JNDE Journal - june issue

Volume 11

issue 1

June 2012

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vol 11 issue 1 June 2012Journal of Non Destructive Testing & Evaluation

from the Chief Editor

DDDDDrrrrr. Krishnan B. Krishnan B. Krishnan B. Krishnan B. Krishnan BalasubramaniamalasubramaniamalasubramaniamalasubramaniamalasubramaniamProfessor

Centre for Non Destructive EvaluationIITMadras, Chennai

[email protected]@gmail.com

URL: http://www.cnde-iitm.net/balas

This second issue in 2012 continues on the previous issue with the several featuressuch as Horizon, Events & News, probe, etc. from the previous issues. The BASICSin this issue covers some of the fundamentals and application potential in the field ofGuided Ultrasonic Waves for NDT. In addition, 4 technical papers are included inthis issue.

I would like to address a topic that is very dear to me. Education in NDE is a keycomponent of the development of manpower that is necessary for the future ofindustries in India. The education opportunities in India are available in two forumsi.e. through Level I, II, and III training and examination, and through formal degreeprograms in a few institutions. While the number of organizations offering Level Iand Level II training program certified under the auspice of ISNT, PCN, or ASNTare many, the Level III programs are limited to a few programs, conducted by ISNTand a few reputed companies. The students enrolled in these programs usuallypossess a Bachelor’s degree in Engineering or Sciences, but do not have any formalexposure to NDE science or technology. Also, many of these short programs arepreparatory in nature and are mostly oriented towards qualification in the respectiveexams. The imparting of the basics and the field experiences, that are necessary forundertaking inspection jobs in the respective areas of NDT, has been left to theconcerned industries in which the individuals are employed. However, there is asignificant gap here that must be filled, since the industries have other pressures anddo not have the luxury to focus on training their employees, particularly in thebasics. The other form of training is through the Master’s and PHD level programsin India which are offered by a select few institutions. These students do undergoformal 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 trainedstudents, after their graduation with a Degree in the field of NDE and often have toaccept jobs that have little or no relation with NDE, unless they too obtain a LevelIII certification. It is hence a situation of irony here that the “employable” are notformally trained and the formally trained ones are not “employable”. As a society, Ihope 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, NewDelhi and we expect it to be a very grand success. It is hoped that all interestedauthors will submit their technical abstracts to the conference well in advance [email protected] in order to avoid any disappointments. For moreinformation, visit www.nde2012.org.

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vol 11 issue 1 June 2012 Journal of Non Destructive Testing & Evaluation

I S N T - National Governing Council

Chapter - Chairman & Secretary

PresidentShri P. Kalyanasundaram

President ElectShri V. Pari

Vice-PresidentsShri D.J. Varde

Shri Swapan ChakrabortyShri N.V. Wagle

Hon. General SecretaryShri R.J.Pardikar

Hon.Jt.SecretariesShri Rajul.R.ParikhDr.B.Venkatraman

Hon. TreasurerShri S.SubramanianHon. Co.Treasurer

Shri Sai Suryanarayana

Immediate Past PresidentShri K.Thambithurai

Past PresidentShri Dilip.P.Takbhate

MembersShri Anil V.Jain

Shri Dara E. RupaShri D.K.Gautam

Shri Diwakar D.JoshiDr. Krishnan Balasubramaniam

Shri Mandar A. VinzeShri B.B.Mate

Prof. G.V.PrabhugaunkarShri B.K.PangareShri M.V.Rajamani

Shri Samir K. ChoksiShri B.K.Shah

Shri S.V.Subba RaoShri Sudipta Dasgupta

Shri R.K.SinghShri A.K.Singh(Kota)

Shri C.AwasthiShri Brig. P.Ganesham

Shri Prabhat KumarShri V.SathyanShri P.Mohan

Shri R.SampathMs. Hemal Thacker

Shri A.K.SinghiShri T.V.K.KidaoShri B. Prahlad Dr. BPC Rao

Dr.Sarmishtha Palit Sagar

Permanent InviteesShri V.A. Chandramouli

Prof. S. RajagopalShri G. Ramachandran

and All Past Presidents,All Chapter Chairmen / Secretaries

Ex-officio MembersChairman NCB,Secretary NCB,Treasurer NCB,

Controller of Examination NCB,President QUNEST,Secretary QUNEST,Treasurer QUNEST

AhmedabadShri D.S. Kushwah, Chairman,NDT Services, 1st Floor, Motilal Estate,Bhairavnath Road, Maninagar,Ahmedabad 380 028. [email protected] Rajeev Vaghmare, Hon. SecretaryC/o Modsonic Instruments Mfg. Co. Pvt. Ltd.Plot No.33, Phase-III, GIDC Industrial EstateNaroda, Ahmedabad-382 330 [email protected]

BangaloreProf.C.R.L.Murthy, ChairmanDept. of Aerospace Engg,Indian Institute of Science, Bangalore 560012Email : [email protected]

ChennaiShri R.Sundar, Chairman,First Floor, North Wing, PWD Office Complex,Chepauk, Chennai - 600 [email protected], [email protected] RG. Ganesan, Hon. Secretary,Chief Executive, BETZ Engineering & Technology49, Vallalar Street, Adambakkam, Chennai - 600 [email protected] ; [email protected]

DelhiShri A.K Singhi, Chairman,MD, IRC Engg Services India Pvt. Ltd612, Chiranjiv Tower 43, New Delhi [email protected] M.C. Giri, Hon.Secretary,Managing Partner, Duplex Nucleo EnterpriseNew Delhi [email protected]

HyderabadShri N. Saibaba, Chairman,Chief Executive, Nuclear Fuel Complex,ECIL PO, [email protected] ; [email protected] M.N.V. Viswanath, Secretary,Dy. Manager, Quality Assurance-Fuels,CFFP Building,Nuclear Fuel Complex,ECIL PO, [email protected] ; [email protected]

JamshedpurDr N Parida, Chairman,Senior Deputy DirectorHead, MSTD, NML, Jamshedpur - 831 [email protected]. GVS Murthy, Hon. Secretary,MSTD, NML, [email protected] / [email protected]

KalpakkamDr. B. Venkatraman, ChairmanAssociate Director, RSEG, & Head, QAD,IGCAR, Kalpakkam 603 [email protected] B. Dhananjaykumar, Hon.SecretaryReprocessing Group, IGCAR,Kalpakkam – 603 102 [email protected]

KochiShri CK Soman, Chairman,Dy. General Manager (P & U),Bharat Petroleum Corporation Ltd. (Kochi Refinery),PO Ambalamugal 682 302. [email protected] V. Sathyan, Hon. Secretary,SM (Project),Bharat Petroleum Corporation Ltd.(Kochi Refinery),PO Ambalamugal-682 302 [email protected]

KolkataShri Swapan Chakraborty, ChairmanPerfect Metal Testing & Inspection Agency,46, Incinerator Road, Dum Dum Cantonment,Kolkata 700 028. [email protected] Dipankar Gautam, Hon. Secretary,4D, Eddis Place, Kolkata-700 [email protected]

KotaShri R.C. Sharma, ChairmanQAS, RAPS - 5 & 6, PO AnushaktiRawatbhata 323 303 [email protected] S.K. Verma, Hon. Secretary,TQAS, RAPS - 5 & 6, PO AnushaktiRawatbhata 323303. [email protected]

MumbaiShri.S.P.Srivastva, Chairman303, Lok Centre, Marol Maroshi Road,Andheri (East), Mumbai 400 059Email: [email protected] ,[email protected] ; [email protected] Samir K. Choksi, Hon. Secretary,Director, Choksi Brothers Pvt. Ltd.,4 & 5, Western India House, Sir P.M.Road,Fort, Mumbai 400 001. [email protected]

NagpurShri Pradeep Choudhari, ChairmanParikshak & Nirikshak, Plot M-9, LaxminagarNagpur - 440 022Mr. Jeevan Ghime, Hon. Secretary,Applies NDT & Tech Services,33, Ingole Nagar, B/s Hotel Pride, Wardha Road,Nagpur 440 005. [email protected]

PuneShri BK Pangare, ChairmanQuality NDT Services, Plot BGA, 1/3 Bhosari, GeneralBlock, MIDC, Bhosari, Pune- 411 [email protected] BB Mate, Hon Secretary,Thermax Ltd., D-13, MIDC Ind. Area, RD AgaRoad, Chinchwad, Pune- 411 [email protected]

SriharikotaShri V Ranganathan, Chairman,Chief General Manager , Solid Propellant Plant,SDSC – SHAR, Sriharikota – [email protected] B KarthikeyanHon. Secretary, ISNT Sriharikota ChapterSci/Eng. NDT/SPROB,SDSC – SHAR, Sriharikota – [email protected]

TarapurShri PG Bhere, Chairman,AFFF, BARC, Tarapur-401 [email protected] Jamal Akhtar, Hon.Secretary,TAPS 1 & 2, NPCIL, Tarapur. [email protected]

TiruchirapalliR.J. PardikarAGM, (NDTL)BHEL Tiruchirapalli 620 014. [email protected] L. Marimuthu, Hon. Secretary,HA-95, Anna NagarTiruchirapalli 620 026. [email protected]

VadodaraShri P M Shah, Chairman,Head-(QA) Nuclear Power Corporation [email protected] S Hemal Thacker, Hon.Secretary,NBCC Plaza, Opp.Utkarsh petrol pump, Kareli Baug,Vadodara-390018. [email protected]

ThiruvananthapuramDr. S. Annamala Pillai, ChairmanGroup Director, Structural Design & Engg Group,VSSC, ISRO, Thiruvananthapuram [email protected]. Binu P. ThomasHon. Secretary, Holography section, EXMD/SDEG,STR Entity, VSSC, Thiruvananthapuram 695 [email protected]

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

Contents

Chief EditorProf. Krishnan Balasubramaniam

e-mail: [email protected]

Dr. BPC [email protected]

Managing EditorSri V Parie-mail: [email protected]

Topical EditorsDr D K BhattacharyaElectromagnetic MethodsDr T Jayakumar,Ultrasonic & Acoustic EmissionMethodsSri P KalyanasundaramAdvanced NDE MethodsSri K ViswanathanRadiation Methods

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About the cover page:

Editorial BoardDr 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 PanelProf 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

ObjectivesThe Journal of Non-Destructive Testing &Evaluation is published quarterly by the IndianSociety for Non-Destructive Testing forpromoting NDT Science and Technology. Theobjective of the Journal is to provide a forumfor dissemination of knowledge in NDE andrelated fields. Papers will be accepted on thebasis of their contribution to the growth ofNDE Science and Technology.

Journal of Non DestructiveTesting & Evaluation

Published byShri RJ Pardikar,General Secretary on behalf ofIndian Society for Non Destructive Testing (ISNT)

The Journal is for private circulation to membersonly. All rights reserved throughout the world.Reproduction in any manner is prohibited. Viewsexpressed in the Journal are those of the authors'alone.

Modules 60 & 61, Readymade GarmentComplex, Guindy, Chennai 600032Phone: (044) 2250 0412Email: [email protected] at VRK Printing [email protected]

Volume 11 issue 1June 2012

The cover page shows the image of a resultobtained using Scanning Eddy CurrentThemography on a graphite epoxycomposite plate with impact damagedefect. Graphite Epoxy composites areused in the aerospace industries for theirimproved performance due to their highstiffness to weight ratio and ability toengineer the performance of these materialsystems. The eddy current thermographyuses an induction coils (that is on the leftside of the image) that is scanned fromright to left over the sample. The change inthe temperature at the surface of thesample is observed using a thermal imaging(IR) camera in the video mode. Due to theconductivity of the graphite epoxy sample,the induction process intoduces heating inthe material. At the locations of the impactdamage in the sample, the electromagneticfield lines are disturbed and thusintroducting local eddy currents at theselocations. The additional eddy currentscauses additional heating, also called asJoule heating, and can be observed usingthe thermal imaging camera. Hence, NDTfor defects in such materials becomesfeasible. Similarly, other conductingmaterials can also be inspected using thishybrid technique.Courtesy:Centre for Nondestructive Evaluation,Indian Institute of Technology Madras,Chennai, India

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Chapter News

Basics - Guided Ultrasonic Wave Techniques

Horizon - Structural Health Monitoring (SHM)

NDE Events

NDE Patents

NDE Puzzle

Technical Papers

Ultrasonic Evaluation of Glass-Epoxy Compositeswith Varied Void ContentShubhendu Verma, Shashwat Anand,C R L Murthy andR M V G K Rao

Low Heat Flux Transient Thermography for DefectDetection in Thick Composite StructuresK Srinivas, T Murugesh and J Lahiri

Detection of fine defects in steam generator tubesof 220 MWe Indian PHWRs using eddy current arrayprobesH.M. Bapat, Manojit Bandyopadhyay, R.K. Puri and ManjitSingh

Automatic Defect Recognition (ADR)System for Real Time Radioscopy (RTR)of Straight Tube Butt (STB) WeldsDeepesh.V, R.J. Pardikar, K.Karthik, A. Sricharan S.Chakravarthy and K. Balasubramaniam

Probe

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Madras Metallurgical Services (P) LtdMetallurgists & Engineers

Serving Industries &Educational

Institutes for thepast 35 years

24, Lalithapuram street, Royapettah, Chennai 600014Ph: 044-28133093 / 28133903 Email: [email protected]

A-3, Mogappair Indl. Area (East) JJ Nagar,Chennai 600037 Phone 044-26564255, 26563370

Email: [email protected];[email protected] www.kidaolabs.com

KIDAO Laboratories

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

NDE Service ProviderProcess and Power Industry, Engineering andFabrication Industries, Concrete Structures,

Nuclear Industries, Stress Relieving

Electro-Magfield Controls & Services &LG Inspection Services

Plot 165, SIDCO Industrial Estate, (Kattur)Thirumullaivoil, Vellanur Village, Ambattur Taluk

Chennai 600062 Phone 044-6515 4664 Email: [email protected]

We manafucture : Magnetic Crack Detectors, Demagnetizers,Magnetic Particles & Accessories, Dye Penetrant Systems etc

Super Stockist & Distributors: M/s Spectonics Corporation, USAfor their complete NDT range of productrs, Black Lights, Intensity

Meters, etc.

Betz Engineering &Technology Zone

An ISO 9001 : 2008 Company

Call M. Nakkeeran, Chief Operations,Lab: C-12, Industrial Estate, Mogappair (West), Chennai 600037

Phone 044-2625 0651 Email: [email protected] ;www.scaanray.com

49, Vellalar Street, near Mount Rail Station, Chennai 600088Mobile 98401 75179, Phone 044 65364123Email: [email protected] / [email protected]

International Training Division21, Dharakeswari Nagar, Tambaram to Velachery Main Road,Sembakkam, Chennai 600073www.betzinternational.com / www.welding-certification.com

NABL Accredited Laboratory carrying out Ultrasonictest, MPL and DP tests, Coating Thickness and

Roughness test. We also do Chemical and Mechnical testsMetallographyStrength of MaterialsNon Destructive TestingFoundry Lab

01J, First Floor, IITM Research Park, Kanagam Road, Taramani,Chennai 600113 India Phone : +91 44 6646 9880

Dhvani R&D Solutions Pvt. Ltd

E-mail: [email protected] www.dhvani-research.com

Classifieds

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

Southern Inspection ServicesNDT Training in all the

following eleven Methods

No.2, 2nd Floor, Govindaraji Naicker Complex,Janaki Nagar, Arcot Road,

Valasaravakkam, Chennai 600 087Tamil Nadu, India

Phone : +91 44-2486 4332, 2486 8785, 4264 7537E-mail: [email protected] and

[email protected] Website: www.sisndt.com/www.ndtsis.com/

wwwpdmsis.com

Transatlantic Systems

Support for NDT ServicesNDT Equipments, Chemicals and Accessories

Call DN Shankar, Manager14, Kanniah Street, Anna Colony,

Saligramam, Chennai 600093Phone 044-26250651 Email: [email protected]

• Inspection Solutions - CUPS, TAPS, CRISP, TASS• Software Products - SIMUT, SIMDR• Training - Guided Waves, PAUT, TOFD• Services & Consultancy - Advanced NDE, Signal Processing

- C-scans, On-line Monitoring

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NATIONAL CERTIFICATION BOARDANNOUNCEMENT

ISNT – Level III Certification Programme

January – February, 2013Pune, India

Dr. M.T. ShyamsunderController of Examinations

National Certification BoardIndian Society for Non-Destructive Testing

All payments shall be made through the means of acrossed Demand Draft drawn favouring

“NCB - ISNT” and payable at “Chennai”.Cheques will not be accepted.

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

Following are the details of the Course Director andcontact person at Pune:

Shri Bhausaheb K. PangareCourse Director

C/o. M/s. Quality NDT ServicesPlot No BGA 1/1,2,3, Bhosari General Block,

MIDC, Bhosari, Pune-411026Ph.: 020-27121843/27119490/8600100700

[email protected] / [email protected] /[email protected]

Indian Society forNon - 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: [email protected] , [email protected] ISNT Invites nominations/applications for the

National NDT Awards fromIndian Nationals for theirsignificant contributions

and excellence in the fieldof NDT and the Best

Chapter award from all thechapters of ISNT. Anannouncement to this

effect is already circulatedto eligible members /

chapters.

The various categories andawards are listed in every

issue of the JNDE.

The nominations /applications are to be sent

in the prescribed formwhich can be downloadedfrom www.isnt.org.in and15 copies of the same areto be sent to the followingaddress by 10th September,

2012.

Shri Dilip P. TakbhateChairman,

Awards Committee,Indian Society for

Non Destructive TestingModule No.60 & 61,

Third Floor,Readymade Garment Complex,

SIDCO Industrial Estate,Guindy, Chennai – 600 [email protected]

I S N TANNUALAWARDS

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O B I T U A RYMr. O.P Singhania, a founder member sinceinception of ISNT passed away on 13th March, 2011.He had a long association with ISNT and hascontributed immensely for the benefit of ISNTs’growth.It’s a great loss to ISNT and our heartfeltcondolences to the bereaved members of his familyfor the irreparable loss.May his soul rest in peace.

