code review_ time-of-flight diffraction and pulse echo line scanning

NDT.net - February 2001, Vol. 6 No. 2

Presented in the Seminar on Automated Ultrasonic Inspection of Welds Using Time-of-Flight Diffraction and Pulse Echo LineScanning at Edison Welding Institute in March 2000

E. GinzelMaterials Research Institute

Waterloo (ON) CanadaM. G. Lozev, Ph.D.

Senior EngineerEdison Welding InstituteNDE Technology Team

1250 Arthur E. Adams Dr. Columbus, OH 43221Phone: 614-688-5188, Fax: 614-688-5001

email: [email protected]

Overview

Definitions: what are CodesCodes vs. GuidesSome Rules applicable to UT or in which UT is applicableCodes and Mechanised UTCodes considering TOFDCode Cases, communications and future requirementsSummary

Codes, Standardes and Procedures

The terms "code", "standard", "specification" and "procedure" are often confused in NDT. In somecases these terms are used inter-changeably. However from a technical point of view each refers to aseparate type of document. To avoid even further confusion we will restrict our definitions to thewritten documents implied by these terms. This caution is due to the common referral to calibrationblocks or test pieces with known defects used to verify accuracy of a test procedure as a "standard".

A standard is a written document assembled by recognized experts, with the purpose of recommendingactions to achieve certain objectives. An example of a standard is the American Society ofNondestructive Testing Recommended Practice No. SNT-TC-1A. This is a standard for qualificationand certification of NDT personnel by employers.

A standard is usually enforced or given authority by an organization or agency (typically professionalsocieties or national institutions). When a set of standards is incorporated into law and therebyenforceable legally it is considered a code. Examples of codes are:

American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel CodeCanadian Standards Association (CSA) Z184 Gas Pipeline Systems.

A federal agency may reference a Standard and thereby give it code status; e.g. 49 CFR 192 is the

Code Review: Time-of-Flight Diffraction and Pulse Echo Line Scanning http://www.ndt.net/article/v06n02/eginzel/eginzel.htm

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American pipeline regulatory document and it references API Standard 1104 Welding of Pipelines andRelated Facilities. Another example is the Canadian Atomic Energy Act (an Act in Parliament)referencing CSA Z-285 General Requirements for Pressure Retaining Systems of Components inCandu Nuclear Power Plants.

When a user or purchaser of a specific product requires assurance of quality level they will oftenarrange their own document describing specific test parameters and acceptance criteria. Such a productspecific standard is considered a specification. Although it may reference other codes and standards itcan require more stringent limits than the more general standards or it may avoid any reference to aCode or Standard.

To avoid the ambiguity of the usually general standards, and to avoid the constant updating ofspecifications that refer to national standards which are constantly revised, companies often develop aprocedure.

The procedure can address the company's needs by setting out its standard practices for the variousaspects of the test method, such as; procurement, processing, periodic controls, approved materials andaccessories.

Finally, as a contracted inspection company applies a test to various parts, the variations available toachieve the desired results can be too myriad to list in the body of a procedure document andgeneralisations are again too vague. A common method of addressing the specific inspectionapplication to a specific part is to use a technique sheet.

In summary, there is a hierarchy of documentation in NDT.

Standards and codes, which provide general guidelines and limits1. Specification, which is a product specific document usually assembled by the customer or owner2. Procedures, which are usually a company's statement of their standard practices or a documentdesigned to address the requirements of a specification issued by the company they are providinga service to.

3.

A technique sheet is often part of a procedure and provides a brief description of test applicationto a specific part.

4.

Code versus Guide

A special condition exists when a technical committee decides that a method of performing a functionshould be better explained. In the American Society for Testing and Materials (ASTM) there is aspecial category of a standard called a Guide.

As defined in the ASTM Blue Book, Form and Style for ASTM Standards a Guide is a compendiumof information or series of options that does not recommend a specific course of action. Whenreferencing ASTM many people refer to the publications as Codes however, the title of the volumesclearly indicates that these are Standards as opposed to Codes; i.e. the Annual Book of ASTMStandards vol. 03.03. This applies to other similar documents and organisations; e.g. British StandardsInstitution.

A careful look at the wording in standards indicates the prevalence of should instead of shall. This isespecially true when the Standard is a standard Guide.


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Some Rules applicable to UT or in which UT is applicable

Time-of-Flight-Diffraction is one of the main concerns of this seminar and is quite simply anotherultrasonic inspection technique. Ultrasonic inspections generally can either be required by a Standardor in turn may be regulated in some way by a Standard.

