)-Flexural, or T(n,m) - Torsional in nature, where the n and the m represent the mode numbers based on Silk and Bainton [6]. For instance when n=0, the mode is axially symmetric, such as the L and T modes. If n>0, then the mode is not axially symmetric. Here, m is the order of the mode of vibration. The multi-mode nature of these wave modes can be an advantage since each mode has different sensitivity to a particular type of defect and hence by comparing the wave propagation of different modes, ie.
The reflectivity of guided waves is governed by very different rules than those for bulk waves; with guided waves, it is possible to find defects whose size is much smaller than a wavelength. At a given defect depth, the reflection coefficient is directly proportional to the circumferential extent of the defect. The reflection coefficient of a half wall thickness notch with a circumferential extent of half a pipe diameter (16% of the pipe circumference) is approximately 5% (-26 dB). If an axially symmetric mode is incident on an axially symmetric feature in the pipe such as a flange, square end or uniform weld, then only axially symmetric modes are reflected. Such a case is illustrated in Figure 3 that represents results from experiments conducted on pipes with welds and corrosion like defects. However, if the feature is non axially symmetric such as a corrosion patch, some non axially symmetric waves will be generated. These propagate back to the transducer rings and can be detected. For instance, if a L(0,2) mode is incident on a defect, the mode conversion is predominantly to the adjacent F(1,3) and F(2,3) modes
Fig. 2 : Representation of anti-symmetric and symmetric flexural guided wave modes in a plate.
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Basics Group velocity cannot be faster than the Phase velocity and the dispersive nature of these modes can be theoretically computed if the material properties are known.
Fig. 3 : Signals from (a) axisymmetric feature e.g. weld; (b) corrosion.[25]
which have similar velocities to the L(0,2) mode in the operating frequency range. The amount of mode conversion obtained depends on the degree of asymmetry, and hence on the circumferential extent of the defect. At low circumferential extent (which is the region of interest for the detection of critical corrosion in practical situations) the mode converted F(1,3) reflection is almost as large as the direct reflection. Thus, if these two reflections are of similar size, it can be concluded that the feature is localised to a small region of the circumference [7-9]. EFFECTS OF DISPERSION
One of the key aspects of guided wave modes is Dispersion, ie. wave velocity is not a constant for a given material. It additionally depends on geometry (thickness) and frequency of the wave. In most cases, this becomes one parameter (f*d). The consequence of dispersion is that a compact broad-banded signal will not retain its shape while propagating and will elongate considerably with distance of travel. This is because of the fact that a broad band ultrasonic pulse comprises of a range of frequencies (depending on the bandwidth and central frequency of the pulse) and since each frequency
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is traveling at a different velocity, the pulse duration increases. This is illustrated in Figure 4 where two modes, one non-dispersive mode (2) and a dispersive mode (3) are shown. The velocity of the wave mode for a single frequency is called as its Phase velocity. It must be apparent that the measurement of phase velocity by traditional velocity measurement techniques (such as pulse-overlap, zero-crossing, etc.) is difficult, due to the change in the pulse shape. Hence, a different definition of velocity called the Group velocity is used while measuring the velocities of a dispersive ultrasonic pulse by traditional methods. The
The dispersion curves used to gain understanding of the types of modes that can be generated and their dispersive nature [10-12]. These curves are also used for interpretation of the signal. The dispersion curves are represented in different manner in literature. The most userful representation for NDE application is shown in Figure 5. In this representation, the velocity of the wave is plotted as a function of frequency of the wave. Each curve represents a guided wave mode. The wave velocity that is plotted can be either phase or group velocity. But, from our previous discussion, it can be concluded that the group velocity is representative of measurements made with dispersive wave pulses and hence is more useful. In Figure 5, it can be seen that there are several (in fact, too many) modes that can be generated in a pipe. All three types of modes are represented (ie. Torsional, Longitudinal and Flexural). The slope of these curves indicate the dispersive nature of the wave mode. Hence, a curve with a steep slope is very dispersive and may be avoided during NDE.
Fig. 4 : Typical guided wave signals (1) Tranmitted pulse, (2) Signal after travel of 30 mm in a component showing no dispersion, and (3) a dispersive mode showing pulse spreading.
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Basics
TYPICAL INSTRUMENTATION
Fig. 5 : Dispersion curves plotting group velocity .vs. frequency for guided waves in a pipe. [25]
Consequently, a flat region of a curve means the mode is non-dispersive and the wave pulse will propagate effectively and measurements are possible. Hence, in Figure 5, the most preferred mode is the L(0,2) which is the 2 nd order axi-symmetric extensional mode within the frequency range between 40kHz. 100 kHz. The dispersion curves can be theoretically computed and plotted using a software package DISPERSE developed by Imperial College, UK. [13] ADVANTAGES AND LIMITATIONS OF THE TECHNIQUE
These guided wave modes represent very different approach to NDE when compared with traditional ultrasonic methods. Some of the key benefits of this technique for pipe and tube inspection is listed below: 1.
Use of multimode, guided, plate waves provides a global longrange inspection technique for characterizing any potential inservice damage (impact and delamination) in typical cylindrical structures.
2.
In the case of pipes with insulation, these modes allow inspection with minimal removal of insulation.
3.
Regions that are inaccessible, such as buried pipes, can be inspected.
4.
The multi-mode cylindrical waves can be utilized to identify regular pipeline features such as welds from localized damage such as corrosion..
Some of the key limitations of the technique are: 1.
This method requires the understanding of multi-mode nature of the guided waves.
2.
The energy that is generated is distributed over a large volume of the structure. Hence, this technique may have difficulty detecting isolated defects such as pin holes, longitudinal cracks, etc., that offer small crosssections for wave reflection.
3.
Signal interpretation is more complex, particularly due to mode conversion effects when wave interacts with damaged area.
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Like any traditional ultrasonic inspection system, the instrumentation for the guided wave technique involves (a) Transducers, (b) Pulser/ receiver with filters and amplifiers, and (c) PC based data acquisition system. The key component is the transducer that is designed specially for generation and reception of guided waves and hence will be discussed in more detail. The Pulser receiver is either an array type or a single channel type depending on the sensor. Usually, an array type is preferred with a capability to change the phase of each signal so that mode selection and tuning is possible. The signal interpretation required precise measurements and interfacing with dispersion relationships. Hence, a PC based system is the best choice for this NDE. The cylindrically guided waves can be generated and measured using several mechanisms. These wave modes can be generated using circular ring-type array transducers [14] for pipes, or comb-transducer configuration [15] for tubes, or like in the regular weld inspection using an array of variable angle beam transducers located around the circumference of the cylinder. Alleyne and Cawley [14] reported the development of a dry coupled piezoelectric transducer system for the excitation of the axially symmetric L(0,m) modes in pipes. It comprises a ring of piezoelectric elements, that are clamped individually to the pipe surface; no coupling fluid is required at the low ultrasonic frequencies used here. The number of elements in the ring should be greater than n where F(n,1) is the highest order flexural mode whose cut off frequency is within the bandwidth of the excitation signal. In the initial configuration, rings of 16 elements were used on 3 inch pipes, while 32 element rings were employed on 6 and 8 inch pipes. vol 11 issue 1 June 2012
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Basics
This gave the possibility of operating at frequencies up to around 100 kHz; in practice, most testing is done at 50 kHz and below, so it has been possible to reduce the number of transducers in a ring. Additionally, non-contact methods using Electro-Magnetic Acoustic Transducers (EMAT) has also been reported [16] These can be located either to the inside or the outside surface of the pipe/tube. The cylindrically guided wave technique has been modified to generate and detect wave-modes without the physical contact with the pipe walls[17]. This is accomplished by using the magnetostrictive property of steel pipes, where the pipe material acts as the transducer, and a coil that encircles the pipe couples the excitation energy into the pipe. A separate receiver coil is used to pick up the signals from the guided waves. These two methods have the advantage of being able to generate and receive waves without physical contact with the structure. But, it has been reported that mode isolation and identification of received signals is more complex. The length of travel of the guided waves will depend on the frequency that is used, the type of wave mode selected and the minimum size of the crack that has to be detected. It is estimated that these modes can travel up to 100 m in length. The smaller the crack size to be detected, the smaller the wavelength, which results in higher frequencies and consequently smaller travel distances due to ultrasound attenuation, which exponentially increases with frequency.
Fig. 6 : Guided wave results from a pipe with simulated defects. A pulse echo type approach was employed. [8,9]
(a) Corrosion detection in Pipelines Significant amount of work has been conducted in the application of this method in the pipeline inspection for corrosion damage in chemical industries [18-20], For example, 70 kHz. guided cylindrical waves in chemical and petroleum pipelines (13 meter diameter and 2-6 inch wall thickness) have detected 25% through wall cracks at a distance of 30 meters. A typical result is shown in Figure 6. From this result, it can be observed that the pipe features such as welds reflect energy while the defects also reflect signals (albeit mode converted) that are significant and detectable. Like in the case of pulse-
echo inspection, the arrival time information will provide the location of the defect and the amplitude will indicate the size of the defect. Figure 7 shows an epoxy painted 4 inch buried pipe at a test position adjacent to a road crossing. This result indicates the ability to test areas of pipes that are inaccessible. The test range extends over more than 20 m on either side of the ring type transducers, which are located in the middle of the plot. A distanceamplitude correction (DAC) curve was computed from the weld indication (using prior information on the weld locations). Then the defect call level by comparison with the weld echo level and the output amplitude were calculated, knowing that a
TYPICAL APPLICATIONS
The applications of the guided waves in NDE are many. A few typical results that have been reported in literature will be used to illustrate this potential. vol 11 issue 1 June 2012
Fig. 7 : Cylindrically guided wave inspection of buried pipe with corrosion damage, under a rail crossing (the crossing is between F2 and F3 as indicated)[25]. Journal of Non Destructive Testing & Evaluation
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Basics typical site weld is a -14 dB reflector. The echo identified as +F2 is the only indication where the mode converted signal is significant compared to the incident mode reflected signal and this indicates possible corrosion at the entry point to a road crossing. (b) Defect detection in Tubes Many results are available for steam generator tube corrosion detection in the nuclear industry [21-22]. Applications of cylindrically guided ultrasonic wave modes have been developed for inspection of flowaccelerated corrosion damage (wall thinning) in LWR feed-water piping, and circumferential stress corrosion cracks in PWR steam generator tubes. In most cases, the wave is generated using a probe that is attached to the inside. Frequency and angle tuning techniques are used to optimize the wave generation and reception mechanisms. In Figure 8, a typical result is illustrated for a steam generator tube inspection. Here, the wave is generated using the Comb transducer. The indications from the corrosion damage defect are clearly identified. (c) Guided Waves in Composite Materials These guided wave are also feasible in tubes and pipes made from composite materials. Although, the calculations must be made using the anisotropic and layered nature of
these advanced material, it has been shown that guided waves are sensitive to damage mechanisms such as fatigue in composites[23]. One of the issues that must be addressed will be that the energy flow of these waves are dependent on the anisotropic nature of the material.[24]. The Lamb modes have been shown to be able to measure elastic moduli of complex anisotropic systems (up to orthotropic symmetry involving 9 elastic constants) as well as the relatively simple isotropic materials. The guided waves can also be used to characterize the elastic properties as a function of temperature up to 2000 C, depending on the material being characterized. (d) Guided waves for Fluid Property Measurements In this method, guided waves are generated through a cylindrical buffer rod/wire and the reflection factor of the guided wave modes are measured. At a solid-fluid interface, the amount of ultrasonic wave energy reflected back into the solid depends upon the operating frequency, the physical properties of the fluid (viscosity and density), and the buffer rod (density and shear modulus). The amount of ultrasonic wave amplitude reflected back into the solid when the buffer rod is in air is used as the reference in the calculation of reflection factor from experimental signals. Reflection Factor is the ratio of peak to peak amplitude of the reflected ultrasonic signals when it is in viscous fluid (whose viscosity is
to be measured) and in air. The viscosity is evaluated from the reflection factor directly by fitting the experimental data to viscosity and reflection factors. Generally, in manufacturing processes, viscosity is taken as a function of temperature and the temperatures are usually measured by another sensor, such as a thermocouple. In the guided wave measurements, the temperature is simultaneously measured, along with viscosity, by measuring change in the time of flight of the ultrasonic wave in the delay line buffer rod at two different temperatures. Recently, this method has been used for flow front monitoring inside molds that have resin systems, fluid level measurements, temperature profile measurements, etc. (e) Air Coupled Guided Wave Ultrasonic NDT Air-coupled ultrasonic transducers can be used to efficiently transmit and receive guided waves in certain materials particularly those having low acoustic impedance such as composite materials. [36-38] Aircoupled ultrasonic inspection is a noncontact or minimally-invasive method. For reasons related to the attenuation of ultrasonic waves in air and the viscoelastic properties of composites most common practical applications of air-coupled ultrasonic inspection are in the frequency range to 50 to 500kHz [41-44] . Extensive studies carried out elsewhere [45, 46]
Fig. 8 : Typical signals from tube inspection from (a) defect free, and (b) defective tubes [15].
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explored characteristics of aircoupled transducers for inspection using guided waves. On this foundation, the concept of single sided inspection for the quantitative identification of delamination was established. Rapid advances in improvements of air-coupled transducer design such as the use of low impedance GMP (gas matrix probes) 1-3 piezo composites as the transducer material and with focusing have made it possible to acquire signals using very little signal averaging. This has opened new areas of NDT applications [47-50] such as the ability to penetrate metals and much thicker composites. Air-coupled ultrasonic transducers are ideally suited for the inspection of composite pipes addressed in this paper. The single sided air-coupled ultrasonic measurement technique using guided wave has received significant interest and new applications have been proposed for detection and visualization of inhomogeneities in composite materials. This study investigated numerically and experimentally the Lamb wave A0 mode interaction with the artificial impact type defects in aerospace honeycomb structures [51]. Single sided techniques have been used successfully for inspecting thick composite wind turbine blades [52]. Recently, there are also few reports on numerical work on using fundamental anti-symmetric A0 Lamb mode generated by air-coupled transducers for detection of delamination sizes in glass fiber reinforced composites using time of arrival of mode converted A0 [53]. In other work by the same authors, Lamb waves based B-scans imaging have been proposed for finding interface delaminations in a composite T-joint using air-coupled transducers. It was shown that it is possible to recognize the geometry of defects and estimated approximate dimensions of the defects from ultrasonic B-scan obtained using aircoupled transducers. vol 11 issue 1 June 2012
Basics
(b) Fig. 9 : Air Coupled single side Lamb wave (A0 mode) B-scan imaging for pipes (a) Scanning setup, and (b) results obtained by dividing circumference of in radial configuration showing detection of defects.
(f) Higher Order Modes Cluster Guided Waves (HOMC-GW): This new phenomenon uses modes clusters that are highly non-dispersive over considerable distance of propagation. This HOMC-GW is a recently explored phenomena which is found to occur at very high frequency-thickness product i.e. 15 MHz.mm to 35 MHz.mm. The HOMC-GW technique appears to have a greater potential for nondestructive inspection of large structures such as pipe support inspection and storage tank annular plate inspection. A typical result is shown in the Fig. 10 where the support location corrosion is mapped using the HOMC technique. The HOMC technique can also be used to detect the presence of corrosion between the sacrificial pad and the pipe at the pipe support locations. This technique has considerable improvement over the low frequency guided wave techniques that are used currently. HOMC-GW appears to have several attractive features for NDE
applications. These are (i) tighter envelope that improves the temporal resolution (ii) shorter wavelength that improves the spatial resolution, (iii) The vanishing surface displacements of the out-of-plane component that is insensitive to surface loading, and (iv) sub-surface defect detectability. Other applications of this technique includes annular plate corrosion inspection in large storage tanks. [39,40]
Fig. 10 : The pipe support inspection using HOMC guided wave technique.
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Basics SUMMARY
Like any non-destructive examination (NDE) method, the guided wave inspection method will have a defined false alarm rate and probability of detection, which will have to be determined. The associated signal-to-noise ratio of the NDE system under a field operating condition is also a factor that must be considered. The effects of the various parameters that influence the technique must be determined, such as accuracy, precision, and sensitivity guidelines, before attempting this technique for solving practical problems. Also, the procedures and limitations of applying this technique must be well understood for the successful implementation of this powerful technique. This method will substantially improve efficiency and reduce the inspection time and cost, especially when utilized as a precursor to a more detailed local inspection. Also, for critical applications, where inaccessible pipes and tubes have to be inspected, this technique provides an opportunity to perform NDE which otherwise may not be possible. REFERENCES
1.
2.
3.
4.
