|NDT.net - February 2001, Vol. 6 No. 2|
The terms "code", "standard", "specification" and "procedure" are often confused in NDT. In some cases these terms are used inter-changeably. However from a technical point of view each refers to a separate type of document. To avoid even further confusion we will restrict our definitions to the written documents implied by these terms. This caution is due to the common referral to calibration blocks 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 recommending actions to achieve certain objectives. An example of a standard is the American Society of Nondestructive Testing Recommended Practice No. SNT-TC-1A. This is a standard for qualification and certification of NDT personnel by employers.
A standard is usually enforced or given authority by an organization or agency (typically professional societies or national institutions). When a set of standards is incorporated into law and thereby enforceable legally it is considered a code. Examples of codes are:
A federal agency may reference a Standard and thereby give it code status; e.g. 49 CFR 192 is the American pipeline regulatory document and it references API Standard 1104 Welding of Pipelines and Related 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 in Candu Nuclear Power Plants.
When a user or purchaser of a specific product requires assurance of quality level they will often arrange their own document describing specific test parameters and acceptance criteria. Such a product specific standard is considered a specification. Although it may reference other codes and standards it can require more stringent limits than the more general standards or it may avoid any reference to a Code or Standard.
To avoid the ambiguity of the usually general standards, and to avoid the constant updating of specifications that refer to national standards which are constantly revised, companies often develop a procedure.
The procedure can address the company's needs by setting out its standard practices for the various aspects of the test method, such as; procurement, processing, periodic controls, approved materials and accessories.
Finally, as a contracted inspection company applies a test to various parts, the variations available to achieve the desired results can be too myriad to list in the body of a procedure document and generalisations are again too vague. A common method of addressing the specific inspection application to a specific part is to use a technique sheet.
In summary, there is a hierarchy of documentation in NDT.
As defined in the ASTM Blue Book, Form and Style for ASTM Standards a Guide is a compendium of information or series of options that does not recommend a specific course of action. When referencing ASTM many people refer to the publications as Codes however, the title of the volumes clearly indicates that these are Standards as opposed to Codes; i.e. the Annual Book of ASTM Standards vol. 03.03. This applies to other similar documents and organisations; e.g. British Standards Institution.
A careful look at the wording in standards indicates the prevalence of should instead of shall. This is especially true when the Standard is a standard Guide.
As an example; ASME Section VIII will require an ultrasonic inspection be carried out on a component and describe the acceptance criteria for the test method. This Section of the Code does not tell you how to perform the inspection. ASME VIII references Section V to describe how inspections are to be set up and carried out.
Therefore we would expect to have to consider two sorts of Standards for TOFD;
In 1979 Maurice Silk and his associates in Harwell introduced the TOFD technique to the world. This came about from the need to more accurately size defects than had been possible using standard amplitude methods. The success of the method as a detection and sizing tool has been well documented throughout the literature.
In spite of the acknowledged success of the TOFD method it has been a 30 year struggle to get meaningful Codes and Standards that allow its use. Still today, it is difficult to apply TOFD and take advantage of its benefits of speed in detection as well as its sizing accuracy because it is not described in most codes referenced by designers.
However, in 1989 the applicable Section V, which would have been referenced for any ultrasonic inspection, would not have even considered computerised imaging techniques (CITs) of which TOFD would be only one subsequently described. CITs were first introduced in the 1992 Addenda (Dec. 31 1992) of the Article 4 in Section V. This finally allowed TOFD to be used (Non-mandatory Appendix E Paragraph E-80). This paragraph acknowledged TOFDs ability to both detect and size volumetric and planar 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 be accepted as a viable ultrasonic tool in a regulatory conscious world. However, as just one of the tools available in Section XI its use was restricted to a tool for sizing and dispositioning flaws found during in-service inspections of pressure vessels.
Parallel development was being carried out to enlighten the potential hands-on users how to carry out TOFD inspections. This led to the development of the Standard Guides.
Standard TOFD Guides include:
Note: Although CEN prEN 583-6 stipulates specific requirements in some aspects of the test, many aspects are addressed as recommendations as opposed to requirements. For this reason we have included it with BS 7706 as a guide.