ED

CHAPTER NEWSKALPAKKAM UT Level-II course was conducted in June, 2012. LT

Level –I & II courses was conducted in June, 2012.SRIHARIKOTA

A Technical Talk on ‘Advances in NDE’ by CV.Krishnamurthy, CNDE, IIT Chennai on 05.06.12

AGM was conducted on 05.06.12.New ExecutiveCommittee 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 Level-I, BARC course (23/04/12 to 11/05/12.One year packageprogramme for BHEL employees wards under progress.

Conducted chapter EC meeting on14/05/12.TRIVANDRUM

ISNT Level II certification course on Ultrasonic TestingISNT Level II certification course in Ultrasonic testing wasorganized 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 chapterin association with M/s Kalva Engineers Pvt Ltd on 16th May2012 at Hotel Horizon Trivandrum.MR Kurup memorial lectureand Annual technical meetDr. B. Venkatraman, AssociateDirector, IGCAR delivered the MR Kurup memorial lecture on23rd June 2012 at Hotel Classic Avenue Trivandum. This wasfollowed by ATM lecture and was delivered by Sr. AnilKesavan INLPTA certified trainer.

Three executive meeting were conducted during March,April and May 2012.A visit to Titanium sponge Plant, KeralaMinerals and Metals Ltd, Chavara was organized by thechapter on 21st April 2012. A Two-day lecture and demo onNDT was organized by ISNT Thiruvananthapuram Chapter tothe instructors and staff members of Government ITI, Attingal,Thiruvananthapuram on 3rd and 4th of March 2012.Two issuesof IMAGE, the quarterly technical bulletin of the chapter, wasreleased for the first and second quarters of 2012AGM washeld on 23rd June 2012 at Hotel Classic avenue. Hon. Secretarypresented the report of activities of 2011-2012 and HonTreasurer presented the audited account of 2011-2012.

CHENNAI MT & PT Level-II (ASNT) course was conducted from

18.05.2012 to 27.05.2012. RT Level-II (ASNT) course wasconducted 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 inassociation with Society of Mechanical Engineers, AnnaUniversity. ISNT DAY was celebrated on 21.04.2012. Memberswith their family participated. Mr. R. Sundar was the Chairmanand Mr. R.G. Ganesan was the Convener. Dr. S. Suresh,General Manger of BHEL was the Chief Guest. EC Meetingheld on 29.04.2012EC Meeting held on 03.06.2012.

DELHICore Committee meeting held on 27th April to discuss and

finalize Venue/ official visit from head office.5th Executivebody meeting held on 4th may 2012 at Indian coffee houseConaught place.Chapter Chairman explained the discussionheld at Durban /Final discussion regarding NDE2012.6thExecutive Committee meeting held at Hotel Majestic East ofKailash in the presence of high official team(Shri.P.Kalyasundaram President-ISNT, Shri V. Pari-President-Elect ISNT, Shri DJ Varde, Vice President -ISNT ) .Finalizationof Venue/Presentation for NDE2012 seminar and other issuesrelated to NDE 2012 were discussed in detail.

HYDERABADExtra-ordinary General body meeting was held on 26th April

2012 and office bearers were elected. Distinguished NDTianof the decade award was presented to Sri JR Joshi, Dy. ProjectDirector- DRDL 23rd April 2012.Our life Member

Sri. MNV Viswanath has bagged Best presentation awardduring 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 atHotel Jewel of Chembur. Conducted Welding Inspectorexamination at ITT, Mahim on 13th May 2012.

EC meeting was held on 30th March, 2012 at ISNT,Mumbai office. APCNDT 2013 committee Meeting was heldon 18th June 2012.JAMSHEDPUR

Materials Evaluation by Magnetic Techniques Speaker : Dr.Amitava Mitra Chief Scientist & Group Leader : NDE &Magnetic Materials. Materials Science & Technology DivisionNational Metallurgy Laboratory Jamshedpur.

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Prof. Krishnan BalasubramaniamProfessor of Mechanical Engineering andHead of Centre for Nondestructive EvaluationDepartment of Mechanical EngineeringIndian Institute of Technology, Madras, Chennai 600 036Email: [email protected]

Basics

Guided Ultrasonic WaveTechniques

ABSTRACT

In this paper, a review of the current status, on the use of guided wave modes, andtheir interaction with cracks and corrosion damage in pipe-like structures will bediscussed. Applications of guided ultrasonic wave modes have been developed forinspection of corrosion damage in pipelines at chemical plants, flow acceleratedcorrosion damage (wall thinning) in feed-water piping, and circumferential stresscorrosion cracks in PWR steam generator tubes. [1-25] It has been demonstratedthat 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 propagationdistance of 100 meters or more is sometimes feasible. The guided waves can beclassified into Long Range, Medium Range and Short Range modes. Intuitively itmust be noted that, the longer the distance (range) of propagation (inspection), thelower will be the frequency used and consequently the lower will be the resolutionof discrete defects. This technique can also be used in the inspection of inaccessibleor buried regions of pipes and tubes.

Also, guided ultrasonic waves can be used for other quantitative NDE applicationssuch asa. 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 roomand at elevated temperatures up to 2000 C. [33,34]

i. Recently, air coupled ultrasonic methods have become viable for low impedancematerials such as fiber reinforced composites, plastics, and even thin metalstructures. A new phenomenon called as “Turning Modes” has been shownto have the ability to detect delaminations in composite structures in regionsthat 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 detectionand sizing of corrosion in hidden regions of pipes and storage tank annularplate.[39,40]

BASICS OF GUIDED WAVES

The ultrasonic guided waves, unlikebulk wave modes like longitudinal andtransverse, are a manifestation ofgeometrical confinement of acousticalwaves by one or more boundaries.[1,2] In many instances, these wavestravel long distances, depending onthe frequency and modecharacteristics of the wave, andfollow the contour of the structure inwhich they are propagating. Usually,these waves not only propagate alongthe length of the structure but alsocover the entire thickness (for plates)and circumference (in the case ofcylinders and rods). The use of guidedwave modes is potentially a veryattractive solution to the problem ofinspecting the embedded portions ofstructures because they can beexcited at one point on the structurepropagated over considerabledistances, and received at a remotepoint on the structure, in a pitch-catchmode, as schematically illustrated inFigure 1 for an elbow pipe. Thereceived signal contains informationabout the integrity of the material thatlies between the transmitting andreceiving transducers. Alternateapproaches, where the receiving andtransmitter are co-located, similar toa pulse-echo method is also possible.

Since there are several types ofguided waves, there are many waysto classify them. The firstclassification can be based on thetype of structure in which it isgenerated. These include (a) Platewaves, (b) Cylindrical waves, (c)Rod waves, etc., depending upon thetype of structure. The waves modecharacteristics for each of the abovewave type is distinctly different, butcan be theoretically predicted if thematerial properties are well known.

The second method of classificationof the wave mode is based on thenature of the particle vibration withrespect to the direction of wavepropagation (like in the case of bulk

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Basics

by using one as a reference modeand the other as a sensing mode,defect and/or materialcharacterisation becomes feasible.

The reflectivity of guided waves isgoverned by very different rules thanthose for bulk waves; with guidedwaves, it is possible to find defectswhose size is much smaller than awavelength. At a given defect depth,the reflection coefficient is directlyproportional to the circumferentialextent of the defect. The reflectioncoefficient of a half wall thicknessnotch with a circumferential extentof half a pipe diameter (16% of thepipe circumference) is approximately5% (-26 dB).

If an axially symmetric mode isincident on an axially symmetricfeature in the pipe such as a flange,square end or uniform weld, thenonly axially symmetric modes arereflected. Such a case is illustratedin Figure 3 that represents resultsfrom experiments conducted on pipeswith welds and corrosion like defects.However, if the feature is non axiallysymmetric such as a corrosion patch,some non axially symmetric waveswill be generated. These propagateback to the transducer rings and canbe detected. For instance, if a L(0,2)mode is incident on a defect, themode conversion is predominantly tothe adjacent F(1,3) and F(2,3) modes

often represented as L(n,m)-Longitudinal, F(n,m)-Flexural, orT(n,m) - Torsional in nature, wherethe n and the m represent the modenumbers based on Silk and Bainton[6]. For instance when n=0, themode is axially symmetric, such asthe L and T modes. If n>0, then themode is not axially symmetric. Here,m is the order of the mode ofvibration.

The multi-mode nature of these wavemodes can be an advantage sinceeach mode has different sensitivityto a particular type of defect andhence by comparing the wavepropagation of different modes, ie.

waves). In this type of classification,the types include (a) Extensional orLongitudinal waves, (b) Shear-horizontal waves, (c) Flexural waves,and (d) Torsional waves [3-5]. Here,the first two types are similar to theLongitudinal and Shear wavevibration. The Flexural waves aremodes where the structure flexes ina wave like pattern and the Torsionwaves exists when the particlemotion is circumferential in naturewhile the wave moves along thestructure.

Also, the wave modes can also bebroadly classified into symmetric andanti-symmetric modes based on thetype of symmetry of the displacementprofile exhibited by the wave duringpropagation. This classification isbased on whether the out-of-planedisplacement in a structure issymmetric about the neutral axis ofthe bounded structure ie. if the twoouter particles simultaneously moveaway from the center axis, then it isa symmetric mode and if they movetogether, then it is anti-symmetric.This is well illustrated in Figure 2.

Finally, for a give type of guidedwave, there are many orders ofmodes that can exist. The modeshave mode shapes are analogous tovibration modes in a beam. Thesemodes are numbered numericallywith zero representing the basicfundamental modes and the higherorder modes representing morecomplex behavior.

Thus, while defining a guided wavemode, a complete description willrequire the specification of all of theabove classifications. For instance, acylindrically guided, flexural, anti-symmetric, fundamental mode wouldrepresent a guided wave that istraveling along the length of acylindrical structure, that has afundamental flexural type particlevibration direction, that is notsymmetric about the axis. Thecylindrically guided wave modes are

Fig. 1 : A schematic comparison betweenbulk wave inspection and guidedwaves using surface mountedtransducers in plates and pipes.The greyed region shows thecoverage of inspection.

Fig. 2 : Representation of anti-symmetric and symmetric flexural guided wave modes ina plate.

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Basics

Group velocity cannot be faster thanthe Phase velocity and the dispersivenature of these modes can betheoretically computed if the materialproperties are known.

The dispersion curves used to gainunderstanding of the types of modesthat can be generated and theirdispersive nature [10-12]. Thesecurves are also used for interpretationof the signal. The dispersion curvesare represented in different mannerin literature. The most userfulrepresentation for NDE applicationis shown in Figure 5. In thisrepresentation, the velocity of thewave is plotted as a function offrequency of the wave. Each curverepresents a guided wave mode. Thewave velocity that is plotted can beeither phase or group velocity. But,from our previous discussion, it canbe concluded that the group velocityis representative of measurementsmade with dispersive wave pulsesand hence is more useful. In Figure5, it can be seen that there areseveral (in fact, too many) modesthat can be generated in a pipe. Allthree types of modes are represented(ie. Torsional, Longitudinal andFlexural). The slope of these curvesindicate the dispersive nature of thewave mode. Hence, a curve with asteep slope is very dispersive andmay be avoided during NDE.

is traveling at a different velocity, thepulse duration increases. This isillustrated in Figure 4 where twomodes, one non-dispersive mode (2)and a dispersive mode (3) are shown.

The velocity of the wave mode fora single frequency is called as itsPhase velocity. It must be apparentthat the measurement of phasevelocity by traditional velocitymeasurement techniques (such aspulse-overlap, zero-crossing, etc.) isdifficult, due to the change in thepulse shape. Hence, a differentdefinition of velocity called the Groupvelocity is used while measuring thevelocities of a dispersive ultrasonicpulse by traditional methods. The

which have similar velocities to theL(0,2) mode in the operatingfrequency range. The amount ofmode conversion obtained dependson the degree of asymmetry, andhence on the circumferential extentof the defect. At low circumferentialextent (which is the region of interestfor the detection of critical corrosionin practical situations) the modeconverted F(1,3) reflection is almostas large as the direct reflection. Thus,if these two reflections are of similarsize, it can be concluded that thefeature is localised to a small regionof the circumference [7-9].

EFFECTS OF DISPERSION

One of the key aspects of guidedwave modes is Dispersion, ie. wavevelocity is not a constant for a givenmaterial. It additionally depends ongeometry (thickness) and frequencyof the wave. In most cases, thisbecomes one parameter (f*d). Theconsequence of dispersion is that acompact broad-banded signal will notretain its shape while propagating andwill elongate considerably withdistance of travel. This is becauseof the fact that a broad bandultrasonic pulse comprises of a rangeof frequencies (depending on thebandwidth and central frequency ofthe pulse) and since each frequency

Fig. 3 : Signals from (a) axisymmetric feature e.g. weld; (b) corrosion.[25]

Fig. 4 : Typical guided wave signals (1) Tranmitted pulse, (2) Signal after travel of 30mm in a component showing no dispersion, and (3) a dispersive mode showingpulse spreading.

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TYPICAL INSTRUMENTATION

Like any traditional ultrasonicinspection system, the instrumentationfor the guided wave techniqueinvolves (a) Transducers, (b) Pulser/receiver with filters and amplifiers,and (c) PC based data acquisitionsystem. The key component is thetransducer that is designed speciallyfor generation and reception of guidedwaves and hence will be discussedin more detail. The Pulser receiveris either an array type or a singlechannel type depending on thesensor. Usually, an array type ispreferred with a capability to changethe phase of each signal so thatmode selection and tuning is possible.The signal interpretation requiredprecise measurements andinterfacing with dispersionrelationships. Hence, a PC basedsystem is the best choice for thisNDE.

The cylindrically guided waves canbe generated and measured usingseveral mechanisms. These wavemodes can be generated usingcircular ring-type array transducers[14] for pipes, or comb-transducerconfiguration [15] for tubes, or likein the regular weld inspection usingan array of variable angle beamtransducers located around thecircumference of the cylinder.Alleyne and Cawley [14] reportedthe development of a dry coupledpiezoelectric transducer system forthe excitation of the axially symmetricL(0,m) modes in pipes. It comprisesa ring of piezoelectric elements, thatare clamped individually to the pipesurface; no coupling fluid is requiredat the low ultrasonic frequencies usedhere. The number of elements in thering should be greater than n whereF(n,1) is the highest order flexuralmode whose cut off frequency iswithin the bandwidth of the excitationsignal. In the initial configuration,rings of 16 elements were used on 3inch pipes, while 32 element ringswere employed on 6 and 8 inch pipes.

2. In the case of pipes withinsulation, these modes allowinspection with minimal removalof insulation.

3. Regions that are inaccessible,such as buried pipes, can beinspected.

4. The multi-mode cylindricalwaves can be utilized to identifyregular pipeline features such aswelds from localized damagesuch as corrosion..

Some of the key limitations of thetechnique are:

1. This method requires theunderstanding of multi-modenature of the guided waves.

2. The energy that is generated isdistributed over a large volumeof the structure. Hence, thistechnique may have difficultydetecting isolated defects suchas pin holes, longitudinal cracks,etc., that offer small cross-sections for wave reflection.

3. Signal interpretation is morecomplex, particularly due tomode conversion effects whenwave interacts with damagedarea.

Consequently, a flat region of a curvemeans the mode is non-dispersiveand the wave pulse will propagateeffectively and measurements arepossible. Hence, in Figure 5, the mostpreferred mode is the L(0,2) whichis the 2nd order axi-symmetricextensional mode within thefrequency range between 40kHz. -100 kHz.

The dispersion curves can betheoretically computed and plottedusing a software packageDISPERSE developed by ImperialCollege, UK. [13]

ADVANTAGES ANDLIMITATIONS OF THETECHNIQUE

These guided wave modes representvery different approach to NDEwhen compared with traditionalultrasonic methods. Some of the keybenefits of this technique for pipeand tube inspection is listed below:

1. Use of multimode, guided, platewaves provides a global long-range inspection technique forcharacterizing any potential in-service damage (impact anddelamination) in typicalcylindrical structures.

Fig. 5 : Dispersion curves plotting group velocity .vs. frequency for guided waves in apipe. [25]

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echo inspection, the arrival timeinformation will provide the locationof the defect and the amplitude willindicate the size of the defect.

Figure 7 shows an epoxy painted 4inch buried pipe at a test positionadjacent to a road crossing. This resultindicates the ability to test areas ofpipes that are inaccessible. The testrange extends over more than 20 mon either side of the ring typetransducers, which are located in themiddle of the plot. A distance-amplitude correction (DAC) curvewas computed from the weldindication (using prior information onthe weld locations). Then the defectcall level by comparison with the weldecho level and the output amplitudewere calculated, knowing that a

(a) Corrosion detection in Pipelines

Significant amount of work has beenconducted in the application of thismethod in the pipeline inspection forcorrosion damage in chemicalindustries [18-20], For example, 70kHz. guided cylindrical waves inchemical and petroleum pipelines (1-3 meter diameter and 2-6 inch wallthickness) have detected 25%through wall cracks at a distance of30 meters.

A typical result is shown in Figure 6.From this result, it can be observedthat the pipe features such as weldsreflect energy while the defects alsoreflect signals (albeit modeconverted) that are significant anddetectable. Like in the case of pulse-

This gave the possibility of operatingat frequencies up to around 100 kHz;in practice, most testing is done at50 kHz and below, so it has beenpossible to reduce the number oftransducers in a ring.