Meeker T.R and A.H. Meitzler, “Guided Wave propagation Elongated Cylinders and Plates,” Physical acoustics, Vol. 1 Part A, 1964, pp. 111-167. Zemmanek J.JR. , “An Experimental and Theoretical Investigation of Elastic Wave Propagation in a Cylinder,” The JASA, Vol. 52, No. 1 (part 2), 1972, pp. 265-283. Moher W. and P. Holler, “On Inspection of Thin Walled Tubes for Transverse and longitudinal Flaws by Guided Ultrasonic Waves,” IEEE Transactions on Sonics and Ultrasonics, Vol. SU23, 1976, pp. 369-374. Gazis, D.C., ‘Three Dimensional Investigation of the Propagation
of Waves in Hollow Circular Cylinders - I. Analytical Foundation,” Journal of the Acoustical Society of America, Vol. 31, No. 5, 1959a pp 568573. 5. Gazis, D.C., ‘Three Dimensional Investigation of the Propagation of Waves in Hollow Circular Cylinders II. Numerical Results,” Journal of the Acoustical Society of America, Vol. 31, No. 5, 1959a pp. 573-578. 6. Silk M.G. and K.F Bainton, “The Propagation Metal Tubing of Ultrasonic Wave Mode Equivalent to Lamb Waves,” Ultrasonics, Vol. 17, 1979, pp. 11-19. 7. Alleyne, D.N., and Cawley, P.,” The interaction of Lamb Waves with Defects,” IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 39, No.3, 1992, pp. 381-397. 8. Alleyne, D.N., Lowe, M.J.S. and Cawley, P. ‘The reflection of guided waves from circumferential notches in pipes’, ASME J Applied Mechanics, Vol 65, pp635-641, 1998. 9. Lowe, M.J.S., Alleyne, D.N. and Cawley, P. ‘The mode conversion of a guided wave by a part-circumferential notch in a pipe’, ASME J Applied Mechanics, Vol 65, pp649-656, 1998. 10. Ditri, J.J., “ Phase and energy velocities of cylindrically crested guided waves”, J. Acoust. Soc. Am., Vol. 97, No. 1, January 1995, pp. 98-107. 11. A. Pilarski, J.L., Rose, and K. Balasubramaniam, “On A Plate/ Surface Wave Mode Selection Criteria for Ultrasonic Evaluation in Layered Structures”, J. of Acoust. Soc. of A., Suppl. 1, Vol 82, S2118, 1987. 12. Alleyne D.N. and Cawley, P., “ Optimization of Lamb Wave inspection techniques,” NDT&E, Vol. 25, No.1, 1992, pp.11-22.
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13. Pavlakovic, B., and Lowe, M., Disperse Software Version 2.0, Imperial College,University of London, 1997. 14. Alleyne, D.N., and P. Cawley, “ The excitation of Lamb waves in pipes using dry-coupled piezoelectric transducers,”, J. of Nondestructive Evaluation, Vol. 15, No.1, 1996, pp. 11-20. 15. Rose, J.L., et. al., N.D.T. &E International, Vol. 27, pp. 307330, 1994. 16. Böttger, W., Schneider, H., and Weingarten, W. ‘Prototype EMAT system for tube inspection with guided ultrasonic waves,’ Nuclear Eng. and Design, Vol 102, pp356-376, 1987. 17. (http://www.swri.org/3pubs/ ttoday/fall00/technics.htm) 18. Alleyne D.N. and P. Cawley, “The long range detection of corrosion in pipes using Lamb waves,” Vol. 14, Rev. of prog in Quant. NDE, N.Y. , Plenum Press,, 1995, pp. 20752080. 19. Alleyne, D.N., et.al.,” The Lamb Wave Inspection of Chemical Plant Pipework,” Proc. 14 th World Conf. On Non-destr. Testing (14th WCNDT), New Delhi, India, Dec. 8-13, 1996, pp. 2303-2306. 20. Alleyne, D.N., Cawley, P., Lank, A.M. and Mudge, P.J. ‘The Lamb Wave Inspection of Chemical Plant Pipework’, Review of Progress in Quantitative NDE, Vol 16, DO Thompson and DE Chimenti (eds), Plenum Press, New York, pp1269-1276, 1997. 21. J. L. Rose, Dale J. and J Spanner, Jr. “Ultrasonic Guided Wave NDE for Piping,” Material Evaluation November 1996. 22. Rose, J.L., Ditri, J.J., Pilarski, A., Rajana, K. and Carr, F.T. ‘A guided wave inspection technique for nuclear steam generator tubing’, NDT & E International, Vol 27, pp307-330, vol 11 issue 1 June 2012
18
23.
24.
25.
26.
27.
28.
29.
Basics 1994. M. D. Seale, B.T. Smith, W. H. Prosser and J. E. Masters, “Lamb Wave Response of Fatigued Composite Samples” , Review of Progress in Quantitative Nondestructive Evaluation, Brunswick Maine, August, 1993, pp. 1261-1266. Sullivan, R, Balasubramaniam, K, and A. G. Bennett, “ Plate wave flow patterns for ply orientation imaging in fiber reinforced composites,” Materials Evaluation, Vol. 54, No. 4, April, pp. 518-523. (1996) D.N. Alleyne, B. Pavlakovic, M.J.S. Lowe, and P. Cawley,” Rapid Long range Inspection of Chemical Plant Pipework Using Guided Waves,” 15th World Conference on Nondestructive Testing, Roma (Italy) 15-21 October 2000. J. Vishnuvardhan, C. V. Krishnamurthy and * K.Balasubramaniam Blind inversion method using Lamb waves for the complete elastic property characterization of anisotropic plates” Journal of Acoustical Society of America 125(2) 761-71 (2009) J Vishnuvardhan, C V Krishnamurthy and K. Balasubramaniam, A Novel Quadrant Array for Material Characterization and SHM of Orthotropic Plate-like Structures, Structural Longevity, 2(1) 49-62 (2009). J. Vishnuvardhan, C. V. Krishnamurthy and K. Balasubramaniam “Structural Health Monitoring of anisotropic plates using ultrasonic guided wave STMR array patches”, NDT and E International 42(3), 193-198 (2009). Sullivan, R, Krishnan Balasubramaniam, and A. G. Bennett, “ Plate wave flow patterns for ply orientation imaging in fiber reinforced
vol 11 issue 1 June 2012
30.
31.
32.
33.
34.
35.
composites,” Materials Evaluation 54(4) 518-523 (1996). Arun, K.; Dhayalan, R.; Balasubramaniam, K.; Maxfield, B.W.; Peres, P.; Barnoncel, D., “An EMAT-based shear horizontal (SH) wave technique for adhesive bond inspection,” AIP Conference Proceedings, Volume 31, 1268-1275 (2012). S.K. Chakrapani, J. Padiyar, and K.Balasubramaniam, “Crack detection in full size Cz-Silicon wafers using Lamb wave Air Coupled Ultrasonic Testing (LAC-UT)” DOI 10.1007/ s10921-011-0119-3, J. of Nondestructive Evaluation (2011). Prabhu R., K. Balasubramaniam, M. Shankar, C.V. Krishnamurthy, “A new approach to inversion of surface wave dispersion relation for determination of depth distribution of non-uniform stresses in elastic materials”, International Journal of Solids and Structures, 42 789-803 (2005). Viswanathan, K., and K. Balasubramaniam, “An Ultrasonic Torsional Guided Wave Sensor for Flow Front Monitoring Inside Molds” Review of Scientific Instruments 78 015110 (2007) V.S.K. Prasad, K. Balasubramaniam, E. Kannan, and K. L Geisinger “Viscosity Measurements of Melts at High Temperatures using Ultrasonic Guided Waves”, J. Materials Processing Technology 207 (1-3), 315-20 (2008) C. Ramadas, K. Balasubramaniam, C.V. Krishnamurthy, and Makarand Joshi, Interaction of Lamb mode (Ao) with structural discontinuity and generation of “Turning modes” in a T-joint, Ultrasonics, 51(5), 586-595 (2011).
36. P. Karthikeyan, M.C. Bharadwaj, and K.Balasubramaniam, NonContact Ultrasonic Sensors for Process Measurements in Composite Fabrication, J. Nondestructive Testing & Evaluation, (2009). 37. C. Ramadas, A. Hood, J. Padiyar, K. Balasubramaniam, and M.Joshi “Sizing of Delamination using Time-ofFlight of the Fundamental Symmetric Lamb modes” Journal of Reinforced Plastics and Composites 30(10) 856863 (2011). 38. J. Padiyar M, C Ramadas, K.Balasubramaniam, “Aircoupled L-scan (Lamb wave scan) Imaging and its application in inspection of composite structures, JNDT&E 10(4) 6067, (2012). 39. Chandrasekaran. J, I. Anto, K. Balasubramaniam and K.S. Venkatraman, Higher order modes cluster (HOMC) guided waves for online defect detection in annular plate region of above ground storage tanks, INSIGHT, 51(11) 606-611 (2009). 40. K. Balasubramaniam, K.S. Venkatraman, I. Anto, HOMCGuided Wave Ultrasonic Technique- A new paradigm for corrosion detection, J. Nondestructive Testing and Evaluation, 9(3), 39-44 (2010). 41. Grandia, W. and Fortunko, C. M, NDE applications of aircoupled ultrasonic transducers, in Ultrasonics Symposium, Levy, M,, Schneider, S. C., and McAvoy, B. R., Ms., Seattle, Washington, November 1995, IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society,pp. 697-709. 42. D.W.Schindel and D.A. Hutchins, Through-thickness characterization of solids by wideband air-coupled ultrasound. Ultrasonics 1995; 33(1):11-17.
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Basics 43. Schindel DW, Forsyth DS, Hutchins DA, Fahr A. Aircoupled ultrasonic NDE of bonded aluminum lap joints. Ultrasonics 1997;35:1-6. 44. Stoessel R, Krohn N, Pfeiderer K, Busse G. Air-coupled ultrasound inspection of various materials. Ultrasonics 2002;40:15963. 45. Castaings M. & Cawley P. The generation, propagation, and detection of Lamb waves in plates using air-coupled ultrasonic transducers. J. Acoust. Soc. Am. 1996;(100):3070-3077. 46. Castaings M., Cawley P, Farlow R. & Hayward G. Single sided inspection of composite materials using air-coupled ultrasound. J.Nondestr. Eval. 1998;17:37-45. 47. Bhardwaj M. C, Non-Contact Ultrasound - The final frontier in non-destructive analysis.
48.
49.
50.
51.
Encyclopedia of Smart Materials, ed. by A. Biderman, John Wiley & Sons, New York, 2001. E. Blomme, D. Bulcaen, F. Declercq, Recent observations with air-coupled NDE in the frequency range of 650 kHz to 1.2 MHz, Ultrasonics 2002; (40):153-157. D. K. Hsu, Nondestructive testing using air-borne ultrasound. Ultrasonics 2006; 44(1):e1019-e1024. T. J. Potter, B. Ghaffari, G.Mozurkewich. Subwavelength resolution in aircoupled ultrasound images of spot welds. NDT & E International 2005;38(5):374-380. Kazys R, Demcenko A, Zukauskas E and Mazeika L Aircoupled ultrasonic investigation of multi-layered composite
materials. Ultrasonics 2006; 44:819-822. 52. R.Raiðutis, E.Jasiûnienë, Zukauskas E. Ultrasonic NDT of wind turbine blades using guided waves. Ultragarsas (Ultrasound) 2008;63:711. 53. Ramadas C, Krishnan Balasubramaniam, Joshi M and Krishnamurthy C V, Interaction of Primary anti-symmetric Lamb mode with symmetric delaminations: Numerical and experimental studies. Smart Mater. Struct 2009;18(8):085011. 54. Ramadas C, Janardhan Padiyar, Balasubramaniam Krishnan, Joshi Makarand, Krishnamurthy CV. Lamb wave based ultrasonic imaging of interface delamination in a composite Tjoint. NDT&E International 2011;44:523-530.
NCB – ISNT ANNOUNCES
ISNT LEVEL – III CERTIFICATION PROGRAMME AT PUNE (7TH JANUARY to 17TH FEBRUARY, 2013) The Announcement for the ISNT Level-III 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
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Horizon Structural Health Monitoring (SHM)
for structural integrity assessment. Under operational conditions, even small fluctuations in forces/couples can initiate a series of events leading to structural failure. Likewise, small oscillations can grow nonlinearly in amplitude leading to failure.
Dr. C V Krishnamurthy
It is clear then that a holistic assessment of the integrity of a structure in a periodic manner using the information provided by a network of local sensors would constitute SHM. More specifically, SHM can be defined as a process of: (i) observing or tracing the performance of a structure under environmental and operational loads by sensors and instrumentation devices, (ii) evaluating the performance of the structure for any development of defect or damage by use of the measured data and analytical tools, and (iii) issuing an alarm when the designated performance criteria are exceeded.