As an example; ASME Section VIII will require an ultrasonic inspection be carried out on a componentand describe the acceptance criteria for the test method. This Section of the Code does not tell you howto perform the inspection. ASME VIII references Section V to describe how inspections are to be setup and carried out.

Therefore we would expect to have to consider two sorts of Standards for TOFD;

One describing how to carry out the test (probes, frequencies, angles, etc.)One describing on what it will be applied and how to evaluate the results of the test.

In practise however, only one of these exists for TOFD and even that is more informative thannominative. Guides on the use of TOFD exist but Standards actually requiring TOFD and providingacceptance criteria are still absent.

Codes and Mechanised UT

Codes and Standards are by their nature, slow to change. The acceptance criteria in many codes wehave to deal with in NDT had their foundation in workmanship standards and these were often basedon the results of what radiography could find. Flaw detection and evaluation by ultrasonic testing wasmore difficult to regulate because its results are heavily dependant on the skills of the manualultrasonic operator. Codes and Standards have been carefully worded to address how to control themanual inspection to ensure good probability of detection and repeatability of results. Asmechanisation of ultrasonic inspection became more common the Codes did not change. Instead, themechanics of inspections were more likely to be adapted to meet the wording of the Code or Standardand the many advantages they could have provided were restricted by the rules for inspection.

In 1979 Maurice Silk and his associates in Harwell introduced the TOFD technique to the world. Thiscame about from the need to more accurately size defects than had been possible using standardamplitude methods. The success of the method as a detection and sizing tool has been well documentedthroughout the literature.

Accurate Crack Depth Measurements in Welded Assemblies,Silk,M.G., Eighth WorldConference on NDT, Cannes, France, 1976

1.

PISC (1979) Evaluation of the PISC Trial Results, volumes I through V of Report No. EUR6371 en, Commision of the European Communities, Brussels

2.

PISC (1986) A Summary of the PISCII Project, Ultrasonic Inspection of Heavy SteelComponents, R.W.Nichols & S.Crutzen, Elsevier Applied Scienc Publishers, London 1988

3.

Depth Measurements in Pressure Tubes Using Crack Tip Diffraction, F.Mastroianni andM.D.C.Moles, Ontario Hydro Research Division Report #86-293-K, 1986

4.

F.H. Dijkstra, T. Bouma, Inspection of complex geometries with Time-of-Flight Diffraction, 14thInternational Conference on NDE in the Nuclear and Pressure Vessel Industries, 24-26 Sept.1996 Conference Proceedings

5.

H. Heckhuser, K.-H. Gischler, Das Zipscan -System bei der Ultraschallprfung an plattierten6.


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Bauteilen und Rohrleitungen, Tagungsband der DGZfP "Automatisierung in derUltraschallprfung , Stand der Technik, Entwicklungstendenzen bei mobilen Prfanlagen"(1989)B. Bttcher, E. Schulz, H. Wstenberg, A new method of crack determination in ultrasonicmaterials testing Proc. 7th Int. Conf. on NDT, Warschau 1973, Reprint No.130

7.

M.G. Silk, B.H. Lidington, Defect Sizing using an Ultrasonic Time Delay Approach, BritishJournal of NDT, March 1975

8.

M.G. Silk, The rapid analysis of TOFD data incorporating the provisions of standards, Proc. 6thEuropean Conf. on NDT, 24.-28. Oct. 1994, Nice France, Vol. 1

9.

T. Just, G. Csapo, Ultrasonic crack depth measurement of surface breaking cracks in piping,UTonline Journal www.ultrasonic.de (also presented at the DGZfP workshop 1994)

10.

Final Draft prENV 583-6, Nondestructive Testing -Ultrasonic Examination- Part 6: Time offlight diffraction, technique as a method for defect detection and sizing

11.

J. Verkooijen, TOFD used to replace radiography, INSIGHT Vol. 37 No 6 June 199512. M.G.Silk, Estimates of the probability of detection of flaws in TOFD data with varying levels ofnoise, INSIGHT Vol. 38 No 1 January 1996

13.

In spite of the acknowledged success of the TOFD method it has been a 30 year struggle to getmeaningful Codes and Standards that allow its use. Still today, it is difficult to apply TOFD and takeadvantage of its benefits of speed in detection as well as its sizing accuracy because it is not describedin most codes referenced by designers.