Additionally, non-contact methodsusing Electro-Magnetic AcousticTransducers (EMAT) has also beenreported [16] These can be locatedeither to the inside or the outsidesurface of the pipe/tube. Thecylindrically guided wave techniquehas been modified to generate anddetect wave-modes without thephysical contact with the pipewalls[17]. This is accomplished byusing the magnetostrictive propertyof steel pipes, where the pipe materialacts as the transducer, and a coilthat encircles the pipe couples theexcitation energy into the pipe. Aseparate receiver coil is used to pickup the signals from the guided waves.These two methods have theadvantage of being able to generateand receive waves without physicalcontact with the structure. But, it hasbeen reported that mode isolation andidentification of received signals ismore complex.

The length of travel of the guidedwaves will depend on the frequencythat is used, the type of wave modeselected and the minimum size of thecrack that has to be detected. It isestimated that these modes can travelup to 100 m in length. The smallerthe crack size to be detected, thesmaller the wavelength, which resultsin higher frequencies andconsequently smaller travel distancesdue to ultrasound attenuation, whichexponentially increases withfrequency.

TYPICAL APPLICATIONS

The applications of the guided wavesin NDE are many. A few typicalresults that have been reported inliterature will be used to illustrate thispotential.

Fig. 6 : Guided wave results from a pipe with simulated defects. A pulse echo typeapproach was employed. [8,9]

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].

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to be measured) and in air. Theviscosity is evaluated from thereflection factor directly by fitting theexperimental data to viscosity andreflection factors.

Generally, in manufacturingprocesses, viscosity is taken as afunction of temperature and thetemperatures are usually measuredby another sensor, such as athermocouple. In the guided wavemeasurements, the temperature issimultaneously measured, along withviscosity, by measuring change in thetime of flight of the ultrasonic wavein the delay line buffer rod at twodifferent temperatures. Recently, thismethod has been used for flow frontmonitoring inside molds that haveresin systems, fluid levelmeasurements, temperature profilemeasurements, etc.

(e) Air Coupled Guided WaveUltrasonic NDT

Air-coupled ultrasonic transducerscan be used to efficiently transmitand receive guided waves in certainmaterials particularly those havinglow acoustic impedance such ascomposite materials. [36-38] Air-coupled ultrasonic inspection is a non-contact or minimally-invasivemethod. For reasons related to theattenuation of ultrasonic waves in airand the viscoelastic properties ofcomposites most common practicalapplications of air-coupled ultrasonicinspection are in the frequency rangeto 50 to 500kHz [41-44] . Extensivestudies carried out elsewhere [45, 46]

these advanced material, it has beenshown that guided waves aresensitive to damage mechanismssuch as fatigue in composites[23].One of the issues that must beaddressed will be that the energyflow of these waves are dependenton the anisotropic nature of thematerial.[24]. The Lamb modes havebeen shown to be able to measureelastic moduli of complex anisotropicsystems (up to orthotropic symmetryinvolving 9 elastic constants) as wellas the relatively simple isotropicmaterials. The guided waves can alsobe used to characterize the elasticproperties as a function oftemperature up to 2000 C, dependingon the material being characterized.

(d) Guided waves for FluidProperty Measurements

In this method, guided waves aregenerated through a cylindrical bufferrod/wire and the reflection factor ofthe guided wave modes aremeasured. At a solid-fluid interface,the amount of ultrasonic wave energyreflected back into the solid dependsupon the operating frequency, thephysical properties of the fluid(viscosity and density), and the bufferrod (density and shear modulus). Theamount of ultrasonic wave amplitudereflected back into the solid whenthe buffer rod is in air is used as thereference in the calculation ofreflection factor from experimentalsignals. Reflection Factor is the ratioof peak to peak amplitude of thereflected ultrasonic signals when itis in viscous fluid (whose viscosity is

typical site weld is a -14 dBreflector. The echo identified as +F2is the only indication where the modeconverted signal is significantcompared to the incident modereflected signal and this indicatespossible corrosion at the entry pointto a road crossing.

(b) Defect detection in Tubes

Many results are available for steamgenerator tube corrosion detection inthe nuclear industry [21-22].Applications of cylindrically guidedultrasonic wave modes have beendeveloped for inspection of flow-accelerated corrosion damage (wallthinning) in LWR feed-water piping,and circumferential stress corrosioncracks in PWR steam generatortubes. In most cases, the wave isgenerated using a probe that isattached to the inside. Frequency andangle tuning techniques are used tooptimize the wave generation andreception mechanisms.

In Figure 8, a typical result isillustrated for a steam generator tubeinspection. Here, the wave isgenerated using the Combtransducer. The indications from thecorrosion damage defect are clearlyidentified.

(c) Guided Waves in CompositeMaterials

These guided wave are also feasiblein tubes and pipes made fromcomposite materials. Although, thecalculations must be made using theanisotropic and layered nature of

Fig. 8 : Typical signals from tube inspection from (a) defect free, and (b) defective tubes [15].

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applications. These are (i) tighterenvelope that improves the temporalresolution (ii) shorter wavelength thatimproves the spatial resolution, (iii)The vanishing surface displacementsof the out-of-plane component thatis insensitive to surface loading, and(iv) sub-surface defect detectability.Other applications of this techniqueincludes annular plate corrosioninspection in large storage tanks.[39,40]

explored characteristics of air-coupled transducers for inspectionusing guided waves. On thisfoundation, the concept of single sidedinspection for the quantitativeidentification of delamination wasestablished. Rapid advances inimprovements of air-coupledtransducer design such as the use oflow impedance GMP (gas matrixprobes) 1-3 piezo composites as thetransducer material and with focusinghave made it possible to acquiresignals using very little signalaveraging. This has opened newareas of NDT applications [47-50]such as the ability to penetratemetals and much thicker composites.Air-coupled ultrasonic transducersare ideally suited for the inspectionof composite pipes addressed in thispaper. The single sided air-coupledultrasonic measurement techniqueusing guided wave has receivedsignificant interest and newapplications have been proposed fordetection and visualization of in-homogeneities in composite materials.This study investigated numericallyand experimentally the Lamb waveA0 mode interaction with the artificialimpact type defects in aerospacehoneycomb structures [51]. Singlesided techniques have been usedsuccessfully for inspecting thickcomposite wind turbine blades [52].Recently, there are also few reportson numerical work on usingfundamental anti-symmetric A0 Lambmode generated by air-coupledtransducers for detection ofdelamination sizes in glass fiberreinforced composites using time ofarrival of mode converted A0 [53].In other work by the same authors,Lamb waves based B-scans imaginghave been proposed for findinginterface delaminations in a compositeT-joint using air-coupled transducers.It was shown that it is possible torecognize the geometry of defectsand estimated approximatedimensions of the defects fromultrasonic B-scan obtained using air-coupled transducers.

(f) Higher Order Modes ClusterGuided Waves (HOMC-GW):

This new phenomenon uses modesclusters that are highly non-dispersiveover considerable distance ofpropagation. This HOMC-GW is arecently explored phenomena whichis found to occur at very highfrequency-thickness product i.e. 15MHz.mm to 35 MHz.mm. TheHOMC-GW technique appears tohave a greater potential fornondestructive inspection of largestructures such as pipe supportinspection and storage tankannular plate inspection. A typicalresult is shown in the Fig. 10 wherethe support location corrosion ismapped using the HOMC technique.The HOMC technique can also beused to detect the presence ofcorrosion between the sacrificial padand the pipe at the pipe supportlocations. This technique hasconsiderable improvement over thelow frequency guided wavetechniques that are used currently.HOMC-GW appears to have severalattractive features for NDE

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 inradial configuration showing detection of defects.

Fig. 10 : The pipe support inspectionusing HOMC guided wavetechnique.

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SUMMARY

Like any non-destructive examination(NDE) method, the guided waveinspection method will have adefined false alarm rate andprobability of detection, which willhave to be determined. Theassociated signal-to-noise ratio of theNDE system under a field operatingcondition is also a factor that mustbe considered. The effects of thevarious parameters that influence thetechnique must be determined, suchas accuracy, precision, and sensitivityguidelines, before attempting thistechnique for solving practicalproblems. Also, the procedures andlimitations of applying this techniquemust be well understood for thesuccessful implementation of thispowerful technique.

This method will substantially improveefficiency and reduce the inspectiontime and cost, especially when utilizedas a precursor to a more detailedlocal inspection. Also, for criticalapplications, where inaccessible pipesand tubes have to be inspected, thistechnique provides an opportunity toperform NDE which otherwise maynot be possible.

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Basics

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22. Rose, J.L., Ditri, J.J., Pilarski,A., Rajana, K. and Carr, F.T.‘A guided wave inspectiontechnique for nuclear steamgenerator tubing’, NDT & EInternational, Vol 27, pp307-330,

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28. J. Vishnuvardhan, C. V.Krishnamurthy and K.Balasubramaniam “StructuralHealth Monitoring of anisotropicplates using ultrasonic guidedwave STMR array patches”,NDT and E International42(3), 193-198 (2009).

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NCB – ISNTANNOUNCES

ISNT LEVEL – III CERTIFICATION PROGRAMMEAT PUNE

(7TH JANUARY to 17TH FEBRUARY, 2013)

The Announcement for the ISNT Level-III Certification programme hasbeen displayed in the web “www.isnt.org.in” with complete details along

with application form. Interested participants may avail thisopportunity for better prospects.

Last date for receipt of application form along with payment isDecember 21, 2012

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Horizon

Dr. C V KrishnamurthyCentre for NDE and Department of Physics, IIT MadrasChennai 600036 Tamilnadu Email ID: [email protected]

Structural HealthMonitoring (SHM)

for structural integrity assessment.Under operational conditions, evensmall fluctuations in forces/couplescan initiate a series of events leadingto structural failure. Likewise, smalloscillations can grow nonlinearly inamplitude leading to failure.

It is clear then that a holisticassessment of the integrity of astructure in a periodic manner usingthe information provided by anetwork of local sensors wouldconstitute SHM. More specifically,SHM can be defined as a processof: (i) observing or tracing theperformance of a structure underenvironmental and operational loadsby sensors and instrumentationdevices, (ii) evaluating theperformance of the structure for anydevelopment of defect or damage byuse of the measured data andanalytical tools, and (iii) issuing analarm when the designatedperformance criteria are exceeded.

Environmental loads on a structuremanifest as a vast range of complexphenomena include diurnal andseasonal weather patterns as well asextreme weather conditions. Theseloads induce physical and chemicalchanges in structural elements todifferent degrees and bring aboutnonlinear responses to routineoperational loads. Thus, whilestructural integrity appears to dealonly with the response of amechanical system to environmentaland operational loads, an SHMsystem is in fact a syntheticapplication of various branches ofengineering and science disciplinessuch as mechanical engineering, civilengineering, optical engineering,electrical engineering, electronicengineering, communicationengineering, software engineering,computer science, material science,information technology, etc.

A perspective emerges by taking alook at some characteristics of

The paradigm behind this new fieldis borrowed from the medicalprofession: by proactively monitoringhealth, by discovering problems intheir early stages before they becomeserious, and by responding quicklyand effectively to accidents, naturalcatastrophes, and other incidents,lives can be saved, structurallifetimes can be extended, and moneycan be saved. This is a diverse fieldwith research and applications inmany areas encompassing disciplinessuch as structural dynamics, materialsand structures, fatigue and fracture,non-destructive testing andevaluation, sensors and actuators,microelectronics, signal processingand possibly much more.

The question, “Is there anythingwrong with a structure?” issurprisingly difficult to answer. Thedifficulty stems from the fact thatconventional non-destructivepractices are seldom used to test astructure for its integrity. Part of thechallenge lies in identifying andmeasuring characteristics that revealthe integrity (or lack of it) of thegiven structure. Part of the challengearises from the variety andcomplexity of the different structuresthat are encountered in real life –ranging from spacecraft, nuclearreactors, ships, dams, pipelines,turbines, and rail tracks to name afew.

Consider a long electrical conductingwire. To check whether it electricallyconnects two distant parts of a

circuit, we apply a small voltageacross the terminals and look for thecurrent flow and ascertain whetherthe circuit is “open” or not. We notethat if it is found “open”, this testwill not help locate the fault alongthe length of the wire. Consider nowa two-dimensional network ofresistors. If one resistor isdisconnected from the network, thesimple test of applying a voltageacross two terminals on the boundarywill not reveal anything abnormal. Acritical number of resistors(percolation threshold) need to bedisconnected (randomly) from thenetwork before the test can revealthe “openness” of the electricalnetwork. In other words, the networkwould behave “normally” even whena few resistors are disconnectedrandomly within the network. Currentflows in alternate pathways to ensure“structural integrity”. This is muchlike the human brain which showsremarkable functionality even whenparts of the brain are either diseasedor damaged – for the pathways aremore complex and richer in threedimensions. We may note that thetesting is done by passing a smallsignal across the whole of thestructure and collecting the response.

Integrity of a mechanical structurewould depend on how loads areredistributed when there a few weaklinks located randomly within thestructure. We can see that the meredetection of weak links on anindividual basis may not be sufficient

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continuous remote monitoring givenbelow:

Sensors are left permanently inplace, so that measurements canbe performed at any time, or atregular specified intervals. Thismeans that sensors must cover allareas of the structure that are tobe monitored.

Sensors must be reliable, asfrequent replacing, repairing, orrecalibrating of sensors throughon-site visits will reduce any costsavings and possibly reduce thesafety margin provided by sensordata.

Individual sensors must berelatively inexpensive, sincemultiple sensors must be dedicatedto a single structure; however, theadvantages of continuous remotemonitoring may justify aconsiderable initial cost.

Sensors must be robust, since theywill often be exposed to theweather and must operate overlong periods of time withoutdegrading.

A reliable communication systemis needed to send the data. Where

wired communication is notpractical, wireless communicationmay have to be used.

A source of power must beavailable for the sensors, dataacquisition, and communicationsequipment. In remote locations thismay require the use of solarpanels.

The amount of raw data that isgenerated by such applications canbecome extremely large, and animportant aspect of the SHM processis developing and deploying the datahandling software so to supportinformed infrastructure management.The types of information generatedby remote continuous monitoring canbe classified into three types, basedon the user:

Web information: For the generalpublic and the media, a simpleoverview of operations thatdescribes whether various aspectsof the system are operatingnormally, abnormally, or not at all.

Viewer information: For managers,much more detailed information forinterpretation by technical experts

to diagnose what is happening andwhat it means for the structure.For example, following an alarmcondition or a sudden change in asignal, this information would guideemergency inspections or repairefforts and decisions about closingthe structure.

Expert information: Researcherscan use raw data from sensors toidentify long-term trends and buildpredictive models of futureperformance.

In addition to providing all these levelsof information reliably and securely,the monitoring system must alsosafely archive the data and ensurethat the system is secure. These aresignificant software requirements,and designing data-handling softwarefor continuous remote monitoringsystems is an important technicalchallenge that must be considered intandem with the instrumentation andcommunication systems.

SHM SCENARIOS

SHM starts with the sensing system- the most common measurementsmade 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 [fromRef.2]

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acceleration (piezoelectric,piezoceramic, fiber opticaccelerometers)

strain (resistive foil, fiber optic,piezoelectric patches)

impedance (piezoelectricpatches)

It is important to recognize that SHMdiffers from NDT in its scope.Techniques employed in NDT do notautomatically become candidates forSHM.

Broadly, SHM systems are classifiedinto two types. A SHM system thatonly used sensors for collecting datais typically referred to as a passivesystem, while a system with built-inactuators, which is designed toproduce diagnostic signals, is referredto as an active system. The passivesystem is most effective in monitoringenvironmental changes to the

structures such as external loads,temperature, etc. An example ofsuch a system is shown below inFigure 1.

The active system is more effectivein detecting cracks, damage andanomalies. An example is the impactdamage detection scheme, shownbelow in Figure 2, carried out withguided acousto-ultrasonic waves incomposites.

Many countries, including India, haveinitiated SHM programs atuniversities and research institutions.Since 1997, Fu-Kuo Chang ofStanford University has beenorganizing International Workshopson SHM. Since 2002, every alternateyear, the European Workshop onSHM is being held at variouslocations in Europe. An InternationalJournal of SHM was launched in2002. Several existing journals have

begun covering topics in SHM. Thereader is directed to the large bodyof research on SHM through thepublications cited at the end of thisoverview.

SHM OF FRCCS

Fiber reinforced cementitiouscomposites (FRCCs) are anemerging high-performance civilengineering material that exhibitsextremely high strength and ductility.FRCCs contain high volumes ofdistributed steel fibers, and thus thematerial has measureableconductivity. Researchers at theUniversity of Michigan have beenexploring the self-sensing capabilityof FRCC as a “smart” materialcapable of distributed sensing forcrack detection. To recreate thespatial distribution of conductivitywithin FRCC materials based onlyon boundary measurements,

Fig. 2 : Network of actuators (PZTs) generate acousto-ultrasonic guided waves which scatter off defects and provide information ofstructural changes [from Speckmann H., Focal point for SHM, IMRBPB Meeting, EASA, Cologne April 22, 2007]

Fig. 3 : Approach to produce impedance tomographs: measurement approach (left) and inverse solution scheme (right)[from Ref. 10]

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electrical impedance tomography iscarried out as illustrated in Figure 3.The image reconstruction process issemi-analytical, requiring iterativematching of analytical andexperimental data to arrive at thefinal impedance image. Micro- andmacrocracks can be imaged in finedetail since cracks can be considerednon-conducting compared to theFRCC material.

SHM OF STEEL CABLES

Magneto-elastic stress sensors utilizethe dependence of the magneticproperties (e.g. the magneticpermeability) of structural steels onthe state of stress of the material.The sensors are comprised of aprincipal and secondary set ofcylindrical wire coils that are woundaround the test object. Magneticfields are generated when electriccurrent is passed through theprincipal coil, and the interaction fieldis detected by the secondary coil.The magneto-elastic sensors functionin a fully contactless manner, and donot touch or alter the inspectedmaterial in any way except bymagnetization. Since the magneticpermeability in ferromagneticmaterials is a function of magnetichistory and applied field, thistechnique measures the internalstress in steel tendons and cablesmore directly as compared to otherNDT methods. Furthermore, only arelatively small length of sample cable(less than 2 m) is needed forlaboratory calibration of the materialused in structure. The sensors allowfor easy installation in new structuresand in-situ installation adaptability forexisting structures. Other attractivefeatures include compact sensor size,ease of operation, high accuracy ofstress determination (within ±3%),and a theoretically unlimited servicelifetime. Researchers at NortheasternUniversity have developed novelmagneto-electric sensors fornondestructively monitoring in situstress in steel pre-stressed tendons

and bridge cables. The sensors havebeen applied to cable-stayed bridges,arch-suspension bridges, post-tensioning concrete box girderbridges, and a large domed spacestructure that contains high tensionsteel bracing cables.