Centre for NDE and Department of Physics, IIT Madras Chennai 600036 Tamilnadu Email ID: cvkm@iitm.ac.in
The paradigm behind this new field is borrowed from the medical profession: by proactively monitoring health, by discovering problems in their early stages before they become serious, and by responding quickly and effectively to accidents, natural catastrophes, and other incidents, lives can be saved, structural lifetimes can be extended, and money can be saved. This is a diverse field with research and applications in many areas encompassing disciplines such as structural dynamics, materials and structures, fatigue and fracture, non-destructive testing and evaluation, sensors and actuators, microelectronics, signal processing and possibly much more. The question, “Is there anything wrong with a structure?” is surprisingly difficult to answer. The difficulty stems from the fact that conventional non-destructive practices are seldom used to test a structure for its integrity. Part of the challenge lies in identifying and measuring characteristics that reveal the integrity (or lack of it) of the given structure. Part of the challenge arises from the variety and complexity of the different structures that are encountered in real life – ranging from spacecraft, nuclear reactors, ships, dams, pipelines, turbines, and rail tracks to name a few. Consider a long electrical conducting wire. To check whether it electrically connects two distant parts of a
circuit, we apply a small voltage across the terminals and look for the current flow and ascertain whether the circuit is “open” or not. We note that if it is found “open”, this test will not help locate the fault along the length of the wire. Consider now a two-dimensional network of resistors. If one resistor is disconnected from the network, the simple test of applying a voltage across two terminals on the boundary will not reveal anything abnormal. A critical number of resistors (percolation threshold) need to be disconnected (randomly) from the network before the test can reveal the “openness” of the electrical network. In other words, the network would behave “normally” even when a few resistors are disconnected randomly within the network. Current flows in alternate pathways to ensure “structural integrity”. This is much like the human brain which shows remarkable functionality even when parts of the brain are either diseased or damaged – for the pathways are more complex and richer in three dimensions. We may note that the testing is done by passing a small signal across the whole of the structure and collecting the response. Integrity of a mechanical structure would depend on how loads are redistributed when there a few weak links located randomly within the structure. We can see that the mere detection of weak links on an individual basis may not be sufficient
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Environmental loads on a structure manifest as a vast range of complex phenomena include diurnal and seasonal weather patterns as well as extreme weather conditions. These loads induce physical and chemical changes in structural elements to different degrees and bring about nonlinear responses to routine operational loads. Thus, while structural integrity appears to deal only with the response of a mechanical system to environmental and operational loads, an SHM system is in fact a synthetic application of various branches of engineering and science disciplines such as mechanical engineering, civil engineering, optical engineering, electrical engineering, electronic engineering, communication engineering, software engineering, computer science, material science, information technology, etc. A perspective emerges by taking a look at some characteristics of vol 11 issue 1 June 2012
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continuous remote monitoring given below: Sensors are left permanently in place, so that measurements can be performed at any time, or at regular specified intervals. This means that sensors must cover all areas of the structure that are to be monitored. Sensors must be reliable, as frequent replacing, repairing, or recalibrating of sensors through on-site visits will reduce any cost savings and possibly reduce the safety margin provided by sensor data. Individual sensors must be relatively inexpensive, since multiple sensors must be dedicated to a single structure; however, the advantages of continuous remote monitoring may justify a considerable initial cost. Sensors must be robust, since they will often be exposed to the weather and must operate over long periods of time without degrading. A reliable communication system is needed to send the data. Where
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wired communication is not practical, wireless communication may have to be used. A source of power must be available for the sensors, data acquisition, and communications equipment. In remote locations this may require the use of solar panels. The amount of raw data that is generated by such applications can become extremely large, and an important aspect of the SHM process is developing and deploying the data handling software so to support informed infrastructure management. The types of information generated by remote continuous monitoring can be classified into three types, based on the user: Web information: For the general public and the media, a simple overview of operations that describes whether various aspects of the system are operating normally, abnormally, or not at all. Viewer information: For managers, much more detailed information for interpretation by technical experts
to diagnose what is happening and what it means for the structure. For example, following an alarm condition or a sudden change in a signal, this information would guide emergency inspections or repair efforts and decisions about closing the structure. Expert information: Researchers can use raw data from sensors to identify long-term trends and build predictive models of future performance. In addition to providing all these levels of information reliably and securely, the monitoring system must also safely archive the data and ensure that the system is secure. These are significant software requirements, and designing data-handling software for continuous remote monitoring systems is an important technical challenge that must be considered in tandem with the instrumentation and communication systems. SHM SCENARIOS SHM starts with the sensing system - the most common measurements made for SHM applications are:
Fig. 1 : Example of a Ubiquitous-Node or U-Node: A sensor module that includes an ADC and ZigBee communication module [from Ref.2] vol 11 issue 1 June 2012
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Fig. 2 : Network of actuators (PZTs) generate acousto-ultrasonic guided waves which scatter off defects and provide information of structural changes [from Speckmann H., Focal point for SHM, IMRBPB Meeting, EASA, Cologne April 22, 2007]
acceleration (piezoelectric, piezoceramic, fiber optic accelerometers) strain (resistive foil, fiber optic, piezoelectric patches) impedance patches)
(piezoelectric
It is important to recognize that SHM differs from NDT in its scope. Techniques employed in NDT do not automatically become candidates for SHM. Broadly, SHM systems are classified into two types. A SHM system that only used sensors for collecting data is typically referred to as a passive system, while a system with built-in actuators, which is designed to produce diagnostic signals, is referred to as an active system. The passive system is most effective in monitoring environmental changes to the
structures such as external loads, temperature, etc. An example of such a system is shown below in Figure 1. The active system is more effective in detecting cracks, damage and anomalies. An example is the impact damage detection scheme, shown below in Figure 2, carried out with guided acousto-ultrasonic waves in composites. Many countries, including India, have initiated SHM programs at universities and research institutions. Since 1997, Fu-Kuo Chang of Stanford University has been organizing International Workshops on SHM. Since 2002, every alternate year, the European Workshop on SHM is being held at various locations in Europe. An International Journal of SHM was launched in 2002. Several existing journals have
begun covering topics in SHM. The reader is directed to the large body of research on SHM through the publications cited at the end of this overview. SHM OF FRCCS Fiber reinforced cementitious composites (FRCCs) are an emerging high-performance civil engineering material that exhibits extremely high strength and ductility. FRCCs contain high volumes of distributed steel fibers, and thus the material has measureable conductivity. Researchers at the University of Michigan have been exploring the self-sensing capability of FRCC as a “smart� material capable of distributed sensing for crack detection. To recreate the spatial distribution of conductivity within FRCC materials based only on boundary measurements,
Fig. 3 : Approach to produce impedance tomographs: measurement approach (left) and inverse solution scheme (right)[from Ref. 10] Journal of Non Destructive Testing & Evaluation
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electrical impedance tomography is carried out as illustrated in Figure 3. The image reconstruction process is semi-analytical, requiring iterative matching of analytical and experimental data to arrive at the final impedance image. Micro- and macrocracks can be imaged in fine detail since cracks can be considered non-conducting compared to the FRCC material. SHM OF STEEL CABLES Magneto-elastic stress sensors utilize the dependence of the magnetic properties (e.g. the magnetic permeability) of structural steels on the state of stress of the material. The sensors are comprised of a principal and secondary set of cylindrical wire coils that are wound around the test object. Magnetic fields are generated when electric current is passed through the principal coil, and the interaction field is detected by the secondary coil. The magneto-elastic sensors function in a fully contactless manner, and do not touch or alter the inspected material in any way except by magnetization. Since the magnetic permeability in ferromagnetic materials is a function of magnetic history and applied field, this technique measures the internal stress in steel tendons and cables more directly as compared to other NDT methods. Furthermore, only a relatively small length of sample cable (less than 2 m) is needed for laboratory calibration of the material used in structure. The sensors allow for easy installation in new structures and in-situ installation adaptability for existing structures. Other attractive features include compact sensor size, ease of operation, high accuracy of stress determination (within Âą3%), and a theoretically unlimited service lifetime. Researchers at Northeastern University have developed novel magneto-electric sensors for nondestructively monitoring in situ stress in steel pre-stressed tendons vol 11 issue 1 June 2012
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and bridge cables. The sensors have been applied to cable-stayed bridges, arch-suspension bridges, posttensioning concrete box girder bridges, and a large domed space structure that contains high tension steel bracing cables. VIBRATION-BASED SHM It is intuitive that damage can be identified by analyzing the changes in vibration features of the structure. The fundamental idea for vibrationbased damage identification is that the damage-induced changes in the physical properties (mass, damping, and stiffness) will cause detectable changes in modal properties (natural frequencies, modal damping, and mode shapes). For instance, reductions in stiffness result from the onset of cracks. Although in vibration test, the excitation and response are always measured and recorded in the form of time history, it is usually difficult to examine the time domain data for damage identification. A more popular method is to examine the modal domain data through modal analysis technique, in which the time domain data is transformed into the frequency domain, and then the modal domain data can be further extracted from the frequency domain data. During the past three decades, great effort has been made in the researches within all three domains (i.e., time, frequency, and modal domains). The modal domain methods are popular because the modal properties (i.e., natural frequencies, modal damping, modal shapes, etc.) have their physical meanings and are thus easier to be interpreted or interrogated than those abstract mathematical features extracted from the time or frequency domain. Modal parameters-based damage identification algorithms for beamtype or plate-type structures can be categorized as natural frequencybased methods, mode shape-based methods, curvature mode shapebased methods and methods using
both mode shape and frequencies. Several algorithms have been proposed and investigated: The single damage indicator (SDI) method has been proposed to locate and quantify a crack in beam-type structures by using changes in a few natural frequencies. A method that depends on experimental data only from damaged structures has recently become a focused research topic in damage identification. These methods do not require a theoretical or numerical model. Their basic assumption is that the mode shape data from a healthy structure contains only lowfrequency signal in spatial domain compared to the damage-induced high-frequency signal change. The generalized fractal dimension (GFD) method is one of the several signal processing technique-based damage detection algorithms studied. It has been shown by many researchers that the displacement mode shape itself is not very sensitive to small damage, even with high density mode shape measurement. It has been found that the mode shape curvature (MSC) or the second derivatives of mode shape, is highly sensitive to damage. A response-based damage detection technique which only requires the mode shapes of the damaged plates, the gapped smoothing method (GSM), is similar to MSC but it does not require the baseline mode shape. Strain energy based damage index method (DIM) The table provides a comparison between various damage detection algorithms.
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Table 1: Capabilities of five comparative damage detection algorithms [from Ref. 3]
ADVANCES IN SENSOR TECHNOLOGY Over the past two decades significant efforts have been made by researchers in order to miniaturize electronic devices for SHM applications and optimize their performance. This “topdown” approach has led to the development of MEMS (microelectromechanical systems) technology, which allows complex circuitry, sensing and actuation mechanisms and advanced computation capabilities to take place in a single microchip. Sensors at the micrometer length scale such as MEMS accelerometers and MEMS ultrasonic transducers have been therefore developed. On the other hand, the opportunities offered by smart materials (that is to say materials characterized by sensing and actuating properties, such as piezoelectric, pyroelectric, electrostrictive, magnetostrictive, piezoresistive and electroactive materials) in the field of SHM have been extensively investigated. However, performance limits in MEMS technology and the high costs associated with fabrication of MEMS sensors in advanced clean rooms have led to an increasing attention towards the opportunities offered in the field of nanotechnology for the development of high performance sensing devices using a “bottom-up” approach. According to it, chemical fabrication parameters can be tailored at the molecular scale to yield macro-scale bulk sensor properties. Smart nanoscale materials are also
able to reduce the typical limitations associated with smart materials (requirement for high voltage or high current, small range of strain or force actuation, brittleness or excessive weight), leading to new solutions to generate and measure motion in devices and structures. Two types of sensors that have attracted considerable attention are the fiber optic sensors and nanostructured carbon based sensors. OPTICAL FIBER SENSORS The main advantages of optical fiber sensors are derived from the particular characteristics of the silica: it is passive, dielectric, and with low losses at optical frequencies. For that reasons, optical fiber sensors are immune to electromagnetic interferences, chemically inert,
biocompatible, withstand high temperatures, and are potentially small and lightweight. Fiber Bragg grating (FBG) sensors are regarded as the most mature grating-based sensors and have already been widely used. An FBG sensor reflects a portion of the incoming light of a particular wavelength, called Bragg wavelength, and leaves the rest of the incoming light pass without altering its property as shown in Figure 4. The Bragg wavelength is defined by the fiber refractive index and grating pitch, which are affected by the external environment changes, such as temperature, strain, vibration and other parameters. All these changes manifest as Bragg wavelength shifts. Therefore, by monitoring the Bragg wavelength shift, several measurands can be monitored using FBG sensors.
Fig. 4 : Illustration of the fiber Bragg grating concept and its optical function.[from Ref. 5]
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precision and stability, SOFO interferometric sensors are the most successful low coherent interferometric sensors for SHM. They have been reported being successfully deployed in more than hundreds of structures so far, including bridges, buildings, oil pipes and tunnels. SOFO interferometric sensors are long-gauge sensors and have a measurement range starting from 0.25 m to 10 m or even up to 100 m with a resolution in the level of micrometer. However, they are only suitable for the measurement of elongations and contractions at a low speed (0.1 Hz–1 Hz) and not capable of detecting the impact damages in aircraft structures.
Fig. 5 : FBG interrogation methods classified by measurement frequency. [from Guo et al., Fiber Optic Sensors for Structural Health Monitoring of Air Platforms, Sensors 11 (2011), 3687-3705]
In the past 20 years, wavelength multiplexing technology has been mature, hundreds (if not thousands) of wavelengths can be multiplexed in one single optic fiber. Current technology makes it possible to multiplex tens or hundreds of FBG strain sensors in one optic fiber and monitor them remotely. With the rapid development in the past few years, FBG sensors have been targeted as the major leading technology in contrast to other competing fiber optic sensor technologies. Besides its wavelength multiplexing capability, FBG sensors have the advantages of low cost,
compact size, and good linearity. The grating length is usually in the order of 10 mm. The resolution is dependent on the wavelength interrogator (see Figure 5 below), which is currently up to 1 pm, corresponding to 1με for strain measurement and 0.1°C for temperature sensing. SOFO (a French acronym for Surveillance d’Ouvrages par Fibres Optiques) is a deformation measurement system based on lowcoherence interferometry using an fiber optic sensor. With features of temperature insensitivity, high
Brillouin fiber optic sensor has the capability to simultaneously measure the strain level and locate the strained point along the sensor. This feature, which has no performance equivalent among the traditional electronic sensors, is extremely valuable. The sensor required by Brillouin technology is an inexpensive, telecom-grade optical fiber that shares most of the typical advantages of fiber optic sensors such as high resistance to moisture and corrosion and immunity to electro-magnetic fields. Some fiber sensors specifically addressed to Brillouin SHM have been developed as shown below in Figure 6. Going beyond the conventional Brillouin sensors, “smart” sensors shown above promise in RC structures. Due to the stiffening
Fig. 6 : Brillouin SHM sensors: RC-embeddable cord (left), woven “smart” FRP(center), and extruded rod (right).[from Ref. 13] vol 11 issue 1 June 2012
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Fig. 7 : Stress distribution over a crack for conventional FO sensor (left) and of “smart” FRP (right).[from Ref. 13]
effect of the structural fibers in the weft, when a crack opens in the substrate to which the “smart” sensor is bonded, the deformation is redistributed in a portion of FRP material longer than the crack width. This process happens because the shear transfer process between the substrate and the FRP material is spread on to a certain “transfer length” that is influenced by its own FRP stiffness and, for carbon-FRP materials, is found to be typically several inches long. Further, the strain distribution on the sensing fiber approaching the crack location becomes much more gradual than the step-like distribution that is assumed without the effect of the stiffening fibers (see Figure 7 below). NANOSTRUCTURAL CARBON SENSORS The superior mechanical performance and conductive properties of CNTs are well-known. From a mechanical point of view, CNTs show an elastic behaviour, with a very high stiffness (Young’s modulus of approximately 1 TPa and a density of about 1.33 g/cm3) and the possibility to bear torsion and bending without breaking. The hexagonally-bonded carbon honeycomb structure of SWCNTs is responsible for their high mechanical strength (tensile strength between 20 GPa and 60 GPa, with maximum
strain up to 10%), which is even better than that of structural steel. From the electrical standpoint, CNTs can be classified as conductors or semi-conductors, depending on the orientation of the carbon atoms in the lattice structure of the tubes. This property depends on the SWCNT one-dimensional structure, which allows electrons to travel greater distances before scattering occurs, approaching a ballistic transport-type behaviour which increases the electrical conductivity. Recently new forms of CNT materials have become available from the Nanoworld Laboratory at the University of Cincinnati. These are arrays, ribbon, thread, yarn, braid, and tape. Tape is the newest material and can be from 1 to 10 cm wide can be used to form a sheet. The functionalized CNT sheet is immersed into an epoxy resin solution to form a CNT sheet pre-preg. CNT sheet pre-pregs are used in forming composites by stacking multiple layers of CNT pre-pregs between two plates and placing into a hot press for curing. All the material forms have the capability to detect damage at the early stages due to their high piezoimpedance sensitivity. The application of these materials for SHM is still in its early development period. Application of the materials for SHM of Civil Infrastructure is
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being investigated by the Structural and Geotechnical Dynamics Laboratory StreGa at the University of Molise, Termoli, Italy. The electrical properties of CNT based composites are influenced by the CNT concentration in the matrix. In fact, its increase leads to more nanotube-to-nanotube junctions, thus providing a greater number of paths for electrical current to flow from one electrode to the other, reducing the overall resistivity of the film. However, beyond a certain concentration (percolation threshold) the benefit in increasing the CNT concentration is lower and lower. Thus, a proper sensor design requires the evaluation of such a threshold in order to optimize sensor performance as a function of CNT concentration. Apart from a few recent studies, very little work has been done on the development of cement based sensors using CNTs. The dispersion of CNTs into a cement matrix leads, first of all, to an improvement of the mechanical properties of the cement composite. In fact, because of their size and aspect ratio, CNTs can be dispersed in a much finer scale than common fibers, thus obtaining a more efficient crack bridging at the initial stage of crack propagation in the composite. Moreover, CNT dispersion in the cement matrix improves also its electrical conductivity, making possible the vol 11 issue 1 June 2012
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development of a smart material whose conductivity is sensitive to the applied strain and which can be therefore used as self-sensing material. The available results about integration of carbon nanotubes and nanofibers (CNF) into cement matrix seem to confirm the higher performance, with respect to traditional carbon fibers, in terms of enhancement of mechanical properties and electrical conductivity of the composite material. CURRENT AND FUTURE DIRECTIONS:
Among the advances being implemented now, or expected in the near term, are these: Use of embedded sensors built into new or substantially rehabilitated structures, Applications of nanotechnology in sensor design to develop lowpowered, area wide sensors Advances in wireless applications that reduce or eliminate dependence on physical connection of sensors or continuous sources of power
Application of new, renewable power sources to support long -term sensor operations With the continuing maturation in both hardware and software technologies, the research on SHM is being devoted to realizing the transition from its diagnostic function to prognostic function. Integrating SHM with condition-based maintenance management (CBMM) for in-service structural systems is another promising direction as it makes the SHM technology more vital. A great diversity of research is required to bridge the current gap between SHM and CBMM. It is clear that structures with a builtin sensing capability have superior advantages over traditional structures due to improved state awareness, enhanced safety and reliability, minimized operation and maintenance, and added multi-functionalities. Structures with sensing capability can detect and monitor their physical health conditions in any given environment and can determine an optimal course of action to maximize their performance and operation with minimal risk in safety. With proper sensing, the performance of many
other added functional capabilities such as shape control, smart antenna, noise reduction, and flow control could be significantly improved or enhanced. Furthermore, it was reported that if the SHM system could be considered at the initial design stage, it could result in a major paradigm change in the structural design for the next generation aircraft and spacecraft. Instead of relying upon uncertainties and safety factors in the design process, the new design with an appropriate SHM system could lead to much more reliable structures without overly conservative weight penalty. As shown in Figure 8 below, a design with structural sensing leads to more benefits, possibilities and potential with a future of more “intelligent” structures. Despite ongoing research, a legitimate question, why is SHM better than schedule-based NDT, needs to be addressed. Every industry needs to recognise demonstrable advantages before SHM makes the transition from research to practice. FOR FURTHER READING: 1. Special issue on interdisciplinary and integration aspects in structural health monitoring, Mechanical Systems and Signal Processing 28 (2012) 2. Chae M.J., Yoo H.S., Kim J.Y., Cho M.Y., Development of a wireless sensor network system for suspension bridge health monitoring, Automation in Construction 21 (2012) 237–252 3. Fan Wei and Qiao Pizhong, Vibrationbased Damage Identification Methods: A Review and Comparative Study,, Structural Health Monitoring 10 (2011), 83-111 4. Rainieri Carlo, Fabbrocino Giovanni, Song Yi, Shanov Vesselin, CNT Composites For SHM: A Literature Review, International Workshop Smart Materials, Structures & NDT In Aerospace Conference NDT, Canada (2011)
Fig. 8 : Diagnostics/prognostics cycle for an intelligent structural health management system [from Ref. 11]. vol 11 issue 1 June 2012
5. López-Higuera José Miguel, Cobo Luis Rodriguez, Incera Antonio Quintela, and Adolfo Cobo, Fiber Optic Sensors in
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HORIZON Structural Health Monitoring, Journal of Lightwave Technology 29 (2011), 587-608 6. Mascarenas David D.L., Flynn Eric B., Todd Michael D., Overly Timothy G., Farinholt Kevin M., Gyuhae Park, Charles R.Farrar, Development of capacitance-based and impedancebased wireless sensors and sensor nodes for structural health monitoring applications, Journal of Sound and Vibration 329 (2010) 2410–2420 7. Wicks Sunny S., deVilloria Roberto Guzman, Wardle Brian L., Ajay Raghavan and Seth Kessler, Tomographic Electrical Resistancebased Damage Sensing in NanoEngineered Composite Structures, 51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 12-15 April 2010, Orlando. FL. 8. Fei Yan, Roger L. Royer, jr and Joseph L. Rose, Ultrasonic Guided Wave Imaging Techniques in Structural Health Monitoring, Journal of
Intelligent Material Systems and Structures 21 (2010), 377-384 9. Staszewsk W.J., Mahzan S., Traynor R., Health monitoring of aerospace composite structures – Active and passive approach, Composites Science and Technology 69 (2009) 1678– 1685 10. Popovics John S., Recent developments in NDT and SHM in the United States, NDTCE’09, Non-Destructive Testing in Civil Engineering, June 30th – July 3rd, 2009, Nantes, France 11. Achenbach Jan D., Structural health monitoring – What is the prescription?, Mechanics Research Communications 36 (2009) 137–142 12. Mousumi Majumder, Tarun Kumar Gangopadhyay, Ashim Kumar Chakraborty, Kamal Dasgupta, D.K. Bhattacharya, Fibre Bragg gratings in structural health monitoring—Present status and applications, Sensors and 47 (2008) 150–164 Actuators A 1 14
13. Bastianini F, Matta F, Rizzo A, Galati N, Nanni A. Overview of recent bridge monitoring applications using distributed Brillouin fiber optic sensors, Journal of Nondestructive Testing 12 12(2007), 269-276 . 14. Brownjohn J.M.W, Structural health monitoring of civil infrastructure, Phil. Trans. R. Soc. A 2007 365 365, 589-622 15. Bhalla S., Yang Y.W., Zhao J., Soh C.K., Structural health monitoring of underground facilities – Technological issues and challenges, Tunnelling and Underground Space Technology 20 (2005) 487–500 16. Chung D.D.L., Structural health monitoring by electrical resistance measurements, Smart Materials and Structures 10 (2001), 624-636 17. Ghoshal Anindya, Sundaresan Mannur J., Schulz Mark J., Frank Pai P., Structural health monitoring techniques for wind turbine blades, Journal of Wind Engineering and Industrial Aerodynamics 85 (2000) 309-324
National NDT Awards
No.