Comparison of BS 7706 and EN 583-6
|Scope||States that it provides guidance and suggests that the linear scan (which it calls a D-scan) is applicable for initial scanning and a B-scan (motion parallel to the plane of the beam axis) is done for accurate sizing||States that it defines general principles for TOFD|
|References||References are both Normative and Informative and all are British Standards||Only Normative standards are referenced and all are EN standards|
|Definitions (several differences)||Several common words are defined such as; hardcopy, probe, transducer, flaw height Some special words are defined: lateral wave, creeping wave, B-scan and D-scan||Several symbols used in diagrams are defined|
Nonparallel scan and S-scan are defined and equate to BS 7706 B-scan and D-scan respectively
|Principles of the Technique (Method in EN 583-6)||General description and several equations to define flaw depths||Very general statement on the principles is made and then requirements for surface condition (as per EN 583-1) and couplant are made. |
Cautions against using TOFD on coarse grained materials
|Personnel||Requires general familiarity and suitable training(Guidelines for training provided in Annex E)||Qualification to EN 473 as a minimum plus additional training in accordance with a written practice|
|Equipment requirements||General guides are suggested as giving suitable results: e.g. pulser, receiver and digitiser parameters.||Minimum parameters are stipulated: e.g. receiver bandwidth, pulser rise time, A-scan sampling rate (minimum 1 sample per mm)|
|Probes||Guidance suggesting short pulse higher frequency probes||Stipulates 2 probes, Same centre frequency within +/-20%, Pulse width not
to exceed 2 cycles Recommends: |
Frequency and refracted angles
|Guiding mechanisms||Not covered||Require the use of mechanics to ensure probe spacing and encoded positioning of probes and accuracy with respect to reference line must be within 10% of probe centre spacing|
|Equipment Set-up Procedures||Extensive discussions on the parameter considerations are given: geometry, surface conditions, probe parameters,temporal and spatial resolution, sensitivity, and digitising rates.||Special note in Section 7 states that the arrangements of probes provided are NOT mandatory. |
Considers probe parameters, digitiser window, sensitivity, scan resolution, scan speed, and checking system performance
|Interpretation and Flaw Analysis||Reporting or Acceptance criteria to be agreed upon by contracting parties
General description of flaw recognition is provided with a more detailed description and examples in Annex D Five classes of flaw:
||Reporting or Acceptance criteria to be agreed upon by contracting parties
Basic information is provided on use of phase analysis to determine flaw extents
Three classifications of flaw:
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 in developing Codes or Standards which can be used with TOFD.
Except for the Guides available there are still no other internationally recognised Standards other than ASME that have incorporated TOFD as part of the acceptance criteria evaluation tools.
Codes and Standards applicable to TOFD include:
ASME VIII Code Case 2235 (2000 Edition)
In the paragraph requiring that ultrasonic examination be performed in accordance with ASME Section V, Article 4 (this article acknowledges CITs of which TOFD is one). A further statement is made. Alternatively, for techniques that do not use amplitude recording levels, acceptable performance is defined as demonstrating that all sized flaws, including the 0.06t flaws have an indicated length equal to or greater than the actual length of the specified flaws in the qualification block. This has clearly opened the door for TOFD to be used on Section VIII pressure vessels.
ASME XI is specific to nuclear applications and is the item Silk pointed to for TOFD to meet the requirements of. Division 1 IWA-3000 requires that flaws detected during In-service inspections that exceed acceptance standards of Section III be evaluated to determine disposition. 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 Flaw Sizing of that appendix it states that flaws must be sized in accordance with Sec. XI Appendix VIII. This essentially requires a statistical assurance that ANY sizing technique meets the lateral and vertical accuracy stipulated in ASME XI.
In 1996 the Dutch NDT Society (KINT) submitted a Draft European Standard, De ontwikkelig von acceptatiecriteria voor de TOFD onderzoemethode (Acceptance Criteria for Time of Flight Diffraction).
This proposes a table of acceptance criteria for all indications that are detected. Detection is based on the 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|
|6mm < d Ł8mm||D||2.5||1|
|8mm< d Ł15mm||D||3||1|
|15mm< d Ł 40mm||15||3||2|
|40mm < d Ł 60mm||20||4||2|
|60mm < d Ł 100mm||25||5||2|
|100mm< d Ł 200mm||50||5||3|
|D > 200mm||70||6||3|
|KINT Acceptance Table|
API Adaptations to TOFD
Draft-API 579 Recommended Practice for Fitness-for-Service (The crack depth, length, angle and distance to other surface breaking or embedded cracks is typically determined using UT examination techniques, either TOFD or angle beam.
Draft-API 580 Risk Based Inspection Recommended Practice (Base Resource Document recommends automated ultrasonic shear wave testing as a highly effective inspection technique for crack detection and 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)
From the apparent lack of Codes and Standards it is difficult to imagine how TOFD became as popular as 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 the necessary venue to prove to the world that TOFD has an important role to play. There are examples of the application of TOFD on many major projects around the world.
Projects using TOFD
A major oil company (Chevron) accepted the test results by a large fabrication company (DaeWoo