VIBRATION-BASED SHM

It is intuitive that damage can beidentified by analyzing the changesin vibration features of the structure.The fundamental idea for vibration-based damage identification is thatthe damage-induced changes in thephysical properties (mass, damping,and stiffness) will cause detectablechanges in modal properties (naturalfrequencies, modal damping, andmode shapes). For instance,reductions in stiffness result from theonset of cracks. Although in vibrationtest, the excitation and response arealways measured and recorded in theform of time history, it is usuallydifficult to examine the time domaindata for damage identification. Amore popular method is to examinethe modal domain data through modalanalysis technique, in which the timedomain data is transformed into thefrequency domain, and then themodal domain data can be furtherextracted from the frequency domaindata. During the past three decades,great effort has been made in theresearches within all three domains(i.e., time, frequency, and modaldomains). The modal domain methodsare popular because the modalproperties (i.e., natural frequencies,modal damping, modal shapes, etc.)have their physical meanings and arethus easier to be interpreted orinterrogated than those abstractmathematical features extractedfrom the time or frequency domain.Modal parameters-based damageidentification algorithms for beam-type or plate-type structures can becategorized as natural frequency-based methods, mode shape-basedmethods, curvature mode shape-based methods and methods using

both mode shape and frequencies.Several algorithms have beenproposed and investigated:

The single damage indicator (SDI)method has been proposed tolocate and quantify a crack inbeam-type structures by usingchanges in a few naturalfrequencies.

A method that depends onexperimental data only fromdamaged structures has recentlybecome a focused research topicin damage identification. Thesemethods do not require atheoretical or numerical model.Their basic assumption is that themode shape data from a healthystructure contains only low-frequency signal in spatial domaincompared to the damage-inducedhigh-frequency signal change. Thegeneralized fractal dimension(GFD) method is one of theseveral signal processingtechnique-based damage detectionalgorithms studied.

It has been shown by manyresearchers that the displacementmode shape itself is not verysensitive to small damage, evenwith high density mode shapemeasurement. It has been foundthat the mode shape curvature(MSC) or the second derivativesof mode shape, is highly sensitiveto damage.

A response-based damagedetection technique which onlyrequires the mode shapes of thedamaged plates, the gappedsmoothing method (GSM), issimilar to MSC but it does notrequire the baseline mode shape.

Strain energy based damage indexmethod (DIM)

The table provides a comparisonbetween various damage detectionalgorithms.

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biocompatible, withstand hightemperatures, and are potentiallysmall and lightweight. Fiber Bragggrating (FBG) sensors are regardedas the most mature grating-basedsensors and have already beenwidely used. An FBG sensor reflectsa portion of the incoming light of aparticular wavelength, called Braggwavelength, and leaves the rest ofthe incoming light pass withoutaltering its property as shown inFigure 4. The Bragg wavelength isdefined by the fiber refractive indexand grating pitch, which are affectedby the external environment changes,such as temperature, strain, vibrationand other parameters. All thesechanges manifest as Braggwavelength shifts. Therefore, bymonitoring the Bragg wavelengthshift, several measurands can bemonitored using FBG sensors.

able to reduce the typical limitationsassociated with smart materials(requirement for high voltage or highcurrent, small range of strain or forceactuation, brittleness or excessiveweight), leading to new solutions togenerate and measure motion indevices and structures.

Two types of sensors that haveattracted considerable attention arethe fiber optic sensors andnanostructured carbon basedsensors.

OPTICAL FIBER SENSORS

The main advantages of optical fibersensors are derived from theparticular characteristics of the silica:it is passive, dielectric, and with lowlosses at optical frequencies. For thatreasons, optical fiber sensors areimmune to electromagneticinterferences, chemically inert,

ADVANCES IN SENSORTECHNOLOGY

Over the past two decades significantefforts have been made byresearchers in order to miniaturizeelectronic devices for SHMapplications and optimize theirperformance. This “topdown”approach has led to the developmentof MEMS (microelectromechanicalsystems) technology, which allowscomplex circuitry, sensing andactuation mechanisms and advancedcomputation capabilities to take placein a single microchip. Sensors at themicrometer length scale such asMEMS accelerometers and MEMSultrasonic transducers have beentherefore developed. On the otherhand, the opportunities offered bysmart materials (that is to saymaterials characterized by sensingand actuating properties, such aspiezoelectric, pyroelectric,electrostrictive, magnetostrictive,piezoresistive and electroactivematerials) in the field of SHM havebeen extensively investigated.

However, performance limits inMEMS technology and the high costsassociated with fabrication of MEMSsensors in advanced clean roomshave led to an increasing attentiontowards the opportunities offered inthe field of nanotechnology for thedevelopment of high performancesensing devices using a “bottom-up”approach. According to it, chemicalfabrication parameters can betailored at the molecular scale to yieldmacro-scale bulk sensor properties.Smart nanoscale materials are also

Table 1: Capabilities of five comparative damage detection algorithms [from Ref. 3]

Fig. 4 : Illustration of the fiber Bragg grating concept and its optical function.[from Ref. 5]

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precision and stability, SOFOinterferometric sensors are the mostsuccessful low coherentinterferometric sensors for SHM.They have been reported beingsuccessfully deployed in more thanhundreds of structures so far,including bridges, buildings, oil pipesand tunnels. SOFO interferometricsensors are long-gauge sensors andhave a measurement range startingfrom 0.25 m to 10 m or even up to100 m with a resolution in the levelof micrometer. However, they areonly suitable for the measurement ofelongations and contractions at a lowspeed (0.1 Hz–1 Hz) and not capableof detecting the impact damages inaircraft structures.

Brillouin fiber optic sensor has thecapability to simultaneously measurethe strain level and locate the strainedpoint along the sensor. This feature,which has no performance equivalentamong the traditional electronicsensors, is extremely valuable. Thesensor required by Brillouintechnology is an inexpensive,telecom-grade optical fiber thatshares most of the typical advantagesof fiber optic sensors such as highresistance to moisture and corrosionand immunity to electro-magneticfields. Some fiber sensors specificallyaddressed to Brillouin SHM havebeen developed as shown below inFigure 6.

Going beyond the conventionalBrillouin sensors, “smart” sensorsshown above promise in RCstructures. Due to the stiffening

compact size, and good linearity. Thegrating length is usually in the orderof 10 mm. The resolution isdependent on the wavelengthinterrogator (see Figure 5 below),which is currently up to 1 pm,corresponding to 1με for strainmeasurement and 0.1°C fortemperature sensing.

SOFO (a French acronym forSurveillance d’Ouvrages par FibresOptiques) is a deformationmeasurement system based on low-coherence interferometry using anfiber optic sensor. With features oftemperature insensitivity, high

In the past 20 years, wavelengthmultiplexing technology has beenmature, hundreds (if not thousands)of wavelengths can be multiplexedin one single optic fiber. Currenttechnology makes it possible tomultiplex tens or hundreds of FBGstrain sensors in one optic fiber andmonitor them remotely. With therapid development in the past fewyears, FBG sensors have beentargeted as the major leadingtechnology in contrast to othercompeting fiber optic sensortechnologies. Besides its wavelengthmultiplexing capability, FBG sensorshave the advantages of low cost,

Fig. 5 : FBG interrogation methods classified by measurement frequency. [from Guo etal., Fiber Optic Sensors for Structural Health Monitoring of Air Platforms,Sensors 11 (2011), 3687-3705]

Fig. 6 : Brillouin SHM sensors: RC-embeddable cord (left), woven “smart” FRP(center), and extruded rod (right).[from Ref. 13]

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effect of the structural fibers in theweft, when a crack opens in thesubstrate to which the “smart”sensor is bonded, the deformation isredistributed in a portion of FRPmaterial longer than the crack width.This process happens because theshear transfer process between thesubstrate and the FRP material isspread on to a certain “transferlength” that is influenced by its ownFRP stiffness and, for carbon-FRPmaterials, is found to be typicallyseveral inches long. Further, the straindistribution on the sensing fiberapproaching the crack locationbecomes much more gradual thanthe step-like distribution that isassumed without the effect of thestiffening fibers (see Figure 7 below).

NANOSTRUCTURALCARBON SENSORS

The superior mechanicalperformance and conductiveproperties of CNTs are well-known.From a mechanical point of view,CNTs show an elastic behaviour,with a very high stiffness (Young’smodulus of approximately 1 TPa anda density of about 1.33 g/cm3) andthe possibility to bear torsion andbending without breaking. Thehexagonally-bonded carbonhoneycomb structure of SWCNTs isresponsible for their high mechanicalstrength (tensile strength between 20GPa and 60 GPa, with maximum

being investigated by the Structuraland Geotechnical DynamicsLaboratory StreGa at the Universityof Molise, Termoli, Italy.

The electrical properties of CNTbased composites are influenced bythe CNT concentration in the matrix.In fact, its increase leads to morenanotube-to-nanotube junctions, thusproviding a greater number of pathsfor electrical current to flow fromone electrode to the other, reducingthe overall resistivity of the film.However, beyond a certainconcentration (percolation threshold)the benefit in increasing the CNTconcentration is lower and lower.Thus, a proper sensor design requiresthe evaluation of such a threshold inorder to optimize sensor performanceas a function of CNT concentration.

Apart from a few recent studies,very little work has been done onthe development of cement basedsensors using CNTs. The dispersionof CNTs into a cement matrix leads,first of all, to an improvement of themechanical properties of the cementcomposite. In fact, because of theirsize and aspect ratio, CNTs can bedispersed in a much finer scale thancommon fibers, thus obtaining a moreefficient crack bridging at the initialstage of crack propagation in thecomposite. Moreover, CNTdispersion in the cement matriximproves also its electricalconductivity, making possible the

Fig. 7 : Stress distribution over a crack for conventional FO sensor (left) and of “smart” FRP (right).[from Ref. 13]

strain up to 10%), which is evenbetter than that of structural steel.

From the electrical standpoint, CNTscan be classified as conductors orsemi-conductors, depending on theorientation of the carbon atoms inthe lattice structure of the tubes. Thisproperty depends on the SWCNTone-dimensional structure, whichallows electrons to travel greaterdistances before scattering occurs,approaching a ballistic transport-typebehaviour which increases theelectrical conductivity.

Recently new forms of CNTmaterials have become availablefrom the Nanoworld Laboratory atthe University of Cincinnati. Theseare arrays, ribbon, thread, yarn, braid,and tape. Tape is the newest materialand can be from 1 to 10 cm widecan be used to form a sheet. Thefunctionalized CNT sheet isimmersed into an epoxy resin solutionto form a CNT sheet pre-preg. CNTsheet pre-pregs are used in formingcomposites by stacking multiplelayers of CNT pre-pregs betweentwo plates and placing into a hotpress for curing. All the materialforms have the capability to detectdamage at the early stages due totheir high piezoimpedance sensitivity.The application of these materials forSHM is still in its early developmentperiod. Application of the materialsfor SHM of Civil Infrastructure is

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development of a smart materialwhose conductivity is sensitive to theapplied strain and which can betherefore used as self-sensingmaterial. The available results aboutintegration of carbon nanotubes andnanofibers (CNF) into cement matrixseem to confirm the higherperformance, with respect totraditional carbon fibers, in terms ofenhancement of mechanicalproperties and electrical conductivityof the composite material.

CURRENT AND FUTUREDIRECTIONS:

Among the advances beingimplemented now, or expected in thenear term, are these:

Use of embedded sensors builtinto new or substantiallyrehabilitated structures,

Applications ofnanotechnology in sensordesign to develop low-powered, area wide sensors

Advances in wirelessapplications that reduce oreliminate dependence onphysical connection of sensorsor continuous sources ofpower

Application of new, renewablepower sources to support long-term sensor operations

With the continuing maturation in bothhardware and software technologies,the research on SHM is beingdevoted to realizing the transitionfrom its diagnostic function toprognostic function. Integrating SHMwith condition-based maintenancemanagement (CBMM) for in-servicestructural systems is anotherpromising direction as it makes theSHM technology more vital. A greatdiversity of research is required tobridge the current gap between SHMand CBMM.

It is clear that structures with a built-in sensing capability have superioradvantages over traditional structuresdue to improved state awareness,enhanced safety and reliability,minimized operation and maintenance,and added multi-functionalities.Structures with sensing capability candetect and monitor their physicalhealth conditions in any givenenvironment and can determine anoptimal course of action to maximizetheir performance and operation withminimal risk in safety. With propersensing, the performance of many

other added functional capabilitiessuch as shape control, smart antenna,noise reduction, and flow controlcould be significantly improved orenhanced.

Furthermore, it was reported that ifthe SHM system could be consideredat the initial design stage, it couldresult in a major paradigm change inthe structural design for the nextgeneration aircraft and spacecraft.Instead of relying upon uncertaintiesand safety factors in the designprocess, the new design with anappropriate SHM system could leadto much more reliable structureswithout overly conservative weightpenalty. As shown in Figure 8 below,a design with structural sensing leadsto more benefits, possibilities andpotential with a future of more“intelligent” structures.

Despite ongoing research, alegitimate question, why is SHMbetter than schedule-based NDT,needs to be addressed. Every industryneeds to recognise demonstrableadvantages before SHM makes thetransition from research to practice.

FOR FURTHER READING:

1. Special issue on interdisciplinary andintegration aspects in structural healthmonitoring, Mechanical Systems andSignal Processing 2828282828 (2012)

2. Chae M.J., Yoo H.S., Kim J.Y., Cho M.Y.,Development of a wireless sensornetwork system for suspension bridgehealth monitoring, Automation inConstruction 2222211111 (2012) 237–252

3. Fan Wei and Qiao Pizhong, Vibration-based Damage Identification Methods:A Review and Comparative Study,,,,,Structural Health Monitoring 10(2011), 83-111

4. Rainieri Carlo, Fabbrocino Giovanni,Song Yi, Shanov Vesselin, CNTComposites For SHM: A LiteratureReview, International Workshop SmartMaterials, Structures & NDT InAerospace Conference NDT, Canada(2011)

5. López-Higuera José Miguel, Cobo LuisRodriguez, Incera Antonio Quintela, andAdolfo Cobo, Fiber Optic Sensors in

Fig. 8 : Diagnostics/prognostics cycle for an intelligent structural health managementsystem [from Ref. 11].

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13. Bastianini F, Matta F, Rizzo A, Galati N,Nanni A. Overview of recent bridgemonitoring applications usingdistributed Brillouin fiber opticsensors, Journal of NondestructiveTesting 1212121212(2007), 269-276 .

14. Brownjohn J.M.W, Structural healthmonitoring of civil infrastructure, Phil.Trans. R. Soc. A 2007 365365365365365, 589-622

15. Bhalla S., Yang Y.W., Zhao J., Soh C.K.,Structural health monitoring ofunderground facilities – Technologicalissues and challenges, Tunnelling andUnderground Space Technology 2020202020(2005) 487–500

16. Chung D.D.L., Structural healthmonitoring by electrical resistancemeasurements, Smart Materials andStructures 1111100000 (2001), 624-636

17. Ghoshal Anindya, Sundaresan MannurJ., Schulz Mark J., Frank Pai P.,Structural health monitoringtechniques for wind turbine blades,Journal of Wind Engineering andIndustrial Aerodynamics 8585858585 (2000)309-324

Structural Health Monitoring, Journalof Lightwave Technology 2929292929 (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 ofcapacitance-based and impedance-based wireless sensors and sensornodes for structural health monitoringapplications, Journal of Sound andVibration 329 (2010) 2410–2420

7. Wicks Sunny S., deVilloria RobertoGuzman, Wardle Brian L., AjayRaghavan and Seth Kessler,Tomographic Electrical Resistance-based Damage Sensing in Nano-Engineered Composite Structures, 51stAIAA/ASME/ASCE/AHS/ASC Structures,Structural Dynamics, and MaterialsConference, 12-15 April 2010,Orlando. FL.

8. Fei Yan, Roger L. Royer, jr and JosephL. Rose, Ultrasonic Guided WaveImaging Techniques in StructuralHealth Monitoring , Journal of

Intelligent Material Systems andStructures 21 (2010), 377-384

9. Staszewsk W.J., Mahzan S., Traynor R.,Health monitoring of aerospacecomposite structures – Active andpassive approach, Composites Scienceand Technology 6969696969 (2009) 1678–1685

10. Popovics John S., Recent developmentsin NDT and SHM in the United States,NDTCE’09, Non-Destructive Testing inCivil Engineering, June 30th – July 3rd,2009, Nantes, France

11. Achenbach Jan D., Structural healthmonitoring – What is the prescription?,Mechanics Research Communications3636363636 (2009) 137–142

12. Mousumi Majumder, Tarun KumarGangopadhyay, Ashim KumarChakraborty, Kamal Dasgupta, D.K.Bhattacharya, Fibre Bragg gratings instructural health monitoring—Presentstatus and applications, Sensors andActuators A 1A 1A 1A 1A 14444477777 (2008) 150–164

National NDT Awards No. Award Name Sponsored by 1. ISNT - EEC M/s. Electronic & Engineering Co., Mumbai

National NDT Award (R&D)

2. ISNT - Modsonic M/s. Modsonic Instruments Mfg. Co. (P) Ltd.,National NDT Award (Industry) Ahmedabad

3. ISNT - Sievert M/s. Sievert India Pvt. Ltd., Navi MumbaiNational NDT Award (NDT Systems)

4. ISNT - IXAR M/s. Industrial X-Ray & Allied RadiographersBest Paper Award in JNDE (R & D) Mumbai

5. ISNT - Eastwest M/s. Eastwest Engineering & Electronics Co.,Best Paper Award in JNDE (Industry) Mumbai

6. ISNT - Pulsecho M/s. Pulsecho Systems (Bombay) Pvt. Ltd.Best Chapter Award for Mumbaithe Best Chapter of ISNT

7. ISNT - Ferroflux M/s. Ferroflux ProductsNational NDT Award (International recognition) Pune

8. ISNT - TECHNOFOURNational NDT Award forYoung 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 professionalenterpreneurs. 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:[email protected]

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NDE eventsWe hope that this new feature added to the journal during the last yearhas been useful for the readers in planning their activities in terms ofpaper submissions, registering for seminars, etc. Please send yourfeedback, comments and suggestions on this section [email protected]

September 2012The 51st Annual Conference of The British Institute ofNon-Destructive TestingSeptember 11- 13, 2012 ; Daventry, Northamptonshire,UKhttp://www.bindt.org/Events/NDT_Conferences_&_Seminars/NDT_2012

30th European Conference on Acoustic Emission Testingand7th International Conference on Acoustic Emission.