Award Name
Sponsored by
1.
ISNT - EEC National NDT Award (R&D)
M/s. Electronic & Engineering Co., Mumbai
2.
ISNT - Modsonic National NDT Award (Industry)
M/s. Modsonic Instruments Mfg. Co. (P) Ltd., Ahmedabad
3.
ISNT - Sievert National NDT Award (NDT Systems)
M/s. Sievert India Pvt. Ltd., Navi Mumbai
4.
ISNT - IXAR Best Paper Award in JNDE (R & D)
M/s. Industrial X-Ray & Allied Radiographers Mumbai
5.
ISNT - Eastwest Best Paper Award in JNDE (Industry)
M/s. Eastwest Engineering & Electronics Co., Mumbai
6.
ISNT - Pulsecho Best Chapter Award for the Best Chapter of ISNT
M/s. Pulsecho Systems (Bombay) Pvt. Ltd. Mumbai
7.
ISNT - Ferroflux National NDT Award
M/s. Ferroflux Products Pune
(International recognition)
8.
ISNT - TECHNOFOUR National NDT Award for Young NDT Scientist / Engineer
9.
ISNT - Lifetime Achievement Award
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
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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 September 2012 The 51st Annual Conference of The British Institute of Non-Destructive Testing September 11- 13, 2012 ; Daventry, Northamptonshire, UK http://www.bindt.org/Events/ NDT_Conferences_&_Seminars/NDT_2012 30th European Conference on Acoustic Emission Testing and 7th International Conference on Acoustic Emission. September 12 – 15, 2012 ; Granada, Spain. http://www.2012.ewgae.eu DACH Jahrestagung 2012 : Joint conference of the German, Austrian and Swiss NDT societies (DGZfP, ÖGfZP and SGZP) September 17 – 19, 2012 ; Graz, Austria. http://jahrestagung.dgzfp.de Conference on Industrial Computed Tomography. (Organised by the CT Research Group, Upper Austrian University of Applied Sciences) September 19 – 21, 2012 ; Wels Campus, Wels, Austria. http://www.3dct.at 55th Annual A4A NDT Forum September 24-28, 2012 ; Seattle, Washington http://www.airlines.org/Pages/ 2012AnnualA4ANDTForum.aspx October 2012 6th Middle East Nondestructive Testing Conference & Exhibition, October 7 – 10, 2012 ; Gulf Hotel Kingdom of Bahrain. http://www.mendt.net/
Maximising the value of modern Non-Destructive Examination (NDE) October 11, 2012 ; Haydock Park Racecourse, Merseyside http://www.imeche.org/events/s1725 2012 IEEE International Ultrasonics Symposium, October 7 - 10, 2012 ; Dresden, Germany https://ius2012.ifw-dresden.de/ 2nd IPC Personnel Certification Conference. October 14-16, 2012 ; Rio de Janeiro, Brazil ASNT Fall Conference and Quality Testing Show 2012. October 29 – November 2, 2012 ; Orlando, Florida, USA. http://www.asnt.org 42nd International Conference and Exhibition – Defektoskopie 2012. October 30 – November 1, 2012 ; Seè, Czech Republic. http://www.cndt.cz November 2012 Workshop on Civil Structural Health Monitoring (CSHM-4) “SHM systems supporting extension of the structures’ service life” November 6 - 8, 2012 ; Berlin, Germany http://www.cshm-4.com/ 4th International Symposium on NDT in Aerospace. November 13-15, 2012 ; Augsburg, Germany. http://www.ndt-aerospace.com 21st International Acoustic Emission Symposium (IAES21). November 27-31, 2012 ; Okinawa, Japan. http://iaes21.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 Journal of Non Destructive Testing & Evaluation
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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/
UNITED STATES PATENT 8,208,344 June 26, 2012 Method, apparatus or software for determining the location of an acoustic emission emitter in a structure Abstract A method, apparatus and software is disclosed in which the location of the origin of a received acoustic emission in a structure is calculated by triangulating the times of flight of the acoustic emission to a distributed set of sensors and using a predetermined acoustic model of the structure. Inventors: Paget; Christophe (Great Britain) Assignee: Airbus Operations Limited (Great Britain)
UNITED STATES PATENT 8,181,526 May 22, 2012 Acoustic emission test sensor fixing device
with the auxiliary supporting arm. The above components are assembled as follows: the main supporting arms of the two sets of supporting mechanisms are connected to the two free ends of the radial positioning mechanism respectively in the way that the auxiliary supporting arms of the two sets of supporting mechanisms are located at the inner sides of the main supporting arms respectively and are arranged axis symmetrically with respect to the central line of the radial positioning mechanism, and the kink shaft members of the two auxiliary supporting arms are respectively inserted into the plugholes of the two bases to form revolute pairs, the two sets of acoustic emission test sensor mounting mechanisms are respectively mounted at the two bases, and the two sets of parallelism adjusting members are respectively mounted on the two main supporting arms and correspond to the positions of the bases. Inventors: Liu; Jianfeng (China), Xie; Heping (China), Xu; Jin (China) Assignee: Sichuan University (China)
Abstract An acoustic emission test sensor fixing device, comprising a radial positioning mechanism, supporting mechanisms, bases, acoustic emission test sensor mounting mechanisms, and parallelism adjusting members, wherein the supporting mechanism comprises a main supporting arm and an auxiliary supporting arm, with one end of the auxiliary supporting arm is fixedly connected to or hinged with the main supporting arm and the other end is provided with kink shaft members which are symmetrical about the auxiliary supporting arm; and the bases are provided with plugholes which form revolute pairs
UNITED STATES PATENT 7,698,943 April 20, 2010 Method for evaluating pressure containers of composite materials by acoustic emission testing
pressure containers from identical production that have been classified as being without defects in predetermined phases of a timecontrolled pressure acting upon the pressure container (AE test procedure) with one or more acoustic emission channels (AE channels) using acoustic emission sensors (AE sensors) of a predetermined position (one AE characteristic per AE sensor or AE channel. Inventors: (Germany), (Germany)
Bohse; Juergen Mair; Georg M.
Assignee: Bam Bundesanstalt Fuer Materialforschung und-Pruefung (Germany)
UNITED STATES PATENT 7,080,555 July 25, 2006 Distributed mode system for real time acoustic emission monitoring Abstract A distributed real time health monitoring system is described for monitoring of acoustic emission signals from different regions of a structure such as aircraft or spacecraft structures. The health monitoring system has its analysis and prognosis intelligence distributed out to the local regions being monitored and therefore does not require extensive cabling systems to carry the high bandwidth information characteristic of acoustic emission. Inventors: Austin; Russell K. (USA), Coughlin; Chris (USA) Assignee: Texas International, Inc. (USA)
Research
Abstract The invention relates to a method for evaluating pressure containers made of a composite material by acoustic emission testing. The method comprises the steps: (a) determining a sufficient number of internal pressure-dependent acoustic emission characteristics (AE characteristics) of
Journal of Non Destructive Testing & Evaluation
UNITED STATES PATENT 6,823,736 November 30, 2004 Nondestructive acoustic emission testing system using electromagnetic excitation and method for using same vol 11 issue 1 June 2012
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vol 11 issue 1 June 2012
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NDE PATENTS Abstract A nondestructive acoustic emission testing system using electromagnetic excitation, comprises: a) an electromagnetic wave generator for generating electromagnetic waves that stimulate a test sample to generate acoustic energy; b) an acoustic energy sensor for detecting the acoustic energy and generating a first output signal that represents the acoustic energy; and c) a data processor for comparing the output signal with a reference and for generating a second output signal that represents a characteristic of the test sample. Inventors: Brock; David W. (USA), Joshi; Narayan R. (USA), Russell; Stephen D. (USA), Lasher; Markham E. (USA), Kasa; Shannon D. (USA) Assignee: The United States of America as represented by the Secretary of the Navy (USA)
UNITED STATES PATENT 7,075,424 July 11, 2006 System for damage location using a single channel continuous acoustic emission sensor Abstract A sensor array for non-destructively monitoring a structure to detect a critical structural event. The sensor array includes a plurality of discrete sensor nodes, each of the discrete sensor nodes producing an electrical signal in response to a structural event. A signal adder is electrically connected to the plurality of discrete sensor nodes for receiving and combining the electrical signal from each of the discrete sensor nodes to form a single sensor array output signal. A signal processing module then receives and processes the single sensor output signal. In the preferred embodiment, the signal processing module uses the time interval between the electrical signals from each of the discrete sensor nodes formed into a single sensor array output signal to calculate the location of the critical structural event. Also, in the preferred embodiment, a data collection system is located downstream of the sensor processing module. Inventors: Sundaresan; Mannur J. (USA), Ghoshal; Anindya (USA), Schulz; Mark J. (USA) Assignee: North Carolina A&T State University (USA)
UNITED STATES PATENT 6,289,143 September 11, 2001 Fiber optic acoustic emission sensor Abstract A fiber optic acoustic emission (FOAE) sensor particularly suitable for vibration sensing in a hostile environment has a pair of optical fibers each having an end face. In one embodiment, a hollow tube or core having opposite open ends receives the end faces of the optical fibers. Means are provided for fixing the optical fibers in the hollow core with the end faces facing each other and spaced by a distance from each other in the core. A signal processing unit is connected to the optical fibers for supplying light to, and for receiving light from, the optical fibers and for measuring variations in optical phase which result in changes in the light intensity due to vibrations of the hollow core. The hollow core is fixed in a resonant cylinder, and the resonant cylinder is fixed in a housing to complete the sensor. Other embodiments dispense with the need for the hollow tube or core and employ means for fixing the optical fibers within a precision hole, advantageously produced by electrical discharge machining (EDM) or similar processes, provided in the resonant cylinder. A system employing these embodiments of the FOAE sensor is also disclosed. Inventors: Berthold; John W. (USA), Roman; Garry W. (USA) Assignee: McDermott Technology, Inc. (USA)
UNITED STATES PATENT 6,173,613 January 16, 2001 Measuring crack growth by acoustic emission Abstract A method and an apparatus for detecting and measuring cracks in plate-like structures using acoustic emission technique are disclosed. A false aperture transducer is designed to provide a criterion for filtering out extraneous noise in the acoustic emission signal based on modal analysis by computing the ratio of the high-frequency peak amplitude to low-frequency peak amplitude of the signal. A calibration curve correlating
Journal of Non Destructive Testing & Evaluation
crack depth to the amplitude ratio can be obtained by simulating crack growth in a fracture specimen coupled to a test structure or field structure, and measuring acoustic emission signal in the structure by the false aperture transducer. The calibration curve correlates simulated crack depth percentage with computed peak amplitude ratio of the measured signal. Using the calibration curve and acoustic emission signal sensed by a false aperture transducer in a field structure, a crack in the structure can be detected and its depth measured by computing the peak amplitude ratio of the signal and identifying the crack depth that correlates with the ratio from the calibration curve. Inventors: Dunegan; Harold L. (USA)
UNITED STATES PATENT 5,528,557 June 18, 1996 Acoustic emission source location by reverse ray tracing Abstract A method of locating an acoustic emission source in a structure by reverse ray tracing. An azimuth acoustic emission sensor is utilized which has an array of individual elemental detectors which independently and sequentially respond to the passage of an acoustic stress wave. The response of each element of the array is electronically monitored, and individual responses to the acoustic stress wave are analyzed to determine the azimuth approach angle of the wave to the azimuth acoustic emission sensor. An accurate measurement of the true location of the acoustic emission signal source is then provided by reverse ray tracing by using a parallel processing arrangement having a plurality of parallel processing elements. The structure is modeled in the computer on a one to one basis, with each parallel processing element simulating and having structural data on one discrete area of the structure. The determined azimuth approach angle is an input to the parallel processing arrangement, such that a simulated wave propagation takes place in the computer model as if it were propagating in the structure, and the actual location of the acoustic emission source is determined by reverse ray tracing by taking into account the structure of the intervening path of the wave and the most probable
vol 11 issue 1 June 2012
36 perturbations of the wave therein. The present invention has particular applicability to aircraft structures, and the method is utilized to locate structural defects therein. Inventors: Horn; Michael (USA) Assignee: Northrop Grumman Corporation (USA)
UNITED STATES PATENT 6,360,608 March 26, 2002
NDE PATENTS
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Transducer for measuring acoustic emission events Abstract A method and an apparatus for detecting and measuring cracks in plate-like structures using acoustic emission technique are disclosed. A false aperture transducer is designed to provide a criterion for filtering out extraneous noise in the acoustic emission signal based on modal analysis by computing the ratio of the high-frequency peak amplitude to low-frequency peak amplitude of the signal. A calibration curve correlating crack depth to the amplitude ratio can be obtained by simulating crack growth in a fracture specimen coupled to a test structure or field structure, and measuring acoustic emission signal in the structure by the false aperture transducer. The calibration curve correlates simulated crack depth percentage with computed peak amplitude ratio of the measured signal. Using the calibration curve and acoustic emission signal sensed by a false aperture transducer in a field structure, a crack in the structure can be detected and its depth measured by computing the peak amplitude ratio of the signal and identifying the crack depth that correlates with the ratio from the calibration curve.