Codes considering TOFD

In 1989, ten years after its introduction, M. Silk wrote a paper on how TOFD could be applied toASME Section XI (Maurice Silk, "Interpretation of TOFD data in the light of ASME XI and similarrules" British Journal of NDT vol 31, May 1989). At that time ASME Section XI allowed theapplication of fracture mechanics to assist in determining the disposition status of a flaw detected byNDT.

However, in 1989 the applicable Section V, which would have been referenced for any ultrasonicinspection, would not have even considered computerised imaging techniques (CITs) of which TOFDwould be only one subsequently described. CITs were first introduced in the 1992 Addenda (Dec. 311992) of the Article 4 in Section V. This finally allowed TOFD to be used (Non-mandatory Appendix EParagraph E-80). This paragraph acknowledged TOFDs ability to both detect and size volumetric andplanar flaws (sizing being limited to planar flaws).

This acceptance in ASME Section XI may be considered the foot in the door that TOFD needed to beaccepted as a viable ultrasonic tool in a regulatory conscious world. However, as just one of the toolsavailable in Section XI its use was restricted to a tool for sizing and dispositioning flaws found duringin-service inspections of pressure vessels.

Parallel development was being carried out to enlighten the potential hands-on users how to carry outTOFD inspections. This led to the development of the Standard Guides.

Standard TOFD Guides include:

BS 7706 (1993) Guide to Calibration and setting-up of the ultrasonic time-of-flightdiffraction (TOFD) technique for detection , location and sizing of flaws.


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British Standards Institute 1993. CEN prEN 583-6 Time of Flight Diffraction Technique as amethod for defect detection and sizing

Note: Although CEN prEN 583-6 stipulates specific requirements in some aspects of the test, manyaspects are addressed as recommendations as opposed to requirements. For this reason we haveincluded it with BS 7706 as a guide.

Comparison of BS 7706 and EN 583-6

Item BS7706 EN 583-6Scope States that it provides guidance and

suggests that the linear scan (which itcalls a D-scan) is applicable for initialscanning and a B-scan (motion parallelto the plane of the beam axis) is donefor accurate sizing

States that it defines generalprinciples for TOFD

References References are both Normative andInformative and all are BritishStandards

Only Normative standards arereferenced and all are ENstandards

Definitions(severaldifferences)

Several common words are definedsuch as; hardcopy, probe, transducer,flaw height Some special words aredefined: lateral wave, creeping wave,B-scan and D-scan

Several symbols used in diagramsare definedNonparallel scan and S-scan aredefined and equate to BS 7706B-scan and D-scan respectively

Principles of theTechnique(Method in EN583-6)

General description and severalequations to define flaw depths

Very general statement on theprinciples is made and thenrequirements for surfacecondition (as per EN 583-1) andcouplant are made.Cautions against using TOFD oncoarse grained materials

Personnel Requires general familiarity andsuitable training(Guidelines for trainingprovided in Annex E)

Qualification to EN 473 as aminimum plus additional trainingin accordance with a writtenpractice

Equipmentrequirements General guides are suggested as giving

suitable results: e.g. pulser, receiver anddigitiser parameters.

Minimum parameters arestipulated: e.g. receiverbandwidth, pulser rise time,A-scan sampling rate (minimum 1sample per mm)

Probes Guidance suggesting short pulse higherfrequency probes

Stipulates 2 probes, Same centrefrequency within +/-20%, Pulsewidth not to exceed 2 cyclesRecommends:Frequency and refracted angles

Guidingmechanisms

Not covered Require the use of mechanics toensure probe spacing and encoded


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positioning of probes andaccuracy with respect to referenceline must be within 10% of probecentre spacing

EquipmentSet-upProcedures

Extensive discussions on the parameterconsiderations are given: geometry,surface conditions, probeparameters,temporal and spatialresolution, sensitivity, and digitisingrates.

Special note in Section 7 statesthat the arrangements of probesprovided are NOT mandatory.Considers probe parameters,digitiser window, sensitivity, scanresolution, scan speed, andchecking system performance

Interpretationand FlawAnalysis

Reporting or Acceptance criteria to beagreed upon by contracting partiesGeneral description of flaw recognitionis provided with a more detaileddescription and examples in Annex DFive classes of flaw:

Planar1. Volumetric2. Threadlike3. Point4. Unclassified5.

Reporting or Acceptance criteriato be agreed upon by contractingparties Basic information isprovided on use of phase analysisto determine flaw extents Threeclassifications of flaw:

Top Surface breaking1. Bottom Surface breaking2. Imbedded3.