September 12 – 15, 2012 ; Granada, Spain.http://www.2012.ewgae.eu

DACH Jahrestagung 2012 : Joint conference of theGerman,Austrian and Swiss NDT societies (DGZfP, ÖGfZP andSGZP)September 17 – 19, 2012 ; Graz, Austria.http://jahrestagung.dgzfp.de

Conference on Industrial Computed Tomography.(Organised by the CT Research Group, Upper AustrianUniversity of Applied Sciences)September 19 – 21, 2012 ; Wels Campus, Wels, Austria.

http://www.3dct.at

55th Annual A4A NDT ForumSeptember 24-28, 2012 ; Seattle, Washingtonhttp://www.airlines.org/Pages/2012AnnualA4ANDTForum.aspx

October 20126th 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-DestructiveExamination (NDE)October 11, 2012 ; Haydock Park Racecourse,Merseysidehttp://www.imeche.org/events/s1725

2012 IEEE International Ultrasonics Symposium,October 7 - 10, 2012 ; Dresden, Germanyhttps://ius2012.ifw-dresden.de/

2nd IPC Personnel Certification Conference.October 14-16, 2012 ; Rio de Janeiro, BrazilASNT 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 2012Workshop on Civil Structural Health Monitoring (CSHM-4)-“SHM systems supporting extension of the structures’service life”November 6 - 8, 2012 ; Berlin, Germanyhttp://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 (IAES-21).November 27-31, 2012 ; Okinawa, Japan.http://iaes21.org

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NDE patentsWe hope that the section on NDE Patents, which featured in the last few issues of thisjournal has continued to trigger your curiosity on this very important topic ofIntellectual property. We continue this section with a few more facts on patents and alisting of a few selected NDE patents. Please send your feedback, comments andsuggestions on this section to [email protected]

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

pressure containers from identicalproduction that have been classifiedas being without defects inpredetermined phases of a time-controlled pressure acting upon thepressure container (AE testprocedure) with one or more acousticemission channels (AE channels) usingacoustic emission sensors (AEsensors) of a predetermined position(one AE characteristic per AE sensoror AE channel.

Inventors: Bohse; Juergen(Germany), Mair; Georg M.(Germany)

Assignee: Bam Bundesanstalt FuerMaterialforschung und-Pruefung(Germany)

UNITED STATES PATENT7,080,555July 25, 2006Distributed mode system forreal time acoustic emissionmonitoring

Abstract

A distributed real time healthmonitoring system is described formonitoring of acoustic emissionsignals from different regions of astructure such as aircraft or spacecraftstructures. The health monitoringsystem has its analysis and prognosisintelligence distributed out to the localregions being monitored and thereforedoes not require extensive cablingsystems to carry the high bandwidthinformation characteristic of acousticemission.

Inventors: Austin; Russell K. (USA),Coughlin; Chris (USA)

Assignee: Texas ResearchInternational, Inc. (USA)

UNITED STATES PATENT6,823,736November 30, 2004Nondestructive acousticemission testing system usingelectromagnetic excitation andmethod for using same

with the auxiliary supporting arm. Theabove components are assembled asfollows: the main supporting arms ofthe two sets of supportingmechanisms are connected to the twofree ends of the radial positioningmechanism respectively in the waythat the auxiliary supporting arms ofthe two sets of supportingmechanisms are located at the innersides of the main supporting armsrespectively and are arranged axissymmetrically with respect to thecentral line of the radial positioningmechanism, and the kink shaftmembers of the two auxil iarysupporting arms are respectivelyinserted into the plugholes of the twobases to form revolute pairs, the twosets of acoustic emission test sensormounting mechanisms arerespectively mounted at the twobases, and the two sets of parallelismadjusting members are respectivelymounted on the two main supportingarms and correspond to the positionsof the bases.

Inventors: Liu; Jianfeng (China),Xie; Heping (China), Xu; Jin (China)

Assignee: Sichuan University (China)

UNITED STATES PATENT7,698,943April 20, 2010Method for evaluating pressurecontainers of compositematerials by acoustic emissiontesting

Abstract

The invention relates to a method forevaluating pressure containers madeof a composite material by acousticemission testing. The methodcomprises the steps: (a) determininga sufficient number of internalpressure-dependent acoustic emissioncharacteristics (AE characteristics) of

UNITED STATES PATENT8,208,344June 26, 2012Method, apparatus or softwarefor determining the location ofan acoustic emission emitter ina structure

Abstract

A method, apparatus and software isdisclosed in which the location of theorigin of a received acoustic emissionin a structure is calculated bytriangulating the times of flight of theacoustic emission to a distributed setof sensors and using a predeterminedacoustic model of the structure.

Inventors: Paget; Christophe (GreatBritain)

Assignee: Airbus Operations Limited(Great Britain)

UNITED STATES PATENT8,181,526May 22, 2012Acoustic emission test sensorfixing device

Abstract

An acoustic emission test sensor fixingdevice, comprising a radial positioningmechanism, supporting mechanisms,bases, acoustic emission test sensormounting mechanisms, andparallelism adjusting members,wherein the supporting mechanismcomprises a main supporting arm andan auxiliary supporting arm, with oneend of the auxiliary supporting arm isfixedly connected to or hinged with themain supporting arm and the otherend is provided with kink shaftmembers which are symmetricalabout the auxiliary supporting arm;and the bases are provided withplugholes which form revolute pairs

Continuing our endeavor to provide you updates on NDE and Inspectionrelated patents, listed below are a few recent patents from a variety ofdifferent areas related to Nondestructive Evaluation and Inspection whichwere issued by USPTO in the last few years. If any of the patents are ofinterest to you, a complete copy of the patent including claims anddrawings may be accessed at http://ep.espacenet.com/

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crack depth to the amplitude ratio canbe obtained by simulating crackgrowth in a fracture specimen coupledto a test structure or field structure,and measuring acoustic emissionsignal in the structure by the falseaperture transducer. The calibrationcurve correlates simulated crackdepth percentage with computed peakamplitude ratio of the measuredsignal. Using the calibration curve andacoustic emission signal sensed by afalse aperture transducer in a fieldstructure, a crack in the structure canbe detected and its depth measuredby computing the peak amplituderatio of the signal and identifying thecrack depth that correlates with theratio from the calibration curve.

Inventors: Dunegan; Harold L.(USA)

UNITED STATES PATENT5,528,557June 18, 1996Acoustic emission sourcelocation by reverse ray tracing

Abstract

A method of locating an acousticemission source in a structure byreverse ray tracing. An azimuthacoustic emission sensor is utilizedwhich has an array of individualelemental detectors whichindependently and sequentiallyrespond to the passage of an acousticstress wave. The response of eachelement of the array is electronicallymonitored, and individual responsesto the acoustic stress wave areanalyzed to determine the azimuthapproach angle of the wave to theazimuth acoustic emission sensor. Anaccurate measurement of the truelocation of the acoustic emission signalsource is then provided by reverse raytracing by using a parallel processingarrangement having a plurality ofparallel processing elements. Thestructure is modeled in the computeron a one to one basis, with eachparallel processing element simulatingand having structural data on onediscrete area of the structure. Thedetermined azimuth approach angleis an input to the parallel processingarrangement, such that a simulatedwave propagation takes place in thecomputer model as if it werepropagating in the structure, and theactual location of the acoustic emissionsource is determined by reverse raytracing by taking into account thestructure of the intervening path of thewave and the most probable

UNITED STATES PATENT6,289,143September 11, 2001Fiber optic acoustic emissionsensor

Abstract

A fiber optic acoustic emission (FOAE)sensor particularly suitable forvibration sensing in a hosti leenvironment has a pair of opticalfibers each having an end face. In oneembodiment, a hollow tube or corehaving opposite open ends receivesthe end faces of the optical fibers.Means are provided for fixing theoptical fibers in the hollow core withthe end faces facing each other andspaced by a distance from each otherin the core. A signal processing unit isconnected to the optical fibers forsupplying light to, and for receivinglight from, the optical fibers and formeasuring variations in optical phasewhich result in changes in the lightintensity due to vibrations of thehollow core. The hollow core is fixedin a resonant cylinder, and theresonant cylinder is fixed in a housingto complete the sensor. Otherembodiments dispense with the needfor the hollow tube or core and employmeans for fixing the optical fiberswithin a precision hole,advantageously produced by electricaldischarge machining (EDM) or similarprocesses, provided in the resonantcylinder. A system employing theseembodiments of the FOAE sensor isalso disclosed.

Inventors: Berthold; John W.(USA), Roman; Garry W. (USA)

Assignee: McDermott Technology,Inc. (USA)

UNITED STATES PATENT6,173,613January 16, 2001Measuring crack growth byacoustic emission

Abstract

A method and an apparatus fordetecting and measuring cracks inplate-like structures using acousticemission technique are disclosed. Afalse aperture transducer is designedto provide a criterion for filtering outextraneous noise in the acousticemission signal based on modalanalysis by computing the ratio of thehigh-frequency peak amplitude tolow-frequency peak amplitude of thesignal. A calibration curve correlating

Abstract

A nondestructive acoustic emissiontesting system using electromagneticexcitation, comprises: a) anelectromagnetic wave generator forgenerating electromagnetic wavesthat stimulate a test sample togenerate acoustic energy; b) anacoustic energy sensor for detectingthe acoustic energy and generating afirst output signal that represents theacoustic energy; and c) a dataprocessor for comparing the outputsignal with a reference and forgenerating a second output signal thatrepresents a characteristic of the testsample.

Inventors: Brock; David W. (USA),Joshi; Narayan R. (USA), Russell;Stephen D. (USA), Lasher; MarkhamE. (USA), Kasa; Shannon D. (USA)

Assignee: The United States ofAmerica as represented by theSecretary of the Navy (USA)

UNITED STATES PATENT7,075,424July 11, 2006System for damage locationusing a single channelcontinuous acoustic emissionsensor

Abstract

A sensor array for non-destructivelymonitoring a structure to detect acritical structural event. The sensorarray includes a plurality of discretesensor nodes, each of the discretesensor nodes producing an electricalsignal in response to a structuralevent. A signal adder is electricallyconnected to the plurality of discretesensor nodes for receiving andcombining the electrical signal fromeach of the discrete sensor nodes toform a single sensor array outputsignal. A signal processing modulethen receives and processes the singlesensor output signal. In the preferredembodiment, the signal processingmodule uses the time interval betweenthe electrical signals from each of thediscrete sensor nodes formed into asingle sensor array output signal tocalculate the location of the criticalstructural event. Also, in the preferredembodiment, a data collection systemis located downstream of the sensorprocessing module.

Inventors: Sundaresan; Mannur J.(USA), Ghoshal; Anindya (USA),Schulz; Mark J. (USA)

Assignee: North Carolina A&T StateUniversity (USA)

NDE PATENTS

36


perturbations of the wave therein. The presentinvention has particular applicability to aircraftstructures, and the method is utilized to locatestructural defects therein.

Inventors: Horn; Michael (USA)

Assignee: Northrop Grumman Corporation (USA)

UNITED STATES PATENT6,360,608March 26, 2002Transducer for measuring acousticemission events

Abstract

A method and an apparatus for detecting andmeasuring cracks in plate-like structures usingacoustic emission technique are disclosed. A falseaperture transducer is designed to provide acriterion for filtering out extraneous noise in theacoustic emission signal based on modal analysisby computing the ratio of the high-frequency peakamplitude to low-frequency peak amplitude of thesignal. A calibration curve correlating crack depthto the amplitude ratio can be obtained by simulatingcrack growth in a fracture specimen coupled to atest structure or field structure, and measuringacoustic emission signal in the structure by the falseaperture transducer. The calibration curvecorrelates simulated crack depth percentage withcomputed peak amplitude ratio of the measuredsignal. Using the calibration curve and acousticemission signal sensed by a false aperturetransducer in a field structure, a crack in thestructure can be detected and its depth measuredby computing the peak amplitude ratio of the signaland identifying the crack depth that correlates withthe ratio from the calibration curve.

Inventors: Dunegan; Harold L. (USA)

Assignee: Dunegan Engineering Consultants, Inc.(USA)

UNITED STATES PATENT5,526,689June 18, 1996Acoustic emission for detection ofcorrosion under insulation

Abstract

A method and apparatus for detecting the presenceof surface corrosion under insulation on a pipingstructure employs artificially generated, broadbandacoustic sound waves to interrogate the pipingstructure. The sound waves are coupled into thepiping structure and detected after they havepropagated through and interacted with a portionof the piping structure. The amplitude of RMS voltagesignals indicative of the detected sound waves isused to determine whether or not surface corrosionis present. Highly corroded pipes have been shownto yield relatively low RMS voltage signals whereasthe lack of corrosion yields relatively high RMSvoltage signals.

Inventors: Coulter; John E. (USA), Robertson;Michael O. (USA), Stevens; Donald M. (USA)

Assignee: The Babcock & Wilcox Company (USA)

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We hope you enjoyed solving the “NDTCrossword Puzzle” which was published inthe last issue. We received many entriesfrom the readers and based on themaximum number of correct wordsidentified, the following are the WINNERS

- Paresh Vaidya, Ex-BARC, Mumbai

- P Selvaraj, ISRO-SHAR, Sriharikota

Congratulations to all the Winners.They will receive their prizes from the ChiefEditor of the journal shortly. The correctanswers to the Puzzle are published below.In this issue, we have another Crosswordpuzzle to continue stimulating your braincells! We hope you will find this sectioninteresting, educative and fun filled. Pleasesend your feedback, comments andsuggestions on this section [email protected] “Crossword Puzzle”, contains morethan thirty (30) words related to EddyCurrent NDE. These include techniques,terminologies, phenomenon, famouspeople, etc. These words are hidden inthe puzzle and may be present horizontally,vertically, diagonally in a forward or reversemanner but always in a straight line.Instructions- All you have to do is identify these

words and mark them on the puzzlewith a black pen

- Preferably you may take a photocopyof the Puzzle sheet and mark youranswers on that (see the markedexample)

- Once completed please scan youranswered puzzle sheet as a PDF file andemail the scanned sheet [email protected] with your name,organization, contact number and emailaddress

Rules & Regulations- Only one submission per person is

allowed- The marked answers should be legible

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The correct answers and the names ofthe prize winners will be published in thenext issue

NAME : ___________________________

ORGANIZATION : __________________________

PHONE : ___________________________

EMAIL ID : ___________________________

ndt puzzlendt puzzleConceptualized & Created by

Dr. M.T. Shyamsunder,GE Global Research, Bangalore

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41Technical Paper

Vol. 11, Issue 1 June 2012 Journal of Non destructive Testing & Evaluation

Ultrasonic Evaluation of Glass-Epoxy Compositeswith 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,India1School of Materials Science and Technology, Indian Institute of Technology, BHU, Varanasi

2Professor, Department of Aerospace Engineering, Indian Institute of Science, Bangalore3 Visiting Scientist, Department of Aerospace Engineering, Indian Institute of Science, Bangalore

*Email: [email protected]

ABSTRACTE-glass-epoxy composites prepared by RT-vacuum bag moulding technique with varied void fractions (5-8%) were investigated using theC-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 qualityranking among these laminates. Results showed good correlation between the LQI and void fractions. Further, A-Scan results were alsoobtained 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)

during service conditions [9-13]. In most of cases theprocessing parameter is not explicitly mentioned. In somecases defects have been artificially induced and theirinfluence on the mechanical properties is studied [14-15].However no studies are reported on quality assessment ofglass-epoxy composites processed by varying vacuum bagmoulding conditions to create exclusively different voidcontents, using both US-C-scan and US-A-scan techniquesat the same time. Further, no report is available indicatinga quantified Laminate Quality Index for such laminatesusing the NDT approach. Such studies become even morecritical as the composite content of an aircraft is increasedin an effort to develop lighter and stronger air vehicles. Inthe present studies, the glass-epoxy laminates intentionallyprepared with different void contents achieved through avariability introduced in the vacuum-time cycles of themoulding process were US-scan evaluated and a relativequality ranking was brought out both through the C-scanand A-scan experiments.