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UNITED STATES PATENT 5,526,689 June 18, 1996
Acoustic emission for detection of corrosion under insulation
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Abstract A method and apparatus for detecting the presence of surface corrosion under insulation on a piping structure employs artificially generated, broadband acoustic sound waves to interrogate the piping structure. The sound waves are coupled into the piping structure and detected after they have propagated through and interacted with a portion of the piping structure. The amplitude of RMS voltage signals indicative of the detected sound waves is used to determine whether or not surface corrosion is present. Highly corroded pipes have been shown to yield relatively low RMS voltage signals whereas the lack of corrosion yields relatively high RMS voltage signals. Inventors: Coulter; John E. (USA), Robertson; Michael O. (USA), Stevens; Donald M. (USA) Assignee: The Babcock & Wilcox Company (USA)
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ndt puzzle We hope you enjoyed solving the “NDT Crossword Puzzle” which was published in the last issue. We received many entries from the readers and based on the maximum number of correct words identified, the following are the WINNERS - Paresh Vaidya, Ex-BARC, Mumbai - P Selvaraj, ISRO-SHAR, Sriharikota
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41
Technical Paper
Ultrasonic Evaluation of Glass-Epoxy Composites with Varied Void Content Shubhendu Verma1, Shashwat Anand1,C R L Murthy2* and R M V G K Rao3 Department of Aerospace Engineering, Indian Institute of Science, Bangalore-560012,India School of Materials Science and Technology, Indian Institute of Technology, BHU, Varanasi 2 Professor, Department of Aerospace Engineering, Indian Institute of Science, Bangalore 3 Visiting Scientist, Department of Aerospace Engineering, Indian Institute of Science, Bangalore *Email: crlmurty@aero.iisc.ernet.in 1
ABSTRACT E-glass-epoxy composites prepared by RT-vacuum bag moulding technique with varied void fractions (5-8%) were investigated using the C-Scan pulse echo technique. Quality of these laminates was assessed using areas of different palette colours obtained from the C-Scans. A new area ratio parameter called “Laminate Quality Index (LQI)” was defined and introduced in these studies to evolve a relative quality ranking among these laminates. Results showed good correlation between the LQI and void fractions. Further, A-Scan results were also obtained on these laminates. The relative quality rankings by both the C-scan and A-scan compared well. Keywords: Glass-Epoxy composites, US-scan(C-Scan, A-scan), Vacuum Bag Moulding, void volume fraction, Laminate Quality Index (LQI)
1. INTRODUCTION Of various continuous fibre based structural composites, the glass fibre reinforced (GFRP) and the carbon fibre reinforced (CFRP) epoxy matrix composites are the most widely used materials in the aerospace and non-aerospace industries. Further, GFRP composites are preferred when it comes to trading off between the functional requirements and product economy. Again Of various fabrication techniques, the vacuum bag moulding offers a very balanced approach of manufacturing large and complex shaped composite products of a well-definable quality and reliability in performance. The all composite two-seater trainer aircraft “HANSA”, developed and demonstrated to high certification standards in 1998 by the National Aerospace Laboratories(NAL-CSIR) was a clear demonstration of the GFRP RT-vacuum bag moulded composite technology for high end applications.[1] However, composites being heterogeneous and layered materials are prone to either inherent manufacturing defects like voids and dry spots in the cured laminate or delaminations or debonds caused due to foreign object impacts (tool drops, projectiles) during the service life of the composite component. Further, the small and scattered voids introduced during manufacturing stage can lead to unpredictable failures in components due to degradation in performance when subjected to service loads and hostile environments. Therefore characterisation and evaluation of such voids assumes considerable importance prior to qualifying the structural composites for practical use. Several investigators have reported extensive studies on CFRP composites invariably autoclave moulded using different NDT techniques including the US-scan technique [2-8]. Many reports on GFRP composites typically address NDT characterisation of defects like delaminations caused Vol. 11, Issue 1 June 2012
during service conditions [9-13]. In most of cases the processing parameter is not explicitly mentioned. In some cases defects have been artificially induced and their influence on the mechanical properties is studied [14-15]. However no studies are reported on quality assessment of glass-epoxy composites processed by varying vacuum bag moulding conditions to create exclusively different void contents, using both US-C-scan and US-A-scan techniques at the same time. Further, no report is available indicating a quantified Laminate Quality Index for such laminates using the NDT approach. Such studies become even more critical as the composite content of an aircraft is increased in an effort to develop lighter and stronger air vehicles. In the present studies, the glass-epoxy laminates intentionally prepared with different void contents achieved through a variability introduced in the vacuum-time cycles of the moulding process were US-scan evaluated and a relative quality ranking was brought out both through the C-scan and A-scan experiments.
2. ULTRASONIC TESTING (UT) Ultrasonic scanning is one of the most efficient Nondestructive evaluation techniques for quality inspection of fibre reinforced composites. The frequency of the ultrasonic wave is generally between 0.1MHz to 15 MHz. The operating system is connected to an ultrasonic transducer which scans the test specimen. The transducer is typically separated from the test specimen by a couplant such as oil or by water, as in immersion testing. US-scan experiments are carried out by either in a pulse echo mode or in a through transmission mode. In the pulse echo mode, (as used in these studies) the transducer performs both the transmission and reception of the pulsed waves as the ultrasound is reflected back to the device. Reflected Journal of Non destructive Testing & Evaluation
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Technical Paper
3. EXPERIMENTAL DETAILS The overall details of the experimental methodology adopted in these studies is presented in a nutshell in Fig.2 3.1 Composite Laminate Fabrication
Fig. 1 : Pulse echo mode with submerged specimen
ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. A schematic of the pulse echo technique is presented in Fig. 1 In the C-scan mode, the response of the test material to the ultrasonic wave is in the form of images with different colour indicating the uniformity of quality (or variations).In the A-scan mode, the response of the material is in a waveform connecting the amplitude of the reflected wave and the time of flight (of the ultrasonic wave) between the front wall and back wall or the discontinuity. Thus while the C-scan provides a clue for the material quality over an area, the A-scan provides the quality across the depth. Together, the C-scan and the A-scan provide a comprehensive picture of the composite laminate quality.
The test laminates studied contained Epoxy resin system (LY5052-HY5052 resin system with the gel time of 100105 minutes supplied by M/s Atul industries, Mumbai, India) and Glass fabric (2x2 twill woven E-glass fabric of 260 gsm supplied by M/s Arun Fabrics Bangalore, India). The 4 test laminates used in these studies were fabricated out of 12 layers of E-Glass Epoxy composite system using RT-vacuum bag moulding technique (schematic diagram Fig. 3). The vacuum-time cycles were varied, to obtain a variability of 5-8% in the laminate void volume fraction, without appreciably affecting the thickness and hence the fibre fraction (65Âą 3%). The laminates were subjected to pressure twice during curing. The pressure of around 300mmHg was applied around 47 mins (half the gel time) for different duration of times (3mins, 7.5mins, 12 mins) to bleed a certain amount of resin. Pressure was applied again at around 100mins till 180mins(curing time) which was inversely proportional in magnitude(660mm Hg, 320mm Hg) to the duration of application in an attempt to render the thickness of the laminates practically insignificant. Further, the vacuum-time cycles employed to affect process variability and obtain the test laminates with different void contents are schematically presented in figure 4a-4d, as presented below.
Fig. 2 : Experimental Plan in a Nutshell Journal of Non destructive Testing & Evaluation
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Technical Paper
1. Vacuum Bag, 2. Release Film, 3. Bleeder, 4.Perforated Film, 5. Composite layup, 6. Seal, 7. Base Plate and 8. Release.
The laminate L2 was fabricated using the same cure-time as used for laminate L1. This laminate however was allowed to bleed from the top and bottom also (surface bleeding) using a bi-directional glass fibre cloth of the same size as bleeder, as it absorbs the resin to a great extent. The laminate L4 was prepared debulked for every 3 layers with full vacuum pressure (as done in autoclave process) in order to take it as a reference laminate. These laminates coded L1(EB), L2(SB), L3(EB), L4(DB), were studied in the US-C-scan and A-scan modes. The abbreviations EB, DB and SB stand for Edge Bleeding, Debulking and Surface Bleeding.
Fig. 3 : Schematic of Vacuum Bag moulding
3.2 US-scan tests
The laminates L1 and L3 were fabricated by edge bleeding technique, which is a very common technique used for prepreg in the autoclave moulding technique, where the resin bleeds only from the side of the laminates (similar to the autoclave moulding technique) sides of the laminates.
The ultrasonic immersion scanning of the specimen was done using pulse echo method with the specimen immersed in normal water. A 5 MHz probe of 12mm diameter was used. The ultrasonic system used was ULTRAN NDC
(a)
(c)
(b)
(d)
Fig. 4 : (a) Vacuum-time Cycle for laminate L1 (b) Curing Cycle of L2 (c) Curing Cycle of L3 (d) Curing Cycle of L4
Fig. 5 : Ultrasonic Experimental Setup Vol. 11, Issue 1 June 2012
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Technical Paper
(a)
(c)
(b)
(d)
Fig. 6 : (a) C-scan Image of laminate L1 (EB) ; (b)C-scan Image of laminate 2(SB) ; (c) C-scan Image of laminate 3 (EB) ; (d) C-Scan Image of laminate L4 (DB)
7000(automated immersion system) in association with ULTRASOFT software for data acquisition, control and imaging. The scanning speed was maintained at 10 mm/s and the resolution was 0.5mm. The laminates were Cscanned over an area of 60x60 mm2. In addition, the specimens were placed on a tripod, which was used as a reflective plane in order to distinguish the back wall echo from any other one. The test set up is shown in figure- 5 below.
Each colour in the C-scan images (as obtained in Fig. 6) represents the relative attenuation of the ultrasound wave in the material e.g. black represents least attenuation whereas white represents maximum attenuation. Therefore the areas of the black region of all the 4 laminates were
As regards the A-scan measurement the specimen was immersed in the tank and 16 readings were taken each from a grid of 20mm by 20mm marked on the specimen. Yokogawa oscilloscope (DLM 2022) of 2000MHz frequency was used to calculate the back wall amplitude.
4. RESULTS AND DISCUSSION 4.1 C-Scan studies
The C-scan images are presented in Fig. 6a-6d. Journal of Non destructive Testing & Evaluation
Fig. 7 : Areas of black region of Glass-Epoxy Laminates Vol. 11, Issue 1 June 2012
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Technical Paper
calculated using a program made in MATLAB by calculating the number of pixels of the region represented by the black colour and the total scanned area of the sample (3600 mm2) and then calculating the ratio of these two areas. Figure 7 shows the column diagram representing the areas of black region for the laminates studied 4.2 LAMINATE QUALITY INDEX (LQI) - A NEW PARAMETER
It is defined as the ratio of the area of black region of the C-scan to the total scanned area of the laminate.
thickness of the laminate and v is velocity of ultrasound in the laminate (3150 m/s for GFRP). The factor 2 in the equation comes because in the pulse echo method the wave travels twice in the medium. Thereafter, a rational statistical averaging approach was evolved, by considering the closest back wall amplitude values (disregarding the very high and very low values) in each case, and the resulting averaged values are presented in table- 2 along with the calculated-TOF(by equation 2) and those (by experiment) corresponding to the nearest of the statistically averaged amplitude. Table 2 : Quality Table of Glass-Epoxy laminates by A-scan
LQ1 =
Area (black of laminate having least attenuation —————————————— Total area of the laminate scanned =
Ab/ΔA
(1)
Based on the above criteria, the quality table of the glassepoxy laminates is generated in terms of the LQI, as presented in Table 1 Table 1 : Quality Table of Glass-Epoxy laminates by C-Scan Specimen
Area of laminate in black(mm2)-ΔAb
LQI (ΔAb/ΔA)
Laminate 1(EB)
0.33
0.000092
Laminate 2(SB)
3563.17
0.99
Laminate 3(EB)
2373.26
0.66
Laminate 4(DB)
1470.76
0.41
>
L3 (EB)
>
L4 (DB)
>
L1 (EB)
4.3 A-Scan Studies
For the A-scan studies, all the 4 test laminates were marked with a 16 square grid, each grid measuring 20mm by 20mm.The A-scans of such laminates were then taken by immersing them in the tank containing water as the liquid couplant and using a 5MHz probe. Accordingly, a total of 4x16 A-scans were generated. In materials with high attenuation (e.g. GFRP) where it is difficult to locate peak amplitude corresponding to the back wall reflection with certainty, the voltage gain was increased and the time of flights (TOF) for each of the 4 laminates were estimated using the following formula: t
=
2s —— v
(2)
Where t is the time of flight (TOF) in seconds, s is the Vol. 11, Issue 1 June 2012
Averaged amplitude (mV)
TOF calculated (ns)
L2
656
1422
1385
L3
360
1440
1435
L4
351
1244
1295
L1
266
1460
1495
TOF experimental (ns)
In this case, the criteria is that, higher the back wall amplitude greater is the energy reflected back and lesser are the discontinuities present in the sample and hence better is the quality of the composite laminate. Thus from the values of the back wall amplitudes of the laminates (Table-2) the A-Scan quality ranking of the Glass-Epoxy laminates stands as follows: L2 (SB)
As can be seen from the above table, and based on the US C-scan results and analysis, the laminate coded L1B with the LQI of 0.99 is the healthiest of all the laminates studied, and the quality ranking of the laminates follows the order given below: L2 (SB)
Laminate code
>
L3 (EB)
>
L4 (DB)
>
L1 (EB)
Typical A-scans for the 4 laminates are presented in figures 8a-8d.
5. QUALITY RANKING BY C-SCAN AND A-SCAN- COMPARATIVE ASSESSMENT Table–3 presented below summarizes the quality ranking of the 4 Glass-Epoxy laminates investigated with different void volume fractions (created with varied process histories) as derived from the Ultrasonic C-Scan and A-scan experimental results. Table 3 : Comparison of laminate quality by US C-scan and A-scan techniques Specimen code
Void volume fraction (%)
LQI (by C-scan)
Back wall amplitude (mV) (By A-scan)
L2
5.60795
0.99
656
L3
5.0797
0.66
360
L4
8.1367
0.41
351
L1
8.2150
0.000092
266
It can be seen that both C-scan and A-scan results give the same relative quality ranking for the Glass-Epoxy laminates investigated. Journal of Non destructive Testing & Evaluation
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Technical Paper
(a)
(c)
(b)
(d)
Fig. 8 : (a) A-scan of laminate L2 ; (b) A-scan of laminate L3 ; (c) A-scan of laminate L4 ; (d) A-scan of laminate L1
6. CONCLUSION
REFERENCES
Glass-Epoxy laminates with different void volume fractions (5-8%) were prepared by RT-vacuum bag moulding process with varied vacuum-time cycles, the thickness and the fibre fractions of all laminates remaining unaffected. Overall, the quality ranking by Ultrasonic NDT technique (both by C-Scan and A-scan modes) for the laminates studied is corelatable with the void volume fraction of the laminates, results clearly confirming that laminates with lower void content (5%) have better quality than those with higher void content (8%).Finally, a good parity was seen between the results obtained from C-scan and A-scan results, both giving the same quality ranking for the glass-epoxy laminates studied.
1. Raja Manuri Venkata Gopala Krishna Rao, “Engineering of Science-The Composite Way” Annals of Indian National Academy of Engineering, Indian National Academy of Engineering, Volume-4 April 2007.
ACKNOWLEGEMENT The authors acknowledge the useful discussions with Mr. Vijay Kumar, Research Scholar, Ms. Nida Ali, Project Assistant and the timely support of Mr. Ranganatha, Senior Technical Officer in laminate preparation, at the Department of Aerospace Engineering, Indian Institute of Science, Bangalore.