Estimation ofFlawDimensions

A special section Section 8 is devoted toa general principles then a detaileddescription for accurate sizing isprovided in Section 9

Analysis of flaw depth and extentare covered in the same section(Section 8) as the interpretation offindingsEN uses the term Estimation inregards to all sizing and providesequations for depth analysis(similar to those found in BS7706Section 4, Principles to themethod)

Limitations Section 11 covers the many variablesthat would cause inaccuracies in thetechnique

Section 10 discusses the variablecausing errors in the accuracy offlaw sizing and location.Caution is also made to notconfuse precision and resolution

Reportingrequirements

Covered in Annex B Allows TOFD without Datarecording in certain simpleapplicationsReporting to conform to EN583-1 as applicable

ReferenceBlocks

Suggests blocks be the same materialsand thickness of the test piece Suggeststhat narrow V-notches or Side DrilledHoles may be used as diffractor targets

RequiresBlocks be of the same material aspart to be testedBlocks must have a wall thicknessequal to or greater than nominal


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wall of test part Width and lengthof block must be adequate forprobe movement over referencediffractors

Annexes A (informative) SpecialTechniques based on TOFDB (informative) Application ofTOFD and reporting criteriaC (informative) Suggested Stepstowards characterisation of flawechoes in TOFDD (informative) Examples oftypical scansE (informative) Suggestedframework for training andqualification

A (normative) Referenceblocks

Table 1

These two guides are very similar, even following a nearly identical outline format.

European countries have been eager to adapt TOFD to inspections but have been no faster indeveloping Codes or Standards which can be used with TOFD.

Except for the Guides available there are still no other internationally recognised Standards other thanASME that have incorporated TOFD as part of the acceptance criteria evaluation tools.

Codes and Standards applicable to TOFD include:

BS 7706 (1993) Guide to Calibration and setting-up of the ultrasonic time-of-flightdiffraction (TOFD) technique for detection, location and sizing of flaws. British StandardsInstitute 1993.Pr EN 583-6 Time of Flight Diffraction Technique as a method for defect detection andsizing

ASME Boiler & Pressure Vessel Code Section V (as a CIT option in Article 4)ASME Boiler & Pressure Vessel Code Section VIII (in a Code Case in lieu of RT)ASME Boiler & Pressure Vessel Code Section XI (for accurate sizing)

ASME adaptations to TOFD are somewhat circuitous.E.g.

ASME V Article 4 Para T-436 Computerized Imaging Techniques (CITs)The paragraph states that, CITs may also be used to perform the basic scanning functions forflaw detection. Then it refers the reader to Appendix E. E-10 lists general requirements for CITsincluding instrument specifics, procedure requirements and the need for the dimensionalinformation on the displays. E-80 then discusses Automated Data and Imaging Technique as oneof the CITs. This describes the basic features of TOFD without calling it a TOFD technique;including scrolling RF waveforms, and sizing using tip diffracted waves.

ASME VIII Code Case 2235 (2000 Edition)


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In the paragraph requiring that ultrasonic examination be performed in accordance with ASMESection V, Article 4 (this article acknowledges CITs of which TOFD is one). A further statementis made. Alternatively, for techniques that do not use amplitude recording levels, acceptableperformance is defined as demonstrating that all sized flaws, including the 0.06t flaws have anindicated length equal to or greater than the actual length of the specified flaws in thequalification block. This has clearly opened the door for TOFD to be used on Section VIIIpressure vessels.

ASME XI is specific to nuclear applications and is the item Silk pointed to for TOFD to meetthe requirements of. Division 1 IWA-3000 requires that flaws detected during In-serviceinspections that exceed acceptance standards of Section III be evaluated to determinedisposition. IWB dealing with Class I components allows acceptance by analytical evaluation(XI Div.1 IWB 3132.3). This requires the calculations as per IWB-3600 (fracture mechanics). Appendix I of ASME XI is the mandatory appendix for Ultrasonic Examination. Under FlawSizing of that appendix it states that flaws must be sized in accordance with Sec. XI AppendixVIII. This essentially requires a statistical assurance that ANY sizing technique meets the lateraland vertical accuracy stipulated in ASME XI.

In 1996 the Dutch NDT Society (KINT) submitted a Draft European Standard, De ontwikkelig vonacceptatiecriteria voor de TOFD onderzoemethode (Acceptance Criteria for Time of FlightDiffraction).