2. ULTRASONIC TESTING (UT)

Ultrasonic scanning is one of the most efficient Non-destructive evaluation techniques for quality inspection offibre reinforced composites. The frequency of the ultrasonicwave is generally between 0.1MHz to 15 MHz. Theoperating system is connected to an ultrasonic transducerwhich scans the test specimen. The transducer is typicallyseparated from the test specimen by a couplant such as oilor by water, as in immersion testing. US-scan experimentsare carried out by either in a pulse echo mode or in athrough transmission mode. In the pulse echo mode, (asused in these studies) the transducer performs both thetransmission and reception of the pulsed waves as theultrasound is reflected back to the device. Reflected

1. INTRODUCTION

Of various continuous fibre based structural composites,the glass fibre reinforced (GFRP) and the carbon fibrereinforced (CFRP) epoxy matrix composites are the mostwidely used materials in the aerospace and non-aerospaceindustries. Further, GFRP composites are preferred whenit comes to trading off between the functional requirementsand product economy. Again Of various fabricationtechniques, the vacuum bag moulding offers a verybalanced approach of manufacturing large and complexshaped composite products of a well-definable quality andreliability in performance. The all composite two-seatertrainer aircraft “HANSA”, developed and demonstrated tohigh certification standards in 1998 by the NationalAerospace Laboratories(NAL-CSIR) was a cleardemonstration of the GFRP RT-vacuum bag mouldedcomposite technology for high end applications.[1]However, composites being heterogeneous and layeredmaterials are prone to either inherent manufacturing defectslike voids and dry spots in the cured laminate ordelaminations or debonds caused due to foreign objectimpacts (tool drops, projectiles) during the service life ofthe composite component. Further, the small and scatteredvoids introduced during manufacturing stage can lead tounpredictable failures in components due to degradationin performance when subjected to service loads and hostileenvironments. Therefore characterisation and evaluationof such voids assumes considerable importance prior toqualifying the structural composites for practical use.Several investigators have reported extensive studies onCFRP composites invariably autoclave moulded usingdifferent NDT techniques including the US-scan technique[2-8]. Many reports on GFRP composites typically addressNDT characterisation of defects like delaminations caused

42 Technical Paper

Journal of Non destructive Testing & Evaluation Vol. 11, Issue 1 June 2012

ultrasound comes from an interface, such as the back wallof the object or from an imperfection within the object. Aschematic of the pulse echo technique is presented in Fig. 1

In the C-scan mode, the response of the test material to theultrasonic wave is in the form of images with differentcolour indicating the uniformity of quality (or variations).Inthe A-scan mode, the response of the material is in awaveform connecting the amplitude of the reflected waveand the time of flight (of the ultrasonic wave) between thefront wall and back wall or the discontinuity. Thus whilethe C-scan provides a clue for the material quality over anarea, the A-scan provides the quality across the depth.Together, the C-scan and the A-scan provide acomprehensive picture of the composite laminate quality.

3. EXPERIMENTAL DETAILS

The overall details of the experimental methodologyadopted in these studies is presented in a nutshell in Fig.2

3.1 Composite Laminate Fabrication

The test laminates studied contained Epoxy resin system(LY5052-HY5052 resin system with the gel time of 100-105 minutes supplied by M/s Atul industries, Mumbai,India) and Glass fabric (2x2 twill woven E-glass fabric of260 gsm supplied by M/s Arun Fabrics Bangalore, India).The 4 test laminates used in these studies were fabricatedout of 12 layers of E-Glass Epoxy composite system usingRT-vacuum bag moulding technique (schematic diagramFig. 3).

The vacuum-time cycles were varied, to obtain a variabilityof 5-8% in the laminate void volume fraction, withoutappreciably affecting the thickness and hence the fibrefraction (65± 3%). The laminates were subjected to pressuretwice during curing. The pressure of around 300mmHgwas applied around 47 mins (half the gel time) for differentduration of times (3mins, 7.5mins, 12 mins) to bleed acertain amount of resin. Pressure was applied again ataround 100mins till 180mins(curing time) which wasinversely proportional in magnitude(660mm Hg, 320mmHg) to the duration of application in an attempt to renderthe thickness of the laminates practically insignificant.Further, the vacuum-time cycles employed to affect processvariability and obtain the test laminates with different voidcontents are schematically presented in figure 4a-4d, aspresented below.

Fig. 1 : Pulse echo mode with submerged specimen

Fig. 2 : Experimental Plan in a Nutshell

43Technical Paper


The laminates L1 and L3 were fabricated by edge bleedingtechnique, which is a very common technique used forprepreg in the autoclave moulding technique, where theresin bleeds only from the side of the laminates (similar tothe autoclave moulding technique) sides of the laminates.

The laminate L2 was fabricated using the same cure-timeas used for laminate L1. This laminate however was allowedto bleed from the top and bottom also (surface bleeding)using a bi-directional glass fibre cloth of the same size asbleeder, as it absorbs the resin to a great extent. Thelaminate L4 was prepared debulked for every 3 layers withfull vacuum pressure (as done in autoclave process) inorder to take it as a reference laminate. These laminatescoded L1(EB), L2(SB), L3(EB), L4(DB), were studied inthe US-C-scan and A-scan modes. The abbreviations EB,DB and SB stand for Edge Bleeding, Debulking andSurface Bleeding.

3.2 US-scan tests

The ultrasonic immersion scanning of the specimen wasdone using pulse echo method with the specimen immersedin normal water. A 5 MHz probe of 12mm diameter wasused. The ultrasonic system used was ULTRAN NDC

1. Vacuum Bag, 2. Release Film, 3. Bleeder, 4.Perforated Film,5. Composite layup, 6. Seal, 7. Base Plate and 8. Release.

Fig. 3 : Schematic of Vacuum Bag moulding

Fig. 4 : (a) Vacuum-time Cycle for laminate L1 (b) Curing Cycle of L2 (c) Curing Cycle of L3 (d) Curing Cycle of L4

(a) (c)

(b) (d)

Fig. 5 : Ultrasonic Experimental Setup

44 Technical Paper


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-ScanImage of laminate L4 (DB)

(a) (c)

(b) (d)

Each colour in the C-scan images (as obtained in Fig. 6)represents the relative attenuation of the ultrasound wavein the material e.g. black represents least attenuationwhereas white represents maximum attenuation. Thereforethe areas of the black region of all the 4 laminates were

7000(automated immersion system) in association withULTRASOFT software for data acquisition, control andimaging. The scanning speed was maintained at 10 mm/sand the resolution was 0.5mm. The laminates were C-scanned over an area of 60x60 mm2. In addition, thespecimens were placed on a tripod, which was used as areflective plane in order to distinguish the back wall echofrom any other one. The test set up is shown in figure- 5below.

As regards the A-scan measurement the specimen wasimmersed in the tank and 16 readings were taken eachfrom a grid of 20mm by 20mm marked on the specimen.Yokogawa oscilloscope (DLM 2022) of 2000MHzfrequency 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. Fig. 7 : Areas of black region of Glass-Epoxy Laminates

45Technical Paper


calculated using a program made in MATLAB bycalculating the number of pixels of the region representedby the black colour and the total scanned area of the sample(3600 mm2) and then calculating the ratio of these twoareas. Figure 7 shows the column diagram representingthe areas of black region for the laminates studied

4.2 LAMINATE QUALITY INDEX (LQI) - A NEWPARAMETER

It is defined as the ratio of the area of black region of theC-scan to the total scanned area of the laminate.

Area (black of laminate havingleast attenuation

LQ1 = ——————————————Total area of the laminate scanned

= Ab/ΔA (1)

Based on the above criteria, the quality table of the glass-epoxy laminates is generated in terms of the LQI, aspresented in Table 1

Table 1 : Quality Table of Glass-Epoxy laminates by C-Scan

Specimen Area of laminate LQIin black(mm2)-ΔAb (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

As can be seen from the above table, and based on the USC-scan results and analysis, the laminate coded L1B withthe LQI of 0.99 is the healthiest of all the laminates studied,and the quality ranking of the laminates follows the ordergiven below:

L2 > L3 > L4 > L1(SB) (EB) (DB) (EB)

4.3 A-Scan Studies

For the A-scan studies, all the 4 test laminates were markedwith a 16 square grid, each grid measuring 20mm by20mm.The A-scans of such laminates were then taken byimmersing them in the tank containing water as the liquidcouplant and using a 5MHz probe. Accordingly, a total of4x16 A-scans were generated. In materials with highattenuation (e.g. GFRP) where it is difficult to locate peakamplitude corresponding to the back wall reflection withcertainty, the voltage gain was increased and the time offlights (TOF) for each of the 4 laminates were estimatedusing the following formula:

2st = —— (2)

vWhere t is the time of flight (TOF) in seconds, s is the

thickness of the laminate and v is velocity of ultrasound inthe laminate (3150 m/s for GFRP). The factor 2 in theequation comes because in the pulse echo method the wavetravels twice in the medium. Thereafter, a rational statisticalaveraging approach was evolved, by considering the closestback wall amplitude values (disregarding the very highand very low values) in each case, and the resultingaveraged values are presented in table- 2 along with thecalculated-TOF(by equation 2) and those (by experiment)corresponding to the nearest of the statistically averagedamplitude.

Table 2 : Quality Table of Glass-Epoxy laminates by A-scan

Laminate code Averaged TOF TOFamplitude calculated experimental

(mV) (ns) (ns)

L2 656 1422 1385

L3 360 1440 1435

L4 351 1244 1295

L1 266 1460 1495

In this case, the criteria is that, higher the back wallamplitude greater is the energy reflected back and lesserare the discontinuities present in the sample and hencebetter is the quality of the composite laminate. Thus fromthe values of the back wall amplitudes of the laminates(Table-2) the A-Scan quality ranking of the Glass-Epoxylaminates stands as follows:

L2 > L3 > L4 > L1(SB) (EB) (DB) (EB)

Typical A-scans for the 4 laminates are presented in figures8a-8d.

5. QUALITY RANKING BY C-SCAN ANDA-SCAN- COMPARATIVE ASSESSMENT

Table–3 presented below summarizes the quality rankingof the 4 Glass-Epoxy laminates investigated with differentvoid volume fractions (created with varied process histories)as derived from the Ultrasonic C-Scan and A-scanexperimental results.

Table 3 : Comparison of laminate quality by US C-scan andA-scan techniques

Specimen Void volume LQI Back wallcode fraction (%) (by C-scan) 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 thesame relative quality ranking for the Glass-Epoxy laminatesinvestigated.

46 Technical Paper


6. CONCLUSION

Glass-Epoxy laminates with different void volume fractions(5-8%) were prepared by RT-vacuum bag moulding processwith varied vacuum-time cycles, the thickness and the fibrefractions of all laminates remaining unaffected. Overall,the quality ranking by Ultrasonic NDT technique (both byC-Scan and A-scan modes) for the laminates studied iscorelatable with the void volume fraction of the laminates,results clearly confirming that laminates with lower voidcontent (5%) have better quality than those with highervoid content (8%).Finally, a good parity was seen betweenthe results obtained from C-scan and A-scan results, bothgiving the same quality ranking for the glass-epoxylaminates studied.

ACKNOWLEGEMENT

The authors acknowledge the useful discussions with Mr.Vijay Kumar, Research Scholar, Ms. Nida Ali, ProjectAssistant and the timely support of Mr. Ranganatha, SeniorTechnical Officer in laminate preparation, at the Departmentof Aerospace Engineering, Indian Institute of Science,Bangalore.

REFERENCES

1. Raja Manuri Venkata Gopala Krishna Rao, “Engineering ofScience-The Composite Way” Annals of Indian National Academyof Engineering, Indian National Academy of Engineering,Volume-4 April 2007.

2. Brian Stephen Wong, Chua Fong Ming Ron, Ow Wing Yoongand Tui Chen Guan, “Non-Destructive Testing Of FiberReinforced Composites and Honeycomb Structures”, Proceedingsof Defense Materials and Mechanics Seminar, Singapore. 1999.

3. Ramanan Sridaran Venkat, Boller Christian, Andrey Bulavinovand Sergey Pudovikov, “Quantitative Non-destructive Evaluationof CFRP Components by Sampling Phased Array”, AeroNDT2010- Emerging Technologies, March 2011.

4. Seung-Joon Lee, Young-Joon Ha, Joon-Hyun Lee and Joon-Hyung Byun, “Expeimental Evaluation of Delaminations in CFRPusing Laser-Based Ultrasound”, The 1st Tohoku-PNU JointWorkshop on Mechanical Science based on Nanotechnology,Busan, Jan 8, 2007

5. M. A. Perez, L. Gil and S. Oller, “Non-destructive testingevaluation of low velocity impact damage in carbon fiber-reinforced laminated composites”, CompNDT 2011 –UltrasonicTechniques for Composite Material, May 2011

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

(a)

(b)

(c)

(d)

47Technical Paper


6. G. Wróbel , Z. Rdzawski, G. Muzia and S. Pawlak, “Quantitativeanalysis of the fibre content distribution in CFRP compositesusing thermal non-destructive testing”, Archives of MaterialsScience and Engineering, Volume 41 Issue 1, Jan 2010, Pages28-36.

7. Johann Kastner, Bernhard Plank, Dietmar Salaberger and JakovSekelja, “Defect and Porosity Determination of Fibre ReinforcedPolymers by X-ray Computed Tomography”, 2nd Internationalsynopsium on NDT in Aerospace, Hamburg, D, Deutschland,2010.

8. K.Koyama, H.Hoshikawa and T.Hirano, “Investigation of ImpactDamage of Carbon Fiber-Reinforced Plastic (CFRP) by EddyCurrent Non-Destructive Testing”, NDT of Canada 2011, Feb,2012.

9. R. Marat-Mendes and M. Freitas, “Non Destructive Evaluationof Delamination in Glassfibre Composites Using C-ScanAnalysis”, 16th International Conference on CompositeStructures, Porto, 2011.

10. G. Wróbel, £. Wierzbicki and S. Pawlak, “A Method for UltrasonicQuality Evaluation of Glass/Polyester Composites”, Archives ofMaterials Science and Engineering, Volume 28 Issue 12 December2007, Pages 729-734.

11. C Scarponi and G Briotti, “Ultrasonic technique for the evaluationof delaminations on CFRP, GFRP, KFRP composite materials”,Composited, Part B 31 (2000) 237-243.

12. Tarapada Roy and Debabrata Chakraborty, “Delamination in FRPlaminates 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 atthe Web/Flange Junction of GFRP I-Beams”, Fourth InternationalConference 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. Srikanthand R.M.V.G.K. Rao, “Studies on Mechanical Behavior of GlassEpoxy Composites with Induced Defects and Correlations withNDT Characterization Parameters”, Journal of Reinforced Plasticsand Composites (2007); 26; 1539.

15. Theodoros Hasiotis, Efstratios Badogiannis and Nicolaos GeorgiosTsouvalis, “Application of Ultrasonic C-Scan Techniques forTracing Defects in Laminated Composite Materials”, Strojniškivestnik - Journal of Mechanical Engineering 57(2011)3, 192-203.

48 Technical Paper


Low Heat Flux Transient Thermography for DefectDetection in Thick Composite Structures

K Srinivas, T Murugesh and J LahiriDoCMP & NDE, Advanced Systems Laboratory, Kanchanbagh PO, Hyderabad-500058

ABSTRACTThick composite structures (thickness ~ 15 mm) with carbon fiber reinforcement (CFRP) and epoxy resin matrix are being used in variousaerospace applications. Quality of final CFRP composite structures are highly process dependent. Several defects such as airgaps anddelaminations manifest within composite structure during processing. Defects such as delaminations and porosity severely affect theperformance of the composite. Hence, defects in thick CFRP composite have to be reliably detected for final applications. Thermographyis one of the most important techniques for NDT of CFRP composites. It is a non-contact, fast and reliable NDT technique for thincomposite structures (thickness ~ 5 mm). For thick composites established techniques do not show all the defects. In order to detect defectswhich are deeper in thick composite structures improved heating systems with complex electronics along with exhaustive data processingtechniques are used. This paper reports, low heat flux (4 kW) transient thermography technique adopted for defect detection (defect depth7.5mm) in thick CFRP composites (thickness upto 15 mm). CFRP composite laminate with implanted air-gaps at several depths withinthe composite laminate has been specially fabricated for simulating defects for these studies. Presence of defects and their defect depthsare correlated with ultrasonic NDT methods. Pulse phase thermography method has been adopted along with improved experimentalprocedures for defect detection. Phase imaging and correlation technique have been adopted for data processing. A comparison of resultsobtained with flash (9.6 KJ), lock-in (with 4KW halogen lamps) and low heat flux pulse phase transient thermography technique ispresented.

Keywords: Composites, NDT, Thermography

PACS: 81.05.QK, 81.70.-q, 81.63.Hg

and thermal diffusivity of the material under test. Variousactive thermography techniques were proposed and beingimplemented for defect detection in composite structures[5-6]. However, all the techniques were restricted to defectdetection for sub-surface defects only. In case of thickCFRP composite structures (thickness~15 mm) defectdetection is severely affected by high lateral diffusivityand low signal to noise ratio (SNR) [7-8].

This paper reports application of low heat flux transientthermography techniques for thick CFRP (thickness ~ 15mm) composite structures for detecting deeper defectswithout damaging the test surface. A comparison of defectdetection 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 chosenfor giving external heat stimulus to the carbon epoxy (CE)laminate for our selected heating time for the presentstudies. These halogen lamps were connected to the outputof the power supply box which can be manipulated by anexternal signal generator. Halogen lamps were chosen fortheir low heat flux values and their ease of adaptability forshop floor applications. The power supply for the halogenlamps has provision to control both voltage and heatingtime for experiments. Four halogen lamps are arranged insuch a way that uniform heating is achieved across the testsurface within an area of 300 mm x 300mm. Figure 1

INTRODUCTION

Active Infrared (IR) thermography NDT is well knowntechnique for defect detection in carbon fiber reinforced(CFRP) composite structures [1-4]. In this technique,external heat stimulus is given on the test surface togenerate thermal wave inside the material. As the thermalwave propagates into the composite, surface temperatureprofile changes due to the reflected thermal wave, whichis then captured using an Infra red (IR) camera for furtheranalysis. IR images are captured either during heating orduring cooling to capture the defect information carried bythe thermal waves onto the test surface. From the capturedsurface thermal profiles, temperature difference betweendefective area and non-defective area is detected forattributing the difference to the presence of defects beneaththe test surface. Since, CFRP has lower thermal diffusivitythan metals, it is possible to use the technique effectivelyfor defect detection by manipulating the various inputparameters such as heating time, heat flux and capturerate. Conditions for effective defect detection in CFRPmaterials being, use of sufficiently high heat flux to generatethermal wave which reaches the defect depth with higheramplitude to show the temperature difference in thereflected thermal wave. During heating, use of higher heatflux is restricted for composite structures as higher heatflux may result in high temperature over the test surfacewhich may be beyond the operating temperature of thematerial. Other condition is that during transientthermography data has to be captured sufficiently long toget the defect signature, this may depend on defect depth

49Technical Paper


shows line diagram of the experimental setup used for thepresent studies. IR power controller card from M/sAutomation Technology, GmBH has been used forcontrolling power supply to the halogen lamps. Signalgenerator, Image acquisition board and IR camera aresynchronized to get correct time signatures for IR imagesbeing captured. However, halogen lamps have requiredwarm up time of at least 1 s for reaching to their highestefficiency, which has been taken careof while applyingheat stimulus for the test object.