Journal of Non destructive Testing & Evaluation
2. Brian Stephen Wong, Chua Fong Ming Ron, Ow Wing Yoong and Tui Chen Guan, “Non-Destructive Testing Of Fiber Reinforced Composites and Honeycomb Structures”, Proceedings of Defense Materials and Mechanics Seminar, Singapore. 1999. 3. Ramanan Sridaran Venkat, Boller Christian, Andrey Bulavinov and Sergey Pudovikov, “Quantitative Non-destructive Evaluation of CFRP Components by Sampling Phased Array”, AeroNDT 2010- Emerging Technologies, March 2011. 4. Seung-Joon Lee, Young-Joon Ha, Joon-Hyun Lee and JoonHyung Byun, “Expeimental Evaluation of Delaminations in CFRP using Laser-Based Ultrasound”, The 1st Tohoku-PNU Joint Workshop on Mechanical Science based on Nanotechnology, Busan, Jan 8, 2007 5. M. A. Perez, L. Gil and S. Oller, “Non-destructive testing evaluation of low velocity impact damage in carbon fiberreinforced laminated composites”, CompNDT 2011 –Ultrasonic Techniques for Composite Material, May 2011
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Technical Paper 6. G. Wróbel , Z. Rdzawski, G. Muzia and S. Pawlak, “Quantitative analysis of the fibre content distribution in CFRP composites using thermal non-destructive testing”, Archives of Materials Science and Engineering, Volume 41 Issue 1, Jan 2010, Pages 28-36. 7. Johann Kastner, Bernhard Plank, Dietmar Salaberger and Jakov Sekelja, “Defect and Porosity Determination of Fibre Reinforced Polymers by X-ray Computed Tomography”, 2nd International synopsium on NDT in Aerospace, Hamburg, D, Deutschland, 2010. 8. K.Koyama, H.Hoshikawa and T.Hirano, “Investigation of Impact Damage of Carbon Fiber-Reinforced Plastic (CFRP) by Eddy Current Non-Destructive Testing”, NDT of Canada 2011, Feb, 2012. 9. R. Marat-Mendes and M. Freitas, “Non Destructive Evaluation of Delamination in Glassfibre Composites Using C-Scan Analysis”, 16th International Conference on Composite Structures, Porto, 2011. 10. G. Wróbel, £. Wierzbicki and S. Pawlak, “A Method for Ultrasonic Quality Evaluation of Glass/Polyester Composites”, Archives of Materials Science and Engineering, Volume 28 Issue 12 December 2007, Pages 729-734.
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47 11. C Scarponi and G Briotti, “Ultrasonic technique for the evaluation of delaminations on CFRP, GFRP, KFRP composite materials”, Composited, Part B 31 (2000) 237-243. 12. Tarapada Roy and Debabrata Chakraborty, “Delamination in FRP laminates with holes under transverse impact”, Materials Design, Elsevier, 2008. 13. C.K.Y. Leung, Y. Jiang, M.Y.M. Ng, M. Motavalli and D. Gsell, “A Fiber-Optics Based Technique for Delamination Detection at the Web/Flange Junction of GFRP I-Beams”, Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland, pages 1-6. 14. D. Pradeep, N. Janardhana Reddy, C. Rahul Kumar, L. Srikanth and R.M.V.G.K. Rao, “Studies on Mechanical Behavior of Glass Epoxy Composites with Induced Defects and Correlations with NDT Characterization Parameters”, Journal of Reinforced Plastics and Composites (2007); 26; 1539. 15. Theodoros Hasiotis, Efstratios Badogiannis and Nicolaos Georgios Tsouvalis, “Application of Ultrasonic C-Scan Techniques for Tracing Defects in Laminated Composite Materials”, Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 192203.
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Technical Paper
Low Heat Flux Transient Thermography for Defect Detection in Thick Composite Structures K Srinivas, T Murugesh and J Lahiri DoCMP & NDE, Advanced Systems Laboratory, Kanchanbagh PO, Hyderabad-500058
ABSTRACT Thick composite structures (thickness ~ 15 mm) with carbon fiber reinforcement (CFRP) and epoxy resin matrix are being used in various aerospace applications. Quality of final CFRP composite structures are highly process dependent. Several defects such as airgaps and delaminations manifest within composite structure during processing. Defects such as delaminations and porosity severely affect the performance of the composite. Hence, defects in thick CFRP composite have to be reliably detected for final applications. Thermography is one of the most important techniques for NDT of CFRP composites. It is a non-contact, fast and reliable NDT technique for thin composite structures (thickness ~ 5 mm). For thick composites established techniques do not show all the defects. In order to detect defects which are deeper in thick composite structures improved heating systems with complex electronics along with exhaustive data processing techniques are used. This paper reports, low heat flux (4 kW) transient thermography technique adopted for defect detection (defect depth 7.5mm) in thick CFRP composites (thickness upto 15 mm). CFRP composite laminate with implanted air-gaps at several depths within the composite laminate has been specially fabricated for simulating defects for these studies. Presence of defects and their defect depths are correlated with ultrasonic NDT methods. Pulse phase thermography method has been adopted along with improved experimental procedures for defect detection. Phase imaging and correlation technique have been adopted for data processing. A comparison of results obtained with flash (9.6 KJ), lock-in (with 4KW halogen lamps) and low heat flux pulse phase transient thermography technique is presented. Keywords: Composites, NDT, Thermography PACS: 81.05.QK, 81.70.-q, 81.63.Hg
INTRODUCTION Active Infrared (IR) thermography NDT is well known technique for defect detection in carbon fiber reinforced (CFRP) composite structures [1-4]. In this technique, external heat stimulus is given on the test surface to generate thermal wave inside the material. As the thermal wave propagates into the composite, surface temperature profile changes due to the reflected thermal wave, which is then captured using an Infra red (IR) camera for further analysis. IR images are captured either during heating or during cooling to capture the defect information carried by the thermal waves onto the test surface. From the captured surface thermal profiles, temperature difference between defective area and non-defective area is detected for attributing the difference to the presence of defects beneath the test surface. Since, CFRP has lower thermal diffusivity than metals, it is possible to use the technique effectively for defect detection by manipulating the various input parameters such as heating time, heat flux and capture rate. Conditions for effective defect detection in CFRP materials being, use of sufficiently high heat flux to generate thermal wave which reaches the defect depth with higher amplitude to show the temperature difference in the reflected thermal wave. During heating, use of higher heat flux is restricted for composite structures as higher heat flux may result in high temperature over the test surface which may be beyond the operating temperature of the material. Other condition is that during transient thermography data has to be captured sufficiently long to get the defect signature, this may depend on defect depth Journal of Non destructive Testing & Evaluation
and thermal diffusivity of the material under test. Various active thermography techniques were proposed and being implemented for defect detection in composite structures [5-6]. However, all the techniques were restricted to defect detection for sub-surface defects only. In case of thick CFRP composite structures (thickness~15 mm) defect detection is severely affected by high lateral diffusivity and low signal to noise ratio (SNR) [7-8]. This paper reports application of low heat flux transient thermography techniques for thick CFRP (thickness ~ 15 mm) composite structures for detecting deeper defects without damaging the test surface. A comparison of defect detection with low heat flux pulse thermography technique, truncated lock-in thermography and high heat flux pulse (flash) thermography is presented.
EXPERIMENTAL Four halogen lamps each with 1 kW power were chosen for giving external heat stimulus to the carbon epoxy (CE) laminate for our selected heating time for the present studies. These halogen lamps were connected to the output of the power supply box which can be manipulated by an external signal generator. Halogen lamps were chosen for their low heat flux values and their ease of adaptability for shop floor applications. The power supply for the halogen lamps has provision to control both voltage and heating time for experiments. Four halogen lamps are arranged in such a way that uniform heating is achieved across the test surface within an area of 300 mm x 300mm. Figure 1 Vol. 11, Issue 1 June 2012
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Fig. 1 : Line diagram of the experimental setup
shows line diagram of the experimental setup used for the present studies. IR power controller card from M/s Automation Technology, GmBH has been used for controlling power supply to the halogen lamps. Signal generator, Image acquisition board and IR camera are synchronized to get correct time signatures for IR images being captured. However, halogen lamps have required warm up time of at least 1 s for reaching to their highest efficiency, which has been taken careof while applying heat stimulus for the test object. Carbon epoxy 0-90 cross ply test laminate (lam1) having 15 mm thickness with embedded air-gaps has been chosen for our studies. Laminate defect configuration, defect sizes and their depths from the test surface are shown in the figure 2(a). Plastic mesh of size 10 mm x 10mm and 20 x 20 mm (thickness ~ 0.5mm) were used for creating delaminations (plastic mesh has been sealed with high temperature plastic sheet to prevent resin flow into the air gaps). Figure 2(b) shows ultrasonic through transmission C-scan image of the test laminate captured using noncontact ultrasonic probes (200 kHz). C-scan image confirms the presence of defects at pre designated locations. Defect depth from the test surface for each of the defects is calculated using ultrasonic pulse echo technique (frequency of 2.5 MHz). Defect depths were calculated to be 1.5 mm,
4.1mm, 7.5 mm, 8.3 mm and 11 mm from the surface. Shape of the defects in ultrasonic C-scan image is distorted due to the use of larger diameter probes (25 mm) for both transmission as well as reception as compared to defect size. The defect of 10 mm x 10 mm size at 11 mm depth has appeared to be smaller than its actual size; this may be due to puncturing of high temperature plastic film over the defect allowing resin flow into the air gap, thus reducing the size of the defect. For the present work defects upto the depth of 8.3 mm were only considered and defects at deeper depth (11 mm) were not covered under the current investigation.
RESULTS AND DISCUSSION LOW HEAT FLUX TRANSIENT THERMOGRAPHY
Low heat flux transient thermography experiment on Carbon epocy (CE) laminate has been performed using 4 kW heat flux (4 halogen lamps each with 1 kW heat flux). Heating is done for 40 s and data is captured for 200 s from the beginning of heating. IR images of the test laminates were recorded with 5 Hz capture rate, so that defect information is completely captured. Figure 3 (a) shows raw image of the test laminate at 50 s from the beginning of heating. Raw image shows appearance of
Fig. 2 : (a) shows line diagram of the carbon epoxy test laminate (CE) with implanted defects (b) Ultrasonic through transmission C-scan image of the test laminate. Defects present in the laminate are clearly seen with higher attenuation in the through transmission signal. Vol. 11, Issue 1 June 2012
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50 defects at 1.5 mm depth clearly, however defects at 4.1 mm are faintly visible. Other deeper defects are not visible in unprocessed raw image due to non-uniform heating pattern as well as higher temporal noise present in the data. Figure 3(b) shows evolution of pixel amplitude over defect and non-defect regions as a function of time. Pixel amplitude shows that defect has higher values as compared to non-defect indicating that defect to be hot (higher temperature) when compared to non-defect. The captured IR images are processed using various data processing techniques [10-12]. Figure 4(a) shows resultant image of IR image at 60 s subtracted with last image (at the end of capture i.e 200s) and normalized with end of heating. Normalized image shows presence of defects at 1.5 mm depth (both 10 x 10 mm and 20 x 20 mm), 4.1 mm depth (both 10 x 10 mm and 20 x 20 mm) and 7.5 mm depth (20 x 20 mm size) clearly. However, deeper defect at 8.3 mm (10 x 10 mm) is faintly visible. All the defects at 4.1 mm depth and 7.5 mm depth are distorted and their boundaries are marked with noise. Table 1 shows SNR of each of the defects at different depths and their defect appearance time during transient thermography. Defects of 20 x 20 mm size at (different depth) appear have higher SNR compared to 10 x 10 mm defects at same depth. In comparison, higher SNR is observed for defect at 7.5 mm depth with low heat flux of 4 kW. However, defect at 8.3 mm depth shows low SNR and appear to be distorted due to (i)nonuniformity of heating (ii) defect size (10 x 10 mm) and (iii) higher surface temporal noise.
Technical Paper
Table 1 : SNR of defects and defect appearance times after data processing with normalization and subtraction Frame no(appearance time ) 6 (1.2 s)
Defect depth-size SNR (4KW) 1.5mm-20X20mm
24.0
1.5mm-10X10mm
16.6
4.1mm-20X20mm
19.7
4.1mm-10X10mm
11.7
316 (63.2 s)
7.5mm-20X20mm
16.9
386 (77.2 s)
8.3mm-10X10mm
7.4
116 (23.2 s)
For improved defect detection, phase difference between defect and non-defect regions was taken, which provided reliable defect information beneath the surface of the test laminate. In order to obtain the phase information for each pixel of the 240 x 320 pixel thermal image, FFT is applied over entire temporal thermal profile of each pixel. The applied FFT algorithm over the temporal thermal history of the pixel separates the phase detail in the frequency domain and provides phase information corresponding to the constituent frequency component [9]. A lab-view (NI, USA) program was written for separating phase and magnitude information for the each of the pixels of the thermal image. The application of one-dimensional FFT on the thermal profile of the pixel results in
(1)
Fig. 3 : (a) shows raw image of the test laminate at 50 s from the beginning of heating (b) evolution of pixel amplitude (i)over defect shown with red line and (ii) over non-defect region during thermography experiment.
Fig. 4 : (a) shows subtracted IR image at 60 s (at maximum contrast time i.e from the beginning of heating) with last IR image (i.e at the end of 200s of capture) and normalized with end of heating (b) shows evolution of pixel amplitude as function of time (normalized with pixel value at the end of heating) Journal of Non destructive Testing & Evaluation
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where k is the sample number (harmonic number) in the obtained FFT of the response profile (y) containing N samples (equivalent to number of frames captured by IR camera). The component phase corresponding to different frequencies can be obtained by computing ϕ(k) = tan–1 (Imk/Rek)
(2)
The frequency of the corresponding component is calculated from the relation (3)
where fk is the frequency of the kth component in the Fourier domain, fs is the sampling frequency (frame rate of IR camera) and N is the number of samples (total number of frames). The phase difference of all the pixels at a particular frequency constitutes the phase image at that frequency [9]. Figure 5(a) below shows phase image (1st harmonic image at 0.006 Hz) after data processing of the above mentioned experimental data. Due to higher frequency resolution of the data, all defects (upto 8.3 mm depth) appear with high phase difference when compared to background. Defects at 1.5 mm depth have highest phase difference when compared with background. However, deeper defect at 8.3 mm (size 10 x 10 mm) shows low phase difference and appears with low contrast in the phase image due to its small size relative to the depth. Table 2 shows SNR of defects present in the test laminate and their relative depths. Highest SNR is observed for shallow defects at 1.5 mm depth. Defect at 8.3 mm depth shows SNR of 10 which is comparable with other data processing techniques such as normalization and subtraction (see table 1 above). In order to reduce the effect of noise present in the data, pixel by pixel data of transient thermography experiment mentioned above has been processed using correlation function. A lab view program for generating correlation image has been written for processing each pixel data with
reference pixel selected from non-defect region. The resultant correlation image is shown in figure 5(b) above. Correlation image shows defects with improved contrast upto a depth of 7.5 mm. However, defect at 8.3 mm appears with poor contrast. Table 2 above shows SNR of defects and their depths from the test surface calculated from correlated image. Defects upto 7.5 mm depth appear with higher SNR when compared with phase imaging. However, 8.3 mm deep defect appears with lower SNR when compared with phase image, this may be due to selection of reference pixel closer to the centre of heating on the test laminate. SNR of deeper defect at 8.3 mm may be lower due to the larger observation time and correspondingly large lateral diffusivity (comparable to signal itself). Table 2 : SNR of defects calculated from phase image and correlation image Defect depth-size
SNR (4KW) From phase image (1st harmonic at 0.006 Hz)
SNR (4 KW) From correlation method
1.5mm-20X20mm
64
375
1.5mm-10X10mm
50
275
4.1mm-20X20mm
32
141
4.1mm-10X10mm
10
48
7.5mm-20X20mm
39
66
8.3mm-10X10mm
15
10
TRUNCATED LOCK-IN THERMOGRAPHY In case of truncated lock-in thermography, experiments were performed at frequencies whose diffusion length (function of thermal diffusivity and frequency) equals to the depth of defect to be investigated. Since for the present laminate thermal diffusivity along thickness direction and defect depths are known, frequency of 0.01 Hz and 0.07 are selected for investigating defects at 4.1 mm depth and 1.5 mm depth respectively. In this technique, IR images were captured during heating cycles and experiment has
Fig. 5 : (a) phase image at 0.006 Hz (1st harmonic image) of laminate (b) shows correlation image of the laminate Vol. 11, Issue 1 June 2012
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while heating the laminate during 3 cycles of thermal excitation. Red line shows defect pixel amplitude evolution and blue line indicates non-defect pixel amplitude as a function of time. The pixel evolutions were observed to be following the halogen lamp heating pattern. It is observed that during cooling lamp filament does not completely cool and radiation which seems to have distorted the sinusoidal excitation (see arrow mark in the figure 6 below).
Fig. 6 : Shows evolution of pixel amplitude as a function of time during truncated lock-in thermography experiment (f=0.01Hz, 3 cycles and 2kW heat flux).
been terminated at the end of heating cycle without waiting for the test surface to reach equilibrium temperature. Figure 6 and 7 shows pixel amplitude evolution as a function of time for truncated lock-in thermography experiment at 0.01 Hz and 0.07Hz excitation frequency with 2 kW heat flux excitation energy. Thermal images were captured at 5 Hz
Figure 8(a) shows phase image of CE laminate (after data processing) obtained from lock-in thermography experiment at 0.01 Hz excitation frequency and 2 kW heat flux. Defects upto a depth of 7.5 mm were clearly visible after processing. Defects at 1.5 mm depth and 4.1 mm depth are with higher contrast when compared to 7.5 mm depth defect. Experiments were repeated with higher heat flux value of 4 kW to improve SNR of the defects. Figure 8 (b) shows phase image of CE laminate obtained from lock-in thermography experiment at 0.01Hz excitation frequency and 4 kW heat flux. Defects at 1.5 mm depth and 4.1 mm depth are clearly visible but deeper defect at 7.5 mm is barely visible in the 1st harmonic phase image. Deeper defect at 7.5 mm depth has appeared after data processing which was not expected as the selected excitation frequency of 0.01 Hz is expected to have diffusion length of 3.7 mm only, however the appearance of deeper defect may be due to presence of low frequency components in the excitation frequency while using halogen lamp heating. This is due to inherent delay in heating as compared to the excitation of power supply. Further investigation is under progress for exploiting the technique.