This proposes a table of acceptance criteria for all indications that are detected. Detection is based onthe settings set out for TOFD in a separate document (prEN 583-6).

Maximum allowable length (lmax)if height does not exceed h2

maximum allowable height (h1)when length exceeds lmax

Thickness Range Lmax h2 h16mm < d 8mm D 2.5 18mm< d 15mm D 3 115mm< d 40mm 15 3 240mm < d 60mm 20 4 260mm < d 100mm 25 5 2100mm< d 200mm 50 5 3D > 200mm 70 6 3

KINT Acceptance Table(*) d = design thickness

API Adaptations to TOFDDraft-API 579 Recommended Practice for Fitness-for-Service (The crack depth, length, angle anddistance to other surface breaking or embedded cracks is typically determined using UT examinationtechniques, either TOFD or angle beam.

Draft-API 580 Risk Based Inspection Recommended Practice (Base Resource Document recommendsautomated ultrasonic shear wave testing as a highly effective inspection technique for crack detectionand sizing. The capability of the AUT technique/type is evaluated using probability of detection(POD) curves from round-robins in the past where TOFD showed the best performance)


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Projects, communications, Code Cases and the future

From the apparent lack of Codes and Standards it is difficult to imagine how TOFD became as popularas it has. But we should keep in mind that there is another level of regulation in the hierarchy;Specifications.

It has been the recognition of individual companies of the advantages of TOFD that has provided thenecessary venue to prove to the world that TOFD has an important role to play. There are examples ofthe application of TOFD on many major projects around the world.

Projects using TOFDA major oil company (Chevron) accepted the test results by a large fabrication company (DaeWooHeavy Industries) on an off-shore oil rig structure in which TOFD was used. Originally therequirements of API RP2X were required (Recommended Practice for ultrasonic Examination ofOffshore Structural Fabrication and Guidelines for Qualification of Ultrasonic Technicians). Thetechnique used combined both TOFD and pulse-echo ultrasonics and did NOT use a raster scan toprovide full volume coverage by the pulse-echo probes. Instead, the pulse-echo was used only for theinner and outer edges while the middle volume was inspected ONLY using TOFD. After verification ofthe ability of the technique on several test samples and the acknowledgement of the regulatory body(DNV) that good probability of detection had been achieved, the project proceeded. Steel butt weldsfrom 25mm to 100mm thick were reliably inspected faster than possible using standard pulse-echo UT,much faster and with much more sensitivity than radiography and without the radiation hazards andother restrictions associated with working around radiation.

Another example of an independent engineering specification using TOFD is in the pipeline industry.In 1997 TransCanada Pipelines expanded their use of mechanised ultrasonic testing to manuallywelded circumferential seams. However, due to the higher probability of off-angle defects in theSMAW process as compared to the mechanised GMAW process, the new specification issued nowrequires the addition of TOFD to carry out any mechanised UT inspections on GMAW welds.

Example of the required display for GMAW welds (includes TOFD and pulse-echo information)

The pipeline application in TCPL eventually led to the development of the new ASTM StandardE-1961. This application with TOFD combines the rapid detection/evaluation abilities associated withmultiple pulse-echo probe arrays used in a linear scan with the characterisation capabilities of TOFD toaid in elimination of false calls. The pulse-echo probes required in the system also ensure that the poorcoverage that can occur in the near zone (upper 3-5mm) and mismatched back-wall regions using onlyTOFD, are adequately inspected.

In another project submerged-arc-welded seams were being inspected using the standard coderequirements found in CSA Z-245 (essentially the same as ASTM E-273 and API 5L). During routinesectioning on a lot, some welds were found to have shrinkage cracks. The ultrasonic technique beingused could not reliably detect these defects so a technique was developed to add a TOFD configurationinto the standard set-up. In this case the shortcomings of pulse-echo are overcome by TOFD.


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Fig:TOFD Results onChevron Qualification Weld48mm Thick

Fig:Defects Placed in the Chevron Qualification Weld

Fig:Example of the"required" display forGMAW welds (includesTOFD and pulse-echoinformation)

Fig : Shrink Cracks in SAW Weld

TOFD significantly improved detection over the code required technique

Number of PipeSeams tested

Number of Indications detected usingMechanised OD UT (>80% reference)

Number of Indicationsdetected using TOFD

149 20 35


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Fig : TOFD indication of a shrink crack

These three examples are typical of the application of TOFD to solve problems that are not adequatelyaddressed by the existing Codes and Standards.