Carbon epoxy 0-90 cross ply test laminate (lam1) having15 mm thickness with embedded air-gaps has been chosenfor our studies. Laminate defect configuration, defect sizesand their depths from the test surface are shown in thefigure 2(a). Plastic mesh of size 10 mm x 10mm and 20x 20 mm (thickness ~ 0.5mm) were used for creatingdelaminations (plastic mesh has been sealed with hightemperature plastic sheet to prevent resin flow into the airgaps). Figure 2(b) shows ultrasonic through transmissionC-scan image of the test laminate captured using non-contact ultrasonic probes (200 kHz). C-scan image confirmsthe presence of defects at pre designated locations. Defectdepth from the test surface for each of the defects iscalculated using ultrasonic pulse echo technique (frequencyof 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 distorteddue to the use of larger diameter probes (25 mm) for bothtransmission as well as reception as compared to defectsize. The defect of 10 mm x 10 mm size at 11 mm depthhas appeared to be smaller than its actual size; this may bedue to puncturing of high temperature plastic film over thedefect allowing resin flow into the air gap, thus reducingthe size of the defect. For the present work defects uptothe depth of 8.3 mm were only considered and defects atdeeper depth (11 mm) were not covered under the currentinvestigation.

RESULTS AND DISCUSSION

LOW HEAT FLUX TRANSIENT THERMOGRAPHY

Low heat flux transient thermography experiment onCarbon epocy (CE) laminate has been performed using 4kW heat flux (4 halogen lamps each with 1 kW heat flux).Heating is done for 40 s and data is captured for 200 sfrom the beginning of heating. IR images of the testlaminates were recorded with 5 Hz capture rate, so thatdefect information is completely captured. Figure 3 (a)shows raw image of the test laminate at 50 s from thebeginning of heating. Raw image shows appearance of

Fig. 1 : Line diagram of the experimental setup

Fig. 2 : (a) shows line diagram of the carbon epoxy test laminate (CE) with implanted defects (b) Ultrasonic through transmission C-scanimage of the test laminate. Defects present in the laminate are clearly seen with higher attenuation in the through transmissionsignal.

50 Technical Paper


defects at 1.5 mm depth clearly, however defects at 4.1mm are faintly visible. Other deeper defects are not visiblein unprocessed raw image due to non-uniform heatingpattern as well as higher temporal noise present in thedata. Figure 3(b) shows evolution of pixel amplitude overdefect and non-defect regions as a function of time. Pixelamplitude shows that defect has higher values as comparedto non-defect indicating that defect to be hot (highertemperature) when compared to non-defect. The capturedIR images are processed using various data processingtechniques [10-12]. Figure 4(a) shows resultant image ofIR image at 60 s subtracted with last image (at the end ofcapture i.e 200s) and normalized with end of heating.Normalized image shows presence of defects at 1.5 mmdepth (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 (20x 20 mm size) clearly. However, deeper defect at 8.3 mm(10 x 10 mm) is faintly visible. All the defects at 4.1 mmdepth and 7.5 mm depth are distorted and their boundariesare marked with noise. Table 1 shows SNR of each of thedefects at different depths and their defect appearance timeduring transient thermography. Defects of 20 x 20 mmsize at (different depth) appear have higher SNR comparedto 10 x 10 mm defects at same depth. In comparison,higher SNR is observed for defect at 7.5 mm depth withlow heat flux of 4 kW. However, defect at 8.3 mm depthshows low SNR and appear to be distorted due to (i)non-uniformity of heating (ii) defect size (10 x 10 mm) and(iii) higher surface temporal noise.

Table 1 : SNR of defects and defect appearance times afterdata processing with normalization and subtraction

Frame no(appearance time ) Defect depth-size SNR (4KW)

6 (1.2 s)1.5mm-20X20mm 24.0

1.5mm-10X10mm 16.6

116 (23.2 s)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

For improved defect detection, phase difference betweendefect and non-defect regions was taken, which providedreliable defect information beneath the surface of the testlaminate. In order to obtain the phase information for eachpixel of the 240 x 320 pixel thermal image, FFT is appliedover entire temporal thermal profile of each pixel. Theapplied FFT algorithm over the temporal thermal historyof the pixel separates the phase detail in the frequencydomain and provides phase information corresponding tothe constituent frequency component [9]. A lab-view (NI,USA) program was written for separating phase andmagnitude information for the each of the pixels of thethermal image. The application of one-dimensional FFTon 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 defectshown 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 atthe 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)

51Technical Paper


where k is the sample number (harmonic number) in theobtained FFT of the response profile (y) containing Nsamples (equivalent to number of frames captured by IRcamera). The component phase corresponding to differentfrequencies can be obtained by computing

ϕ(k) = tan–1 (Imk/Rek) (2)

The frequency of the corresponding component iscalculated from the relation

(3)

where fk is the frequency of the kth component in theFourier domain, fs is the sampling frequency (frame rateof IR camera) and N is the number of samples (total numberof frames). The phase difference of all the pixels at aparticular frequency constitutes the phase image at thatfrequency [9].

Figure 5(a) below shows phase image (1st harmonic imageat 0.006 Hz) after data processing of the above mentionedexperimental data. Due to higher frequency resolution ofthe data, all defects (upto 8.3 mm depth) appear with highphase difference when compared to background. Defectsat 1.5 mm depth have highest phase difference whencompared with background. However, deeper defect at 8.3mm (size 10 x 10 mm) shows low phase difference andappears with low contrast in the phase image due to itssmall size relative to the depth. Table 2 shows SNR ofdefects present in the test laminate and their relative depths.Highest SNR is observed for shallow defects at 1.5 mmdepth. Defect at 8.3 mm depth shows SNR of 10 which iscomparable with other data processing techniques such asnormalization 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 experimentmentioned above has been processed using correlationfunction. A lab view program for generating correlationimage has been written for processing each pixel data with

reference pixel selected from non-defect region. Theresultant correlation image is shown in figure 5(b) above.Correlation image shows defects with improved contrastupto a depth of 7.5 mm. However, defect at 8.3 mm appearswith poor contrast. Table 2 above shows SNR of defectsand their depths from the test surface calculated fromcorrelated image. Defects upto 7.5 mm depth appear withhigher SNR when compared with phase imaging. However,8.3 mm deep defect appears with lower SNR whencompared with phase image, this may be due to selectionof reference pixel closer to the centre of heating on thetest laminate. SNR of deeper defect at 8.3 mm may belower due to the larger observation time andcorrespondingly large lateral diffusivity (comparable tosignal itself).

Table 2 : SNR of defects calculated from phase image andcorrelation image

Defect SNR (4KW) SNR (4 KW)depth-size From phase image From correlation

(1st harmonic methodat 0.006 Hz)

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, experimentswere performed at frequencies whose diffusion length(function of thermal diffusivity and frequency) equals tothe depth of defect to be investigated. Since for the presentlaminate thermal diffusivity along thickness direction anddefect depths are known, frequency of 0.01 Hz and 0.07are selected for investigating defects at 4.1 mm depth and1.5 mm depth respectively. In this technique, IR imageswere 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

52 Technical Paper


while heating the laminate during 3 cycles of thermalexcitation. Red line shows defect pixel amplitude evolutionand blue line indicates non-defect pixel amplitude as afunction of time. The pixel evolutions were observed to befollowing the halogen lamp heating pattern. It is observedthat during cooling lamp filament does not completely cooland radiation which seems to have distorted the sinusoidalexcitation (see arrow mark in the figure 6 below).

Figure 8(a) shows phase image of CE laminate (after dataprocessing) obtained from lock-in thermography experimentat 0.01 Hz excitation frequency and 2 kW heat flux. Defectsupto a depth of 7.5 mm were clearly visible after processing.Defects at 1.5 mm depth and 4.1 mm depth are with highercontrast when compared to 7.5 mm depth defect.Experiments were repeated with higher heat flux value of4 kW to improve SNR of the defects. Figure 8 (b) showsphase image of CE laminate obtained from lock-inthermography experiment at 0.01Hz excitation frequencyand 4 kW heat flux. Defects at 1.5 mm depth and 4.1 mmdepth are clearly visible but deeper defect at 7.5 mm isbarely visible in the 1st harmonic phase image. Deeperdefect at 7.5 mm depth has appeared after data processingwhich was not expected as the selected excitation frequencyof 0.01 Hz is expected to have diffusion length of 3.7 mmonly, however the appearance of deeper defect may be dueto presence of low frequency components in the excitationfrequency while using halogen lamp heating. This is dueto inherent delay in heating as compared to the excitationof power supply. Further investigation is under progressfor exploiting the technique.

been terminated at the end of heating cycle without waitingfor the test surface to reach equilibrium temperature. Figure6 and 7 shows pixel amplitude evolution as a function oftime for truncated lock-in thermography experiment at 0.01Hz and 0.07Hz excitation frequency with 2 kW heat fluxexcitation energy. Thermal images were captured at 5 Hz

Fig. 6 : Shows evolution of pixel amplitude as a function of timeduring truncated lock-in thermography experiment(f=0.01Hz, 3 cycles and 2kW heat flux).

Fig. 7 : Shows evolution of pixel amplitude as a function of time during truncated lock-in thermography experiment (f=0.07Hz, 3 cyclesand 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-inthermography (0.01) phase image of CE laminate with 4 kW halogen lamp heating.

53Technical Paper


Figure 9(a) shows phase image of CE laminate obtainedfrom truncated lock-in thermography experiment at 0.07Hz and 2 kW heat flux. Defects at 1.5 mm depth areclearly visible with 2 kW heating after data processing.Figure 9(b) shows phase image of CE laminate after heatingwith 4 kW. Defects upto 1.5 mm depth are visible, howeverdeeper defects were not visible even with 4 kW heat flux.Table 3 shows SNR of 1.5 mm depth defect and 4.1 mmdepth defect captured calculated from truncated lock-inthermography experiment performed at 0.01 Hz and 0.07Hz excitation frequency. No increase in SNR of defects at1.5 mm depth was observed with increase in heat fluxfrom 2kW to 4 kW. SNR of defect at 4.1 mm depth isobserved to be lower than transient thermographytechnique. Lower frequency truncated lock-in thermographymay be useful for detecting deeper defects upto 8.3 mm,since the experimental time required for lower frequencyis long we have not explored this for the present paper. Incase of shallow defects at 1.5 mm depth, truncated lock-in thermography after data processing is observed to showdefects with best defect features such as shape and size.Truncated lock-in technique is observed to be in-sensitiveto non-uniform heating.

Table 3 : SNR of defects as a function of heat flux of halogenlamps in Lock-in thermography experiment

Lock-in Defect SNR SNRfrequency depth-size (4KW) (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

0.01Hz 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

FLASH THERMOGRAPHY

For comparison flash pulse thermography has beenperformed using 9.6 kJ heat flux flash lamps for 5 ms,data has been captured upto 200 s at 5 Hz capturefrequency. Figure 10(a) shows raw thermal image of testlaminate with flash pulse thermography at the end of 15 sof capture. Defects upto 1.5 mm depth are clearly visiblewithout data processing. However, defects which are deeperare not visible. Raw data of each pixel has been furtherprocessed with FFT for obtaining phase information. Figure10 (b) shows phase image at 0.053 Hz after data processing.Defects at 1.5 mm depth are clearly visible; however defectat 4.1 mm is faintly visible. SNR for each of the defect hasbeen calculated and shown in table 4 below. Shallow defectsat 1.5 mm depth appear with higher SNR when comparedwith low heat flux truncated lock-in thermography due touse of higher heat flux for excitation. However, theexcitation energy dissipates much faster than both transientmethod 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

Table 4 : SNR of defects as a function of depth in flash pulsethermography

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 selectionof heating time and data capture rate is observed to beuseful for detecting deeper defects upto 8.3 mm (size 10x 10 mm) in thick carbon epoxy laminates.SNR for defectsobserved from low heat flux transient thermographytechnique appear to be higher than other techniques withcomparative heat flux values. Truncated lock-inthermography method is observed to be useful technique

Fig. 9 : (a) shows Lock-in thermography (0.07 Hz) phase image of CE laminate with 2 kW halogen lamp heating (b) shows Lock-inthermography (0.07 Hz) phase image of CE laminate with 4 kW halogen lamp heating.

54 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 annualconference, Kolkotta (2005)

8. K.Srinivas,A.O. Siddiqui and J.Lahiri, Proc. ISNT NDE annualconference, 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 EInternational, 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).

for defect detection upto 7.5 mm depth with 4 kW heatflux heating systems. High energy flash thermography incomparison is observed to be useful for detecting sub-surface 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 fornon-destructive testing, John Wiley & Sons, New York, USA(2001).

55Technical Paper


Detection of fine defects in steam generator tubes of220 MWe Indian PHWRs using eddy current array

probes

H.M. Bapat, Manojit Bandyopadhyay, R.K. Puri and Manjit SinghDivision of Remote Handling and Robotics,

Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, INDIA

ABSTRACTThe inspection of components used in nuclear industry is a critical issue for the safety of reactor. One of the main component in nuclearreactor is steam generator. Steam generator tubes need to be inspected regularly during In-service inspections. At present standard eddycurrent 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 andcircumferentially oriented defects. These very fine defects are detected using hydrogen/helium leak test and based on results of leak testtubes are plugged or removed. So it is necessary to detect very fine cracks and circumferentially oriented defects using eddy currenttechnique. This paper describes an arrayed multi-coil probe technique, newly developed for the testing of steam generator tubes by eddycurrent 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 isa critical issue for the safety of reactor. One of the importantcomponent in nuclear reactor is steam generator. A varietyof degradation modes can affect the integrity of steamgenerator tube bundles, resulting in expensive repairs, tubeplugging or replacement of tube bundles. One keycomponent for ensuring tube integrity is inspection andmonitoring for detection and characterization of thedegradation. In-service inspection of steam generator tubebundles is usually carried out using eddy current (EC)bobbin coils, which are adequate for the detection ofvolumetric degradations. They are quite reliable and providerepeatable results, being able to reliably detect and sizevolumetric flaws such as fretting wear and pitting corrosion.However, they are ineffective in detecting circumferentiallyoriented cracks because the induced current in the tubewall circulates parallel to the coil windings and is inherentlyunaffected by the presence of such cracks. These probesare 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 generatedby geometrical tube-wall distortions significantly reducedetectability.

Because of this shortcoming with bobbin coil probes,mechanically rotating pancake coil (RPC) probes have beenimplemented world-wide for inspecting tubes that aresuspected to have circumferential cracks. Eddy currentsinduced by these probes have circumferential and axialcomponents that interact with cracks oriented in alldirections.

However scanning by these probes is very time consumingand costly process. Axial scanning speed is about 200times slower that of bobbin probes. These mechanicallyrotating pancake probes are usually spring loaded tominimize lift-off, which makes them prone to failure. Thisis especially evident in situations where the presence ofinternal tube deposits can reduce probe life significantly.To overcome all these problems Transmit/Receive arrayedmulti-coil probes are developed. These Transmit/Receive(T/R) array probes take advantage of the superior propertiesof T/R technology compared to impedance probetechnology. Transmit-receive (T/R) array probes offer highdefect detectability in conjunction with fast and reliableinspection capabilities. They can detect both axial andcircumferentially oriented defects. The data is displayed inC-Scan format which helps in better interpretation ofresults.

EDDY CURRENT ARRAY PROBE

Eddy current array and conventional eddy currenttechnology share the same basic principle. Alternatingcurrent injected into a coil creates a magnetic field. Whenthe coil is placed over a conductive part, opposedalternating currents (eddy currents) are generated. Defectsin the part disturb the path of the eddy currents. Thisdisturbance can be measured by the coil.

Eddy current array probe consists of number of small eddycurrent coils placed side by side in form of an array insame probe assembly. The probe used by us consists of 16elements (16 small surface coils) arranged in two rows. 8elements are mounted on top row and 8 elements are in

56 Technical Paper


tubes is Incoloy 800. Outside diameter of these tubes is 16mm and thickness is 1mm. Since machining is very difficultin the inner surface of tubes, notches were fabricated onouter surface. These notches have 200 micron and 300micron depth. The dimensions of these notches are givenin table 1. The images of these defect standards along witharray 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 elementsis used for picking up EDM notches fabricated on tube ofsteam generator. The results of scanning are shown as 2Dand 3D images in transverse and longitudinal mode.Circumferentially oriented notches are best picked up andshown in transverse 2D & 3D image while axially orientednotches are best picked up and shown in longitudinal 2D& 3D image. Scanning is carried out at 200kHz. All majorcracks, even the narrowest ones (200 ìm), show a verygood signal to noise ratio. Signals obtained from Very fineEDM notches are clearly visible as shown in Figure (darkzone on the CSCAN image). The experimental data showthe good results in terms of detection obtained with thearray probes for small cracks. The probe is able to pickupnotches from outer surface of tubes, where sensitivity is

bottom row. The elements are arranged in such a way thattop and bottom row elements are diagonal to each other.Eddy current instrument INSIS-EX is used for testing theprobe.