Fig. 7 : Shows evolution of pixel amplitude as a function of time during truncated lock-in thermography experiment (f=0.07Hz, 3 cycles and 2kW heat flux).
Fig. 8 : (a) shows Lock-in thermography (0.01 Hz) phase image of CE laminate with 2 kW halogen lamp heating (b) shows Lock-in thermography (0.01) phase image of CE laminate with 4 kW halogen lamp heating. Journal of Non destructive Testing & Evaluation
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Fig. 9 : (a) shows Lock-in thermography (0.07 Hz) phase image of CE laminate with 2 kW halogen lamp heating (b) shows Lock-in thermography (0.07 Hz) phase image of CE laminate with 4 kW halogen lamp heating.
Figure 9(a) shows phase image of CE laminate obtained from truncated lock-in thermography experiment at 0.07 Hz and 2 kW heat flux. Defects at 1.5 mm depth are clearly visible with 2 kW heating after data processing. Figure 9(b) shows phase image of CE laminate after heating with 4 kW. Defects upto 1.5 mm depth are visible, however deeper defects were not visible even with 4 kW heat flux. Table 3 shows SNR of 1.5 mm depth defect and 4.1 mm depth defect captured calculated from truncated lock-in thermography experiment performed at 0.01 Hz and 0.07 Hz excitation frequency. No increase in SNR of defects at 1.5 mm depth was observed with increase in heat flux from 2kW to 4 kW. SNR of defect at 4.1 mm depth is observed to be lower than transient thermography technique. Lower frequency truncated lock-in thermography may be useful for detecting deeper defects upto 8.3 mm, since the experimental time required for lower frequency is long we have not explored this for the present paper. In case of shallow defects at 1.5 mm depth, truncated lockin thermography after data processing is observed to show defects with best defect features such as shape and size. Truncated lock-in technique is observed to be in-sensitive to non-uniform heating.
FLASH THERMOGRAPHY
Table 3 : SNR of defects as a function of heat flux of halogen lamps in Lock-in thermography experiment
Table 4 : SNR of defects as a function of depth in flash pulse thermography
Lock-in frequency
Defect depth-size
SNR (4KW)
SNR (2KW)
0.07Hz
1.5mm-20X20mm
26.3
26.1
1.5mm-10X10mm
24.3
23.4
4.1mm-20X20mm
Not visible
Not visible
4.1mm-10X10mm
Not visible
Not visible
1.5mm-20X20mm
6.1
4.3
1.5mm-10X10mm
8.5
7.5
4.1mm-20X20mm
10.6
10.4
4.1mm-10X10mm
6.6
5.8
0.01Hz
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For comparison flash pulse thermography has been performed using 9.6 kJ heat flux flash lamps for 5 ms, data has been captured upto 200 s at 5 Hz capture frequency. Figure 10(a) shows raw thermal image of test laminate with flash pulse thermography at the end of 15 s of capture. Defects upto 1.5 mm depth are clearly visible without data processing. However, defects which are deeper are not visible. Raw data of each pixel has been further processed with FFT for obtaining phase information. Figure 10 (b) shows phase image at 0.053 Hz after data processing. Defects at 1.5 mm depth are clearly visible; however defect at 4.1 mm is faintly visible. SNR for each of the defect has been calculated and shown in table 4 below. Shallow defects at 1.5 mm depth appear with higher SNR when compared with low heat flux truncated lock-in thermography due to use of higher heat flux for excitation. However, the excitation energy dissipates much faster than both transient method as well as truncated lock-in thermography method, thus resulting in lower SNR values for deeper defects. Fig. 10 : (a) shows raw image of the test laminate at the end of 15 s (b) phase image at 0.053 Hz
Phase image at
Defect depth-size
SNR
Frequency 0.053Hz
1.5mm-20X20mm
42
Frequency 0.0093Hz
4.1mm-20X20mm
11.5
CONCLUSIONS Low heat flux transient thermography with careful selection of heating time and data capture rate is observed to be useful for detecting deeper defects upto 8.3 mm (size 10 x 10 mm) in thick carbon epoxy laminates.SNR for defects observed from low heat flux transient thermography technique appear to be higher than other techniques with comparative heat flux values. Truncated lock-in thermography method is observed to be useful technique
Journal of Non destructive Testing & Evaluation
54 for defect detection upto 7.5 mm depth with 4 kW heat flux heating systems. High energy flash thermography in comparison is observed to be useful for detecting subsurface defects upto 4.1 mm depth.
REFERENCES 1. X.P.V.Maldague and S.Marinetti, J.Appl. Phys. 79, pp. 2694 (1996). 2. V.P.Vavilov and D.Burleigh, Nondestructive testing handbook, ASNT,(2001),3, pp. 54-75. 3. V.P.Vavilov, Nondestructive testing monographs and traces, Gordon and Breach science publishers, Great Britain, (1992). 7,pp. 131-210. 4. X.P.V.Maldague, Theory and practice of Infrared technology for non-destructive testing, John Wiley & Sons, New York, USA (2001).
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Technical Paper 5. X.P.V.Maldague, Materials Evaluation, 9, pp. 1060-1073 (2002). 6. V.P.Vavilov, Thermosense XXII, SPIE, 4020,pp. 152-162 (2000). 7. A.O.Siddiqui ,K.Srinivas and J.Lahiri Proc. ISNT NDE annual conference, Kolkotta (2005) 8. K.Srinivas,A.O. Siddiqui and J.Lahiri, Proc. ISNT NDE annual conference, Kolkotta (2005) 9. Busse G, Wu D, and W.Karpen, J. Appl. Phys. 71,pp. 3962-3965 (1992). 10. M.Choi, K.Kang, J.Park, W.Kim, and K.Kim NDT and E International, 41(2), pp.119–124 (2008) 11. R.Mulaveesala and S.Tuli Applied Physics Letters, 89, 19, 1913 (2006). 12. V.S.Ghali and R.Mulaveesala Sens. Imaging 12, pp.15-33 (2011).
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Technical Paper
Detection of fine defects in steam generator tubes of 220 MWe Indian PHWRs using eddy current array probes H.M. Bapat, Manojit Bandyopadhyay, R.K. Puri and Manjit Singh Division of Remote Handling and Robotics, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, INDIA
ABSTRACT The inspection of components used in nuclear industry is a critical issue for the safety of reactor. One of the main component in nuclear reactor is steam generator. Steam generator tubes need to be inspected regularly during In-service inspections. At present standard eddy current bobbin probes are used for inspection of these tubes. This Present bobbin coil technique mostly detects only wall thinning (Volumetric degradation). An acceptance criterion is less than 40%. This bobbin coil technique cannot detect very fine cracks and circumferentially oriented defects. These very fine defects are detected using hydrogen/helium leak test and based on results of leak test tubes are plugged or removed. So it is necessary to detect very fine cracks and circumferentially oriented defects using eddy current technique. This paper describes an arrayed multi-coil probe technique, newly developed for the testing of steam generator tubes by eddy current testing (ECT). This probe can detect both axial and circumferentially oriented fine defects. Because of the multi-coil arrangement, the arrayed probe has a high detection speed around the whole tube, without the need for rotation. Keywords :Eddy current, T/R array probe, EDM notches, C-Scan display
INTRODUCTION The inspection of components used in nuclear industry is a critical issue for the safety of reactor. One of the important component in nuclear reactor is steam generator. A variety of degradation modes can affect the integrity of steam generator tube bundles, resulting in expensive repairs, tube plugging or replacement of tube bundles. One key component for ensuring tube integrity is inspection and monitoring for detection and characterization of the degradation. In-service inspection of steam generator tube bundles is usually carried out using eddy current (EC) bobbin coils, which are adequate for the detection of volumetric degradations. They are quite reliable and provide repeatable results, being able to reliably detect and size volumetric flaws such as fretting wear and pitting corrosion. However, they are ineffective in detecting circumferentially oriented cracks because the induced current in the tube wall circulates parallel to the coil windings and is inherently unaffected by the presence of such cracks. These probes are sensitive to axial cracks at straight tube sections; however, at defect-prone areas such as top of tubesheet (TTS) and U-bend transition, the large signals generated by geometrical tube-wall distortions significantly reduce detectability. Because of this shortcoming with bobbin coil probes, mechanically rotating pancake coil (RPC) probes have been implemented world-wide for inspecting tubes that are suspected to have circumferential cracks. Eddy currents induced by these probes have circumferential and axial components that interact with cracks oriented in all directions. Vol. 11, Issue 1 June 2012
However scanning by these probes is very time consuming and costly process. Axial scanning speed is about 200 times slower that of bobbin probes. These mechanically rotating pancake probes are usually spring loaded to minimize lift-off, which makes them prone to failure. This is especially evident in situations where the presence of internal tube deposits can reduce probe life significantly. To overcome all these problems Transmit/Receive arrayed multi-coil probes are developed. These Transmit/Receive (T/R) array probes take advantage of the superior properties of T/R technology compared to impedance probe technology. Transmit-receive (T/R) array probes offer high defect detectability in conjunction with fast and reliable inspection capabilities. They can detect both axial and circumferentially oriented defects. The data is displayed in C-Scan format which helps in better interpretation of results.
EDDY CURRENT ARRAY PROBE Eddy current array and conventional eddy current technology share the same basic principle. Alternating current injected into a coil creates a magnetic field. When the coil is placed over a conductive part, opposed alternating currents (eddy currents) are generated. Defects in the part disturb the path of the eddy currents. This disturbance can be measured by the coil. Eddy current array probe consists of number of small eddy current coils placed side by side in form of an array in same probe assembly. The probe used by us consists of 16 elements (16 small surface coils) arranged in two rows. 8 elements are mounted on top row and 8 elements are in Journal of Non destructive Testing & Evaluation
56 bottom row. The elements are arranged in such a way that top and bottom row elements are diagonal to each other. Eddy current instrument INSIS-EX is used for testing the probe. The instrument has the ability to electronically drive multiple eddy current coils placed side by side in the same probe assembly. Data acquisition is performed by multiplexing the eddy current coils in a special pattern to avoid mutual inductance between the individual coils. To simplify the process of signal analysis and make it more user-friendly, the data is displayed in C-scan format. This display method is a valuable tool in helping to visualize flaw morphology and location while reducing the number of data channels to be analyzed retaining all the original data. Eddy current array probe along with reference defect standard and instrument is shown in Fig. 1. Defect standards
Axially and circumferentially oriented notches were fabricated using EDM technique on outer surface of steam generator tubes of 220MWe reactor. The material of these
Fig. 1 : The complete experimental setup with probe, defect standard and instrument
Technical Paper
tubes is Incoloy 800. Outside diameter of these tubes is 16 mm and thickness is 1mm. Since machining is very difficult in the inner surface of tubes, notches were fabricated on outer surface. These notches have 200 micron and 300 micron depth. The dimensions of these notches are given in table 1. The images of these defect standards along with array probe is shown in Fig. 2. Table 1 Sr No.
Length
Width
Depth
1.
6 mm
150 micron
200 micron
2.
6mm
150 micron
300 micron
RESULTS AND DISCUSSION A flexible T/R array probe composed of sixteen elements is used for picking up EDM notches fabricated on tube of steam generator. The results of scanning are shown as 2D and 3D images in transverse and longitudinal mode. Circumferentially oriented notches are best picked up and shown in transverse 2D & 3D image while axially oriented notches are best picked up and shown in longitudinal 2D & 3D image. Scanning is carried out at 200kHz. All major cracks, even the narrowest ones (200 ĂŹm), show a very good signal to noise ratio. Signals obtained from Very fine EDM notches are clearly visible as shown in Figure (dark zone on the CSCAN image). The experimental data show the good results in terms of detection obtained with the array probes for small cracks. The probe is able to pickup notches from outer surface of tubes, where sensitivity is
Fig. 2 : Reference defect standard along with array probe
Fig. 3 : C-Scan image and strip chart of 200 micron depth Circumferential notch on outer diameter of Steam Generator tube Journal of Non destructive Testing & Evaluation
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Fig. 4 : C-Scan image and XY plot of 300 micron depth axial and Circumferential notch on outer diameter of Steam Generator tube
Fig. 5 : XY plot and strip chart record of 300 micron depth axial and Circumferential notch on outer diameter of Steam Generator tube
Fig. 6 : C-Scan image of defects of conventional reference defect standard of Steam Generator tube Vol. 11, Issue 1 June 2012
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58 less. This means probe will have more sensitivity for defects on inner surface of tube. Test results obtained by scanning tube having defect standards with array probe are shown in Figures 3 to 6. Figure 3 shows C-Scan image and strip chart of 200 micron depth Circumferential notch on OD of Steam Generator tube. From strip chart it is clear that absolute channel A07 picked up the signal from circumferential notch. Figure 4 shows C-Scan image and XY plot of 300 micron depth axial and Circumferential notch on OD of Steam Generator tube. Axial notch is picked up and shown in Longitudinal 2D and 3D Image and circumferential notch is picked up and shown in Transverse 2D and 3D image. Figure 5 shows XY plot and strip chart record of 300 micron depth axial and Circumferential notch on OD of Steam Generator tube. The array probe is also used to scan conventional reference defect standard having various defects in terms of percentage wall thinning ( support ring(baffle plate), 20%, 40% ,60%, 80% of wall thickness and through hole). The CScan display of scanning results are shown in Fig. 6.
CONCLUSIONS Transmit/receive array probes can detect all degradation mechanisms in a single scan at a scanning speed similar to that of bobbin probes. Circumferential defects which are difficult to detect using conventional bobbin coil technique can be easily detected by array probes. Simultaneous detection and discrimination of circumferential and axial cracks can help reduce the need for re-inspection, tube replacement and forced outages. The C-Scan displays ease
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the interpretation. Since it is an eddy current technology, the surface preparation is minimal and inspection can be done without direct contact, or any liquid coupling or penetrant.
ACKNOWLEDGMENTS The authors are thankful to M/s Technofour, Pune for help in fabrication of reference defect standards and eddy current array probes.
REFERENCES 1. Obrutsky L.S., Cecco V.S., Sullivan S.P., Humphrey D., “Transmit-Receive Eddy Current Probes For Circumferential Cracks In Heat Exchanger Tubes”, Materials Evaluation, Vol. 54, No 1, pp. 93-98. The American Society for Nondestructive Testing, Inc. (January 1996). 2. Haoyu Huang, Nozomu Sakurai, Toshiyuki Takagi, Tetsuya Uchimoto, “Design of an eddy-current array probe for crack sizing in steam generator tubes” ,NDT&E, Volume 36, Issue 7, Oct 2003, Pages 515-522 3. Obrutsky L.S., Watson N., Fogal C., Cantin M., Cecco V.S., Lakhan J.R. and Sullivan S.P., “Fast Single-Pass Eddy Current Array Probe For Steam Generator Inspection”, Proceedings of the 4th CNS International Steam Generator Conference, Toronto (2002). 4. Obrutsky L.S., Watson N.J., Fogal C.H., Cantin M., Cecco V.S., Lakhan R.J. and Sullivan S.P. “Experiences and Applications of the X-Probe for CANDU Steam Generators”, 20th EPRI Steam Generator NDE Workshop, Orlando, Florida (July 9-11, 2001).