What all these examples have in common is a linear scan. A raster scan is the traditional way ofcarrying out a manual inspection. This is done by moving the probe perpendicular to the weld axisallowing the volume to be covered.

Fig : Raster scan volumecoverage of a weld usingforward and backwards

motion.

When mechanised this process moves the probe in a fixture with a series of motions similar to themanual movement and data collection is done on the forward and backward motions.

Fig : Traditional raster scan in a mechanised set-up

A linear scan moves the probe parallel to the long axis of the weld. Data collection is done on the scanparallel to the weld and the raster step may not be required if multiple probes are used or if the probesused provide the coverage required (e.g. TOFD and limited pulse-echo coverage).


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Fig : Linear scan for increased data collection speed

CommunicationIn addition to the insistence of various industries to push ahead with the advantages of TOFD there arealso efforts being made to get Codes and Standards changed or made to recognise and incorporateTOFD more fully.

In November 1996 one of the authors submitted a formal Technical Inquiry to the ASME Section Vcommittee concerning TOFD. The Question was stated in the required format. This included abackground comment, the question, then a proposed answer and rationale:

Comment:Article 5 paragraph T542.4.3 references Article 4 Appendices B and C as examples of generaltechniques for performing a weld inspection. The paragraph also states; Other techniques may beused.

Question:Does this imply that Article 5 paragraph T542.4.3 allows non-amplitude based techniques, such asTime of Flight Diffraction Technique, to be used as detection methods instead of pulse echo ?

Proposed answer :YES. Provided that the technique can be demonstrated to detect the basic calibration reflectors in thebasic calibration block and the technique is demonstrated to provide the volume coverage required bythe referencing code section.

The ASME reply was received in Nov. 1997. They disappointingly stated No.This implied that TOFD was not recognised as a DETECTION method by the committee.

However, in the same letter to the ASME an enquiry as to the use of a linear scan technique was made:

Comment:Both Article 4 paragraph T-424.1 and Article 5 paragraph T-523.1 consider examination coverage. Thewording assumes that only simple pulse-echo scanning with a raster style scan pattern will be used.

Question:Are the requirements of Article 4 paragraph T-424.1 and Article 5 paragraph T-523.1 met bytechniques employing a single pass; such as Time of Flight Diffraction, multiprobe arrays and phasedarrays?

Proposed answer :YES. Provided that the technique can be demonstrated to detect the basic calibration reflectors in the


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basic calibration block and the technique is demonstrated to provide the volume coverage required bythe referencing code section.

In this case the committee agreed! With the ASTM E-1961 Standard then in draft before the ASTM committee it made the case for anon-raster technique described in E-1961 suitable for ASME style inspections. The standard raster scanmoves the probe perpendicular to the weld axis for the scan step and the small raster parallel to theweld axis is a non-data collection step. With the usually short scan length of the data collection scan(typically 50-200mm) the ramp-up and ramp-down requirements in motor controllers makes this a veryslow process. When the main axis of data collection can be the scan parallel to the weld axis themaximum scan speed can me sustained for a long period of time and less time is wasted in the smallincrement step. When phased arrays or multiple probe arrays can be arranged to ensure the coverage ina linear scan the scan times can be significantly reduced.

There is a difference between the two scans. The traditional raster scan allows the operator to see thesignal peaked in the centre of the beam. Whereas the linear scan may result in a less than maximumamplitude if the step positions the probe beam at a point either side of the maximised reflection point(but the same point could be made for length sizing with the traditional raster motion).

A third point was raised to the ASME Section V committee and also received a disappointing negative.

Comment:Article 5 paragraph T-542.7.2.3 requires scanning to be performed at a gain setting at least twotimes the primary reference level. Computerized Imaging Techniques in conjunction with TimeCorrected Gain permit display of signals over a set threshold and can even allow variability ofthis threshold after the data has been collected if waveforms are stored. Scanning at a gain settingover reference could result in saturating signals unless logarithmic amplifiers are used.

Question:If Computerized Imaging Techniques in conjunction with Time Corrected Gain can therequirement of Article 5 paragraph T-542.7.2.3 for scanning with extra gain be met by settingappropriate thresholds?

Proposed answer :YES. Article 5 paragraph T-542.7.2.3 also states that Evaluation will be performed with respectto primary reference level. Since the computerized image is the recording that would be used forevaluation it should correctly indicate amplitude at the primary reference level.

ASME Section V decided that NO was the correct answer but no rationale was provided explaininghow scanning at 6dB over the reference was fundamentally different from reducing the datacollection/evaluation threshold of the raw data by 6dB.