The instrument has the ability to electronically drivemultiple eddy current coils placed side by side in the sameprobe assembly. Data acquisition is performed bymultiplexing the eddy current coils in a special pattern toavoid mutual inductance between the individual coils.

To simplify the process of signal analysis and make itmore user-friendly, the data is displayed in C-scan format.This display method is a valuable tool in helping to visualizeflaw morphology and location while reducing the numberof data channels to be analyzed retaining all the originaldata. Eddy current array probe along with reference defectstandard and instrument is shown in Fig. 1.

Defect standards

Axially and circumferentially oriented notches werefabricated using EDM technique on outer surface of steamgenerator tubes of 220MWe reactor. The material of these

Fig. 1 : The complete experimental setup with probe, defectstandard and instrument

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

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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

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the interpretation. Since it is an eddy current technology,the surface preparation is minimal and inspection can bedone without direct contact, or any liquid coupling orpenetrant.

ACKNOWLEDGMENTS

The authors are thankful to M/s Technofour, Pune for helpin fabrication of reference defect standards and eddy currentarray probes.

REFERENCES

1. Obrutsky L.S., Cecco V.S., Sullivan S.P., Humphrey D.,“Transmit-Receive Eddy Current Probes For CircumferentialCracks In Heat Exchanger Tubes”, Materials Evaluation, Vol.54, No 1, pp. 93-98. The American Society for NondestructiveTesting, Inc. (January 1996).

2. Haoyu Huang, Nozomu Sakurai, Toshiyuki Takagi, TetsuyaUchimoto, “Design of an eddy-current array probe for cracksizing 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 CurrentArray Probe For Steam Generator Inspection”, Proceedings ofthe 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 ofthe X-Probe for CANDU Steam Generators”, 20th EPRI SteamGenerator NDE Workshop, Orlando, Florida (July 9-11, 2001).

less. This means probe will have more sensitivity for defectson inner surface of tube. Test results obtained by scanningtube having defect standards with array probe are shownin Figures 3 to 6. Figure 3 shows C-Scan image and stripchart of 200 micron depth Circumferential notch on OD ofSteam Generator tube. From strip chart it is clear thatabsolute channel A07 picked up the signal fromcircumferential notch. Figure 4 shows C-Scan image andXY plot of 300 micron depth axial and Circumferentialnotch on OD of Steam Generator tube. Axial notch ispicked up and shown in Longitudinal 2D and 3D Imageand circumferential notch is picked up and shown inTransverse 2D and 3D image. Figure 5 shows XY plot andstrip chart record of 300 micron depth axial andCircumferential notch on OD of Steam Generator tube.The array probe is also used to scan conventional referencedefect standard having various defects in terms ofpercentage wall thinning ( support ring(baffle plate), 20%,40% ,60%, 80% of wall thickness and through hole). TheCScan display of scanning results are shown in Fig. 6.

CONCLUSIONS

Transmit/receive array probes can detect all degradationmechanisms in a single scan at a scanning speed similar tothat of bobbin probes. Circumferential defects which aredifficult to detect using conventional bobbin coil techniquecan be easily detected by array probes. Simultaneousdetection and discrimination of circumferential and axialcracks can help reduce the need for re-inspection, tubereplacement and forced outages. The C-Scan displays ease

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INTRODUCTION

Radiography is very well established as an NDT technique,using both film and electronic X-Ray detection systems.Mainly used in petroleum, petrochemical, nuclear andpower generation industries, for inspection of welds,radiography has played an important role in the qualityassurance of the piece or component, in conformity withthe requirements of the standards, specifications and codesof manufacturing. Most radiographic exposures and filminterpretations in RT are still carried out manually (1).Human interpretation of weld defects, however, is tedious,subjective and is dependent upon the experience andknowledge of the inspector (2). Human inspectors are notalways consistent and effective evaluators of productsbecause inspection tasks are monotonous and exhausting.It has been reported that human visual inspection is at best80% effective. In addition, achieving human ‘100%-inspection’, where it is necessary to check every productthoroughly, typically requires high level of redundancy,thus increasing the cost and time for inspection (3). Herecomes the importance of automation of evaluation, whichreduces human involvement, thus making the inspectionmore reliable and faster.

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

1 BHEL, Tiruchirapalli-620014,India2 Indian Institute of Technology Madras, Chennai 600036, India

ABSTRACTNon 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 wellestablished for Digital Radiographic inspection of cast and machined components, ADR is still considered a challenge in case of manytypes of weld joints, mainly due to the non-uniformity in the radiographic images of weld joints. This paper introduces an indigenouslydeveloped Automatic Defect Recognition System for Real Time Radioscopy (RTR) of Straight Tube Butt Weld (STBW) joints, which arethe critical joints of tubular components like Economiser, Super heater and Reheater of a Boiler. RTR system for inspection of STB weldsconsists of a constant potential X-Ray equipment with swiveling arrangement, as the X-Ray source, Digital Flat Panel (DFP) as the Imagingdevice with its associated image acquisition and review software, along with the ADR software for defect recognition, classification andthereby evaluation of STB welds. ADR Algorithm, scans through the Digital X-Ray image of the STB Weld joint, and detects the defectspresent 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 enhancethe reliability of defect detection and evaluation. The preprocessing uses concepts from digital image processing, image analysis, andpattern recognition. The development of this system involves validation with a wide range of weld samples with various types ofdiscontinuities. The system has been implemented in one of the RTR stations in BHEL and the ANN training has so far resulted in over95% accuracy level. This system replaces the hitherto used, manual evaluation procedure and removes its inherent limitations likesubjectivity, 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.

REAL TIME RADIOSCOPY (RTR) OF STB WELDS

Tubular products form an important part of SteamGenerators in thermal power plants. These include mainlySuper heater, Reheater, Economizer coils, water wall panelsetc. These components consist of tube assembly of severalmeters length and the required length is achieved by StraightTube Butt (STB) welding process. These tubes are madeof carbon steel, alloy steel etc. It is a pulsed Metal InertGas welding process using spray type of metal transferboth at average current levels and low current levels whichare very much suitable for out of position welding andthin gauge material welding. The major defects which occurin this weld are porosity, gas hole, incomplete penetration,lack of fusion, excess penetration, burnthrough, etc.

STB weld joints are subjected to online monitoring systemfor quality assessment. This system is called Real TimeRadioscopy system (RTR). These systems consist constantpotential duel focal (large focal size 3 mm x 3 mm andsmall focus 0.8x 0.8 mm) X-Ray machine with capacity320kV, 10mA as the X-Ray source and a Digital Flat PanelDetector (FPD) as the Imaging device. X-Rays from thesource penetrate through the weld thickness and thedifferential absorption of radiation gives a two dimensional

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X-Ray image of the weld joints. Incoming X-rays firststrike a Cesium Iodide scintillator of the Digital Flat Panel,which converts the X-Rays into light. The light then passesthrough a photodiode matrix of amorphous silicon, and isconverted into electrical signals, which are amplified anddigitized (4).The light is directed onto silicon without lateraldiffusion, which ensures image sharpness (5).The digitaldata is then processed into images via a correspondinggray value table, and is displayed, printed or sent tocomputer as required. The system offers the additionaladvantages of image post-processing and archiving.Compared to other imaging devices FPD provides highquality digital images, better signal to noise ratio anddynamic range of 12 to 16 bit (6), which provides highsensitivity for radiographic application. The present RTRsystem in BHEL uses an Amorphous Silicon Flat Panel(model: DXR250RT).The images obtained by Flat PanelImage Acquisition and Review computers are in DICONDEformat.

These images are evaluated by experienced, qualified NDEpersonnel; the decision of acceptance/rejection taken asper standards (7) and the feedback is given to the welder.The thickness range usually is 4 mm to 12 mm. TheRadiographic technique used here is Double Wall DoubleImage (8).

AUTOMATIC DEFECT RECOGNITION (ADR)OF STB WELDS

The manual evaluation has certain limitations likesubjectivity, and humane dependency, which affect theproductivity and reliability. Here comes the importance ofautomation in evaluation, which reduces humaninvolvement, thus making the inspection more reliable andfaster. The ADR system scans through the Digital X-RayImage of the STB weld joint, and recognizes the defectsand takes the decision of acceptance/rejection, based onthe Acceptance Standards. ADR technology is alreadyavailable for Castings (9) especially Aluminuium Wheels,Magnesium components (10) and weld joints (11). Thereare also ADR systems available for general NDT methods(12). However, there is no customized package availableas such, for ADR of Straight Tube Butt welds. Thistriggered the development of an ADR system for integrityassessment of STB welds. The new system is differentfrom other ADR systems in the aspect that, it uses differentdetection approaches for different class of defects whereas many of the other ADR systems do defect classificationonly after defect detection.

ADR ALGORITHM (13-19)

The first stage of operations performed by ADR algorithmis preprocessing of the input image. This stage involvesenhancement of the image properties to such a level that,pattern recognition can be executed without errors. In caseof the STB weld image, the Region of Interest (ROI)includes the elliptical weld region and adjacent raw materialregion of joining tubes. The next step is the extraction ofthe ROI; the boundaries of ROI are found by analyzingthe vertical summation of the extracted tube for regions ofheightened activity.

The next stage is feature extraction. Since each defectmanifests in a different manner, different feature vectorshave to be used for each defect. These feature vectors aresubsequently presented to a Radial Basis Function NeuralNetwork (RBFNN) for defect classification. In the presentsystem, the defects are classified into three broad categories.

Fig. 1&2 : RTR System and Control Station

Fig. 3 : RTR image of STB welds

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One set covers all the defects like Gas Holes, Porositiesand other rounded indications. The second type includesIncomplete Penetration (ICP) and Lack of Fusion (LF),and the third type includes Burn-through (BT) and ExcessPenetration (EP). This classification is not based on thenature of the defect or its metallurgical behavior. It ischosen based on the gray level variation of the defectregion with respect to the surroundings. Gas Holes are

seen as prominent white dots in the ROI. Detection of Gasholes involves smoothening of the image by medianfiltering and self-subtraction in order to scrutinize thesmaller white regions. However since it may contain verysmall white spots which may be noise, further filteringneeds to be carried out using median filtering with a smallerkernel. The next step is conversion to binary form, followedby extraction of features like aspect ratio, area, length,breadth and roundness. These features are then passedindividually to the RBFNN for classification. If even onegas hole is detected, the output of this block is the RBFNN’soutput for that gas hole.

For ICP the most prominent indication is a break in theroot line. Here Median filtering and image subtraction isdone to extract the weld root. This is followed by smallkernel median filtering and subsequent conversion to binaryimage. The feature vector for RBFNN is the vertical grayvalue sum of the weld root portion. Since the morphologicaldifference between ICP and non-defective images is oftensubtle, and if the quality of the input images is inadequate,in some cases there is ambiguity regarding the classificationof images as ICP or non-defective. In such cases, anexception is raised to the operator and the final decisionis passed to them. This ensures that the risk of falsenegatives is mitigated, as well as weeding out false positivesfrom the final classification. The feature used to detect thethird category is a vector formed by concatenating the

Fig. 4 : A schematic of the proposed ADR system.

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

(a) (b)

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vertical and horizontal summation of the contour of theROI. The RBFNN network is trained on this vector. Thefinal decision regarding the classification is made by thefinal RBFNN, which takes the output of the three previousRBFNNs as inputs.

EXPERIMENTAL RESULTS

Table 1 : ADR trial results

Defect Type No of test No of Per-samples Correct cent-

Classifications 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 imagesamples. These welds include non-defective as well asdefective weld joints covering a wide range of defects ofdifferent size, shape, location and orientations. The resultsof the trial are shown in table 1.0. Defect recognitionalgorithm based on RBF can efficiently overcome theshortcomings of traditional methods, which require largenumber of samples.

CONCLUSIONS AND SUMMARY

Usually the approach for common ADR algorithm is therecognition of the defect, followed by classification basedon the features. However the proposed algorithm appliesdifferent techniques for detection of different classes ofdefects and hence defect detection and classification areparallel processes. The trials carried out give good resultsfor class I (Gas hole and porosity) and class II (ICP andLF). However the performance needs to be improved inthe case of the third category (BT and EP). ANN trainingwith sufficient quantity of images is being carried out toenhance the detection level further. Application of othernetworks like support vector machine (SVM) is also beingstudied so that probability of detection of class III can beenhanced. In case of training samples, the size of the samplewith one class of defect should also take in to account theprobability of occurrence of that class during actual fieldtrial. It is also important to note that RTR offers real timeimage and by rotation of the tube and swiveling, differentimages of the same weld joint can be captured. Somedefects which are not prominent in one image becomeprominent in a different orientation of the weld with respectto the source and detector. Hence a defect detectionapproach based on multi image or direct frame analysis ofa video image can enhance the detection and classificationlevel significantly.

Fig. 7 : ADR GUI during normal operation

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REFERENCES

1. F Fucsok and M Scharmach, ‘Human factors: The NDE reliabilityof routine radiographic film evaluation’, Proceedings of 15thWorld Conference on Non-Destructive Testing, Roma 2000.

2. T Y Lim,MM Ratnam and M A Khalid, “Automatic classificationof weld defect using simulated data and an MLP neuralnetwork”,Insight Vol 49 No.3 ,march 2007,p 154-159

3. Domingo merry, Workshop on Digital Radiography, GE GlobalResearch Centre, Bangalore, June 27, 2005

4. Dr.P.R.Vaidya-Flat Panel Detectors for Industrial Radiography-International 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.,‘Detectivequantum efficiency of CZT semiconductor detectors for digitalradiography’, Instrumentation and Measurement,IEEETransactionsVolume 53, Issue 6, Dec. 2004, P 1479 – 1484.

6. V.R.Ravindran, ‘Digital Radiography Using Flat Panel Detectorfor t e Non-Destructive Evaluation of Space Vehicle Components’,Journal of Non-Destructive Testing & Evaluation , Vol.4, Issue2, September 2005.

7. ASME Section I.

8. ASME Section V.

9. Frank Herold, Rolf-Rainer Grigat, “A New Analysis andClassification Method for Automatic Defect Recognition in X-Ray Images of Castings”, Paper presented at the 8th ECNDT,Barcelona, June 2002

10. Veronique Rebuffel, Subash Sood, “Defect Detection Method inDigital Radiography for Porosity in Magnesium Castings”.

11. G. Bonser and S. W. Lawson, “Defect detection in partiallycomplete SAW and TIG welds using on-line radioscopy andimage 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 todiscrimination of defect type in automatic radiographic testingof welds. ISIJ Int 1999;39(10):1081–7.

15. Daum W, Rose P, Heidt H, Builtjes JH. Automatic recognitionof 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 radiographicimages 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 automaticweld defect identification system for radiographic testing. WeldWorld 1992;30(7/8):182–8.

18. Gayer, A, Saya, A, Shiloh, A. (1990). Automatic recognition ofwelding defects in real-time radiography. NDT International,23(4):131–136.

19. Lawson, S W, Parker, G A, (1994). Intelligent segmentation ofindustrial radiographic images using neural networks. In MachineVision Applications and Systems Integration III, Proc. of SPIE,volume 2347, pages 245–255, November 1994.

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PROBEPossibility thinking follows Perception – Third in the list. Mind over matter! How many times have weheard this? Yogic culture says that there are two parts of our brain, 1) The Right Brain and the other 2) theleft Brain. One is for logic and the other for intuition. Science depends mostly on logic, even though thereare several instances of intuition helping discoveries (inventions)- Fritjof Capra. (The Tao of Physics). ButYOGA believes both in logic and intuition. We generally reiey on logic and all our decisions are dependenton it. The intuition (the gut feeling) is often neglected. Let us not forget that logic is based on our pastexperiences – the data collected by our mind and the experience of others forced on us. But change istaking place continuously. We neglect this aspect. Our decisions shall be based on a judicial mix of bothlogic and intuition. This means that both our left and right brains shall function in unison. How do weachieve 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 beforearriving at any conclusion. Keep your index finger underneath your nose and observe your breathing. Thebreadth will go in out through one of the nostrils. Repeat the experiment after about 40 minutes. Thebreath would have changed its path to the other nostril. To make both right and left brains to work inunison we shall make the breath move in and out through both nostrils all the time. Want to learn how todo it? Continue reading.

Birbal was meditating as usual in the morning, when Akbar paid him an unannounced visit. Birbal’s wifewas 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 inmeditation. Akbar waited and was getting impatient. After considerable amount of time when Birbal cameout at last, Akbar was almost boiling with rage and wanted to know if meditation was more important toBirbal than the emperor himself. Birblal replied that meditation is a process for self realization. Akbarwanted to know the process. But Birbal politely declined telling him that process shall be initiated only bya learned Guru and not by anybody and everybody. Hearing this Akbar got more angry and challengedBirbal that he will learn what Birbal was practicing and adopt it. By threatening the ministers Akbar got toknow what Birbal was practicing and started to follow the procedure. Days passed but Akbar could not feelany tangible benefit. So he sent word for Birbal. Birbal knew all that had happened. When he entered theroom apart from Akbar and himself only the guard was present. As soon as he entered he ordered theguard to arrest the emperor. The guard was transfixed. Hearing this Akbar got angry and ordered the guardto arrest Birbal and the guard did so. Immediately Birbal said “See oh! King the words by themselves donot have any effect, but it acquires the effect when it is uttered by the proper source. That is why I wishedthat 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 benefitfrom yogic Kriyas as they lead you to possibility thinking. Modern day sports psychology is full of it (exampleCarl 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, Isat in front of a mountain and I prayed for days, but the mountain did not move ( Prayers can movemountains). I knew that it will not happen”. The Guru replied “if you knew it why did you waste yourtime?”. 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 bybrick through a positive frame of mind aroused by possibility thinking. With possibility thinking you realizethat 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.Ram.Ram.Ram.Ram.

www.apcndt2013.in

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