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Automatic Defect Recognition (ADR) System for Real Time Radioscopy (RTR) of Straight Tube Butt (STB) Welds Deepesh.V1, R.J. Pardikar1, K.Karthik 2, A. Sricharan 2 S. Chakravarthy 2 and K. Balasubramaniam 2 BHEL, Tiruchirapalli-620014,India Indian Institute of Technology Madras, Chennai 600036, India 1
2
ABSTRACT Non Destructive Evaluation (NDE) Methods, in particular Digital Radiography (DR), incorporated with Automatic Defect Recognition (ADR), for industrial applications is a rapidly progressing area of research across the globe. Though ADR technology has been well established for Digital Radiographic inspection of cast and machined components, ADR is still considered a challenge in case of many types of weld joints, mainly due to the non-uniformity in the radiographic images of weld joints. This paper introduces an indigenously developed Automatic Defect Recognition System for Real Time Radioscopy (RTR) of Straight Tube Butt Weld (STBW) joints, which are the critical joints of tubular components like Economiser, Super heater and Reheater of a Boiler. RTR system for inspection of STB welds consists of a constant potential X-Ray equipment with swiveling arrangement, as the X-Ray source, Digital Flat Panel (DFP) as the Imaging device with its associated image acquisition and review software, along with the ADR software for defect recognition, classification and thereby evaluation of STB welds. ADR Algorithm, scans through the Digital X-Ray image of the STB Weld joint, and detects the defects present and takes the decision of Acceptance / Rejection, based on the Acceptance Standards for STB welds. Artificial Neural Network (ANN) techniques enable the ADR system to continuously learn and grow more efficient with every joint it evaluates. This will enhance the reliability of defect detection and evaluation. The preprocessing uses concepts from digital image processing, image analysis, and pattern recognition. The development of this system involves validation with a wide range of weld samples with various types of discontinuities. The system has been implemented in one of the RTR stations in BHEL and the ANN training has so far resulted in over 95% accuracy level. This system replaces the hitherto used, manual evaluation procedure and removes its inherent limitations like subjectivity, inconsistency, and fatigue and accomplishes a faster and more reliable evaluation. Keywords: Automatic Defect Recognition, Real Time Radioscopy, Digital Radiography, Digital Image Processing, Pattern Recognition, Artificial Neural Network, Radial Basis Function.
INTRODUCTION
REAL TIME RADIOSCOPY (RTR) OF STB WELDS
Radiography is very well established as an NDT technique, using both film and electronic X-Ray detection systems. Mainly used in petroleum, petrochemical, nuclear and power generation industries, for inspection of welds, radiography has played an important role in the quality assurance of the piece or component, in conformity with the requirements of the standards, specifications and codes of manufacturing. Most radiographic exposures and film interpretations in RT are still carried out manually (1). Human interpretation of weld defects, however, is tedious, subjective and is dependent upon the experience and knowledge of the inspector (2). Human inspectors are not always consistent and effective evaluators of products because inspection tasks are monotonous and exhausting. It has been reported that human visual inspection is at best 80% effective. In addition, achieving human ‘100%inspection’, where it is necessary to check every product thoroughly, typically requires high level of redundancy, thus increasing the cost and time for inspection (3). Here comes the importance of automation of evaluation, which reduces human involvement, thus making the inspection more reliable and faster.
Tubular products form an important part of Steam Generators in thermal power plants. These include mainly Super heater, Reheater, Economizer coils, water wall panels etc. These components consist of tube assembly of several meters length and the required length is achieved by Straight Tube Butt (STB) welding process. These tubes are made of carbon steel, alloy steel etc. It is a pulsed Metal Inert Gas welding process using spray type of metal transfer both at average current levels and low current levels which are very much suitable for out of position welding and thin gauge material welding. The major defects which occur in this weld are porosity, gas hole, incomplete penetration, lack of fusion, excess penetration, burnthrough, etc.
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STB weld joints are subjected to online monitoring system for quality assessment. This system is called Real Time Radioscopy system (RTR). These systems consist constant potential duel focal (large focal size 3 mm x 3 mm and small focus 0.8x 0.8 mm) X-Ray machine with capacity 320kV, 10mA as the X-Ray source and a Digital Flat Panel Detector (FPD) as the Imaging device. X-Rays from the source penetrate through the weld thickness and the differential absorption of radiation gives a two dimensional
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Fig. 1&2 :
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RTR System and Control Station
X-Ray image of the weld joints. Incoming X-rays first strike a Cesium Iodide scintillator of the Digital Flat Panel, which converts the X-Rays into light. The light then passes through a photodiode matrix of amorphous silicon, and is converted into electrical signals, which are amplified and digitized (4).The light is directed onto silicon without lateral diffusion, which ensures image sharpness (5).The digital data is then processed into images via a corresponding gray value table, and is displayed, printed or sent to computer as required. The system offers the additional advantages of image post-processing and archiving. Compared to other imaging devices FPD provides high quality digital images, better signal to noise ratio and dynamic range of 12 to 16 bit (6), which provides high sensitivity for radiographic application. The present RTR system in BHEL uses an Amorphous Silicon Flat Panel (model: DXR250RT).The images obtained by Flat Panel Image Acquisition and Review computers are in DICONDE format. These images are evaluated by experienced, qualified NDE personnel; the decision of acceptance/rejection taken as per standards (7) and the feedback is given to the welder. The thickness range usually is 4 mm to 12 mm. The Radiographic technique used here is Double Wall Double Image (8).
AUTOMATIC DEFECT RECOGNITION (ADR) OF STB WELDS The manual evaluation has certain limitations like subjectivity, and humane dependency, which affect the productivity and reliability. Here comes the importance of automation in evaluation, which reduces human involvement, thus making the inspection more reliable and faster. The ADR system scans through the Digital X-Ray Image of the STB weld joint, and recognizes the defects and takes the decision of acceptance/rejection, based on the Acceptance Standards. ADR technology is already available for Castings (9) especially Aluminuium Wheels, Magnesium components (10) and weld joints (11). There are also ADR systems available for general NDT methods (12). However, there is no customized package available as such, for ADR of Straight Tube Butt welds. This triggered the development of an ADR system for integrity assessment of STB welds. The new system is different from other ADR systems in the aspect that, it uses different detection approaches for different class of defects where as many of the other ADR systems do defect classification only after defect detection.
ADR ALGORITHM (13-19) The first stage of operations performed by ADR algorithm is preprocessing of the input image. This stage involves enhancement of the image properties to such a level that, pattern recognition can be executed without errors. In case of the STB weld image, the Region of Interest (ROI) includes the elliptical weld region and adjacent raw material region of joining tubes. The next step is the extraction of the ROI; the boundaries of ROI are found by analyzing the vertical summation of the extracted tube for regions of heightened activity.
Fig. 3 : RTR image of STB welds Journal of Non destructive Testing & Evaluation
The next stage is feature extraction. Since each defect manifests in a different manner, different feature vectors have to be used for each defect. These feature vectors are subsequently presented to a Radial Basis Function Neural Network (RBFNN) for defect classification. In the present system, the defects are classified into three broad categories. Vol. 11, Issue 1 June 2012
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seen as prominent white dots in the ROI. Detection of Gas holes involves smoothening of the image by median filtering and self-subtraction in order to scrutinize the smaller white regions. However since it may contain very small white spots which may be noise, further filtering needs to be carried out using median filtering with a smaller kernel. The next step is conversion to binary form, followed by extraction of features like aspect ratio, area, length, breadth and roundness. These features are then passed individually to the RBFNN for classification. If even one gas hole is detected, the output of this block is the RBFNN’s output for that gas hole.
Fig. 4 : A schematic of the proposed ADR system.
One set covers all the defects like Gas Holes, Porosities and other rounded indications. The second type includes Incomplete Penetration (ICP) and Lack of Fusion (LF), and the third type includes Burn-through (BT) and Excess Penetration (EP). This classification is not based on the nature of the defect or its metallurgical behavior. It is chosen based on the gray level variation of the defect region with respect to the surroundings. Gas Holes are
(a)
For ICP the most prominent indication is a break in the root line. Here Median filtering and image subtraction is done to extract the weld root. This is followed by small kernel median filtering and subsequent conversion to binary image. The feature vector for RBFNN is the vertical gray value sum of the weld root portion. Since the morphological difference between ICP and non-defective images is often subtle, and if the quality of the input images is inadequate, in some cases there is ambiguity regarding the classification of images as ICP or non-defective. In such cases, an exception is raised to the operator and the final decision is passed to them. This ensures that the risk of false negatives is mitigated, as well as weeding out false positives from the final classification. The feature used to detect the third category is a vector formed by concatenating the
(b)
Fig. 5 : Input image (a) after contrast enhancement. (b) ROI extracted from contrast enhanced image.
Fig. 6 : ROI extracted images of welds with (a) BT, (b) GH and (c) ICP respectively. (d) Binary image of ICP feature extraction Vol. 11, Issue 1 June 2012
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Fig. 7 : ADR GUI during normal operation
vertical and horizontal summation of the contour of the ROI. The RBFNN network is trained on this vector. The final decision regarding the classification is made by the final RBFNN, which takes the output of the three previous RBFNNs as inputs.
EXPERIMENTAL RESULTS Table 1 : ADR trial results Defect Type
No of test No of Persamples Correct centClassifications age
Category I(Gas Hole & Pores)
560
546
97.5%
Category II(ICP & LF)
95
92
96.8%
Category III(Burn Through & Excess Penetration)
100
85
85%
Non defective joints
840
823
98%
The code was tested initially on a set of 1500 weld image samples. These welds include non-defective as well as defective weld joints covering a wide range of defects of different size, shape, location and orientations. The results of the trial are shown in table 1.0. Defect recognition algorithm based on RBF can efficiently overcome the shortcomings of traditional methods, which require large number of samples.
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CONCLUSIONS AND SUMMARY Usually the approach for common ADR algorithm is the recognition of the defect, followed by classification based on the features. However the proposed algorithm applies different techniques for detection of different classes of defects and hence defect detection and classification are parallel processes. The trials carried out give good results for class I (Gas hole and porosity) and class II (ICP and LF). However the performance needs to be improved in the case of the third category (BT and EP). ANN training with sufficient quantity of images is being carried out to enhance the detection level further. Application of other networks like support vector machine (SVM) is also being studied so that probability of detection of class III can be enhanced. In case of training samples, the size of the sample with one class of defect should also take in to account the probability of occurrence of that class during actual field trial. It is also important to note that RTR offers real time image and by rotation of the tube and swiveling, different images of the same weld joint can be captured. Some defects which are not prominent in one image become prominent in a different orientation of the weld with respect to the source and detector. Hence a defect detection approach based on multi image or direct frame analysis of a video image can enhance the detection and classification level significantly.
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REFERENCES 1. F Fucsok and M Scharmach, ‘Human factors: The NDE reliability of routine radiographic film evaluation’, Proceedings of 15th World Conference on Non-Destructive Testing, Roma 2000. 2.
T Y Lim,MM Ratnam and M A Khalid, “Automatic classification of weld defect using simulated data and an MLP neural network”,Insight Vol 49 No.3 ,march 2007,p 154-159
3. Domingo merry, Workshop on Digital Radiography, GE Global Research Centre, Bangalore, June 27, 2005 4. Dr.P.R.Vaidya-Flat Panel Detectors for Industrial RadiographyInternational Workshop on Imaging NDE- 2007, April 25-28, 2007,Kalpakkam,Chennai,India. 5. Giakos,G.C.; Suryanarayanan,S.;Guntupalli, R.Odogba,J.; Shah,N.; Vedantham,S.; Chowdhury,S.; Mehta,K.; Sumrain,S.; Patnekar,N.; Moholkar,A.; Kumar,V.; Endorf, R.E.,‘Detective quantum efficiency of CZT semiconductor detectors for digital radiography’, Instrumentation and Measurement,IEEE TransactionsVolume 53, Issue 6, Dec. 2004, P 1479 – 1484. 6. V.R.Ravindran, ‘Digital Radiography Using Flat Panel Detector for t e Non-Destructive Evaluation of Space Vehicle Components’, Journal of Non-Destructive Testing & Evaluation , Vol.4, Issue 2, September 2005. 7. ASME Section I. 8. ASME Section V. 9. Frank Herold, Rolf-Rainer Grigat, “A New Analysis and Classification Method for Automatic Defect Recognition in XRay Images of Castings”, Paper presented at the 8th ECNDT, Barcelona, June 2002
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10. Veronique Rebuffel, Subash Sood, “Defect Detection Method in Digital Radiography for Porosity in Magnesium Castings”. 11. G. Bonser and S. W. Lawson, “Defect detection in partially complete SAW and TIG welds using on-line radioscopy and image processing”, Miguel Carrasco and Domingo Mery, “Segmentation of welding defects using a robust algorithm”. 12. US Patent No: 4896278,dt: 23rd January1990. 13. Inoue, K. and Sakai, M., “Automation of inspection for weld”, Trans. Of Japanese Welding Research Institute,Osaka University, vol. 14(1), pp. 35-44, 1985. 14. Aoki L, Suga Y. Application of artificial neural network to discrimination of defect type in automatic radiographic testing of welds. ISIJ Int 1999;39(10):1081–7. 15. Daum W, Rose P, Heidt H, Builtjes JH. Automatic recognition of weld defects in X-Ray inspection. Br J NDT 1987;29(2):79– 82. 16. Liao TW, Li DM, Li YM. Extraction of welds from radiographic images using fuzzy classifiers. Inform Sci 2000;126:21–42. 17. Kato Y, Okumura T, Matsui S, Itoga K, Harada T, Sugimoto K, Michiba K, Iuchi S, Kawano S. Development of an automatic weld defect identification system for radiographic testing. Weld World 1992;30(7/8):182–8. 18. Gayer, A, Saya, A, Shiloh, A. (1990). Automatic recognition of welding defects in real-time radiography. NDT International, 23(4):131–136. 19. Lawson, S W, Parker, G A, (1994). Intelligent segmentation of industrial radiographic images using neural networks. In Machine Vision Applications and Systems Integration III, Proc. of SPIE, volume 2347, pages 245–255, November 1994.
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PROBE Possibility thinking follows Perception – Third in the list. Mind over matter! How many times have we heard this? Yogic culture says that there are two parts of our brain, 1) The Right Brain and the other 2) the left Brain. One is for logic and the other for intuition. Science depends mostly on logic, even though there are several instances of intuition helping discoveries (inventions)- Fritjof Capra. (The Tao of Physics). But YOGA believes both in logic and intuition. We generally reiey on logic and all our decisions are dependent on it. The intuition (the gut feeling) is often neglected. Let us not forget that logic is based on our past experiences – the data collected by our mind and the experience of others forced on us. But change is taking place continuously. We neglect this aspect. Our decisions shall be based on a judicial mix of both logic and intuition. This means that both our left and right brains shall function in unison. How do we achieve this? It is achieved by adopting the technique of alternate nostril breathing. I can see some of you smirking as you are reading this. I request you to perform this experiment before arriving at any conclusion. Keep your index finger underneath your nose and observe your breathing. The breadth will go in out through one of the nostrils. Repeat the experiment after about 40 minutes. The breath would have changed its path to the other nostril. To make both right and left brains to work in unison we shall make the breath move in and out through both nostrils all the time. Want to learn how to do it? Continue reading. Birbal was meditating as usual in the morning, when Akbar paid him an unannounced visit. Birbal’s wife was taken aback as she had been strictly instructed by Birbal not to be disturbed during meditation time. So she received the emperor with due respect and requested the emperor to wait while Birbal was in meditation. Akbar waited and was getting impatient. After considerable amount of time when Birbal came out at last, Akbar was almost boiling with rage and wanted to know if meditation was more important to Birbal than the emperor himself. Birblal replied that meditation is a process for self realization. Akbar wanted to know the process. But Birbal politely declined telling him that process shall be initiated only by a learned Guru and not by anybody and everybody. Hearing this Akbar got more angry and challenged Birbal that he will learn what Birbal was practicing and adopt it. By threatening the ministers Akbar got to know what Birbal was practicing and started to follow the procedure. Days passed but Akbar could not feel any tangible benefit. So he sent word for Birbal. Birbal knew all that had happened. When he entered the room apart from Akbar and himself only the guard was present. As soon as he entered he ordered the guard to arrest the emperor. The guard was transfixed. Hearing this Akbar got angry and ordered the guard to arrest Birbal and the guard did so. Immediately Birbal said “See oh! King the words by themselves do not have any effect, but it acquires the effect when it is uttered by the proper source. That is why I wished that you be initiated into meditation by a proper Guru. I narrated this story to impress upon you to learn meditation from a proper Guru, to derive the benefit from yogic Kriyas as they lead you to possibility thinking. Modern day sports psychology is full of it (example Carl Lewis). History is strewn with examples of developing possibility thinkers who became successful Aristotle Onasis. At this point of time I recall the story of the old woman who goes to the Guru and says, I sat in front of a mountain and I prayed for days, but the mountain did not move ( Prayers can move mountains). I knew that it will not happen”. The Guru replied “if you knew it why did you waste your time?”. The old lady did not posses faith in herself. Faith in self is the back bone of any action or inaction. Without faith nothing can be achieved – Faith in yourself and in your capabilities. Faith is built brick by brick through a positive frame of mind aroused by possibility thinking. With possibility thinking you realize that your potential is limitless as you are a mini universe and you posses the capacity to create. Possibility thinking is a prerequisite to lead a life of ecstasy.
Ram. Journal of Non destructive Testing & Evaluation
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