Code CasA year later a different committee (ASME Section VIII SC-VIII SG&I) was approached with arevision to a Code Case. The Code Case 2235 was titled: Use of Ultrasonic Examination in Lieu ofRadiography Section VIII, Division 1 and 2. It was issued in December 1996 and then applied tomaterials 4 inches thick and greater using standard UT methods described in Section V. A more recentvisitation of this Code Case was made to extend the thicknesses it was applicable to and to considernon-amplitude based ultrasonic techniques.


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Case number 2235 has since passed the approval of the committee and is due to be incorporated intoCode.

Its inquiry asked: Under what conditions and limitations may an ultrasonic examination be used in lieuof radiography, when radiography is required in accordance with Section VIII, Division 2, TableAF-241.1?

It the reply, the code case states that: It is the opinion of the Committee that all welds in material 1/2 in.or greater in thickness in pressure vessels may be examined using the ultrasonic (UT) method in lieu ofthe radiography (RT) method, provided that all of the following requirements are met:

It goes on to make several requirements including that ultrasonic examination be performed inaccordance with ASME Section V, Article 4. This would seem to again limit the use of TOFD based onthe Section V committee reply of 1997. However, in the same paragraph requiring that ultrasonicexamination be performed in accordance with ASME Section V, Article 4 a further statement is made. Alternatively, for techniques that do not use amplitude recording levels, acceptable performance isdefined as demonstrating that all sized flaws, including the 0.06t flaws have an indicated length equalto or greater than the actual length of the specified flaws in the qualification block.

Other references are also made in the Code Case 2235 to methods or techniques that do not useamplitude recording levels. This has clearly opened the door for TOFD to be used on pressure vessels.

Where are we heading now

In a Paper from 1991, Weld Metal Fabrication v 59 n 8 Oct 1991 3p ISSN: 0043-2245, J. Lilley and P.Osborne examined the potential of time-of-flight diffraction (TOFD) to replace the traditional methodsof inspecting fabricated tubular components.

They speculated in this article that TOFD would eventually replace radiography and traditionalultrasonic testing as the primary method for detecting and sentencing on tubular components. They alsocaution that this is a very large step partially because of the lack of suitable codes.

Lilley also suggests that it would be possible to introduce codes and acceptance criteria but it wouldtake a great deal of time and money for the validation process. As a result, industry will need to waitmany years before the benefits of the method are realised.

Lilley proposed a step-by-step method of introducing TOFD and warned that it would be industrydriven.

This iterative process would involve using TOFD as a screening method for manualultrasonics. Having proven the detection capability of the calibration holes (as in a standard ASMEblock with side drilled holes) TOFD would be used to detect any indications with a length greater thanallowed by the manual Code. Having found any such indications the manual techniques would be usedto disposition the indication. Comparing TOFD results and manual UT results to excavation resultswould be used to establish a database from which a TOFD acceptance criteria could be established.

A parallel programme comparing radiographic results and TOFD to excavations and metallographywould similarly be used to assure industry of TOFDs ability to be used in lieu of radiography.

In 1995 Det Norske Veritas (DNV) adopted a document by Olav Forli, et al (Nordtest). The document


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entitled Guidelines for Replacing NDE Techniques with One Another describes options for industriesinterested in using one NDE method in lieu of another. This process is much the same as Lilleysuggested in 1991 that industry should do to establish a credibility and assurance of detection forTOFD to be used in lieu of manual UT or radiography.

The DNV paper goes into lengthy descriptions of how to set up programmes and establish Probabilityof Detection (POD) curves.

This document was in fact what was used for the heavywall inspection of the off-shore structure project(DHI) described above. Having substantiated the abilities of the TOFD method according to this set ofrules the DNV inspection team overseeing the project was able to accept the inspection technique andresults submitted.

Summary

A technical hierarchy of rules exists: Codes, Standards, Specifications, Procedures and TechniquesUntil recently no Code existed that recognised TOFD so its use was restricted Two documents (BSI &CEN) are well known TOFD Standards but both are GuidesA Code using TOFD specific acceptance criteria has been drafted but is as yet still in formulation stagePresently an ASME Code Case to replace RT with UT has resulted in incorporating TOFD intopressure vessel work for both detection and sizing of flaws.Results of the many TOFD projects can be used in a format of statistical studies to allow TOFD toreplace manual UT or radiography.

TOFD has been used on projects where:

Codes are not applicable but POD requirements are highTOFDsizing can be used in conjunction with fracture mechanicsexistingcode requirements have been known to miss defectsspeed of detection is an important consideration

Code/Standard Title Comment

BS 7706(1993)

Guide to Calibration and setting-up of the ultrasonictime-of-flight diffraction (TOFD) technique fordetection ,location and sizing of flaws

Guide only

Pr EN 583-6 Time of Flight Diffraction Technique as a methodfor defect detection and sizing Guide only

ASME Section V as a CIT option inArticle 4

ASME Section VIII in a Code Case in lieuof RT)ASME Section XI (for accurate sizing)

KINT norm pr 9ExxxDraft standardsubmitted to CEN basedon 1998 report

List of TOFD Related Codes, Standards and Draft Standards at this time

Finally; after over 20 years TOFD is being recognised as a powerful tool for NDT. But no single NDT


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method finds ALL defects. Each method has its advantages and limitations. With the regulatory bodiesnow gradually recognising the strengths of TOFD for detection and sizing, it is likely that the financialbenefits of TOFD will now drive industry to promote its use.

In a more general respect, it is obvious to more and more users that automated ultrasonic inspectionsare becoming superior to manual UT in many regards, particularly speed and repeatability.

In 1989 the IIW published a Guide entitled Automated Ultrasonic Inspection of Welds; Guidance on itsMerits, Performance Requirements, Selection and Applications.

British Standards has on its listings a Standard BS 3923 part 2 1972 Ultrasonic examination of welds.Automated examination of fusion welded butt joints in ferritic steel. Presently this standard is stillcurrent but it is acknowledged by most as being very out of date. The Welding Institute proposed adraft revision to this (Revision #6 was in March 1995 BSI Committee WEE/46/-/13) but as yet noreplacement exists. (The existing standard covers requirements for equipment, surface condition, parentmetal examination, weld examination, evaluation of imperfections, test plates and presentation ofresults. Appendices on determination of probe characteristics, use of DGS diagram and method forsetting sensitivities.)

In 1998 ASTM E-1961 became one of the first American standards to be dedicated to the applicationof mechanised UT to weld inspections.

Other Standards in which mechanised UT and linear scanning is applicable include:

ASTM E-1961 Mechanised ultrasonic inspection of girth welds using zonal discriminationwith focused search unitsAPI 5L Steel Line PipeCSA Z-245.1 Steel Line Pipe

BS 3923 part2 Ultrasonic examination of welds. Automated examination of fusion weldedbutt joints in ferritic steelASTM E-273 Ultrasonic examination of longitudinal welded pipe and tubing

In a recent meeting of the Commission V of the IIW, 22/07/99, there was a micro-seminar - 'AutomatedUT and TOFD - Techniques. - Acceptance Criteria, Reliability, Cost-effectiveness, Human Factor andQualification' (V-1142-99). At this session a German proposal for a new European standard wasintroduced in the Sub-commission VC discussion.

Guide draft, September 1998

Ultrasonic testing systems for automatic inspections(prepared by the German Society for NDT)

This is a guide for users of automatic ultrasonic inspection systems. This guide should help in selectionand application of such systems.

The guideline will give instructive and helpful information to users considering relevant requirementsof clients and of existing inspection standards to

Probes and sensors


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Manipulation and control systemsUltrasonic devicesData recording and data visualisationData processing and evaluationDocumentation

Reference

Maurice Silk, Interpretation of TOFD data in the light of ASME XI and similar rules British Journal ofNDT vol 31, May 1989

1.

Lilley. J. and Osborne P., Weld Metal Fabrication v 59 n 8 Oct 1991 3p2. BS 7706 (1993) Guide to Calibration and setting-up of the ultrasonic time-of-flight diffraction (TOFD)technique for detection , location and sizing of flaws. British Standards Institute 1993.

3.

Pr EN 583-6, Time of Flight Diffraction Technique as a method for defect detection and sizing4. ASME Boiler & Pressure Vessel Code, Section V (CIT option in Article 4)5. ASME Boiler & Pressure Vessel Code, Section VIII (Code Case 2235 UT in lieu of RT)6. ASME Boiler & Pressure Vessel Code, Section XI (for accurate sizing)7. Olav Forli, et al, Guidelines for Replacing NDE Techniques with One Another Nordtest. 19948. KINT pr EN xxx draft Standard, De ontwikkelig von acceptatiecriteria voor de TOFDonderzoemethode (Acceptance Criteria for Time of Flight Diffraction).

9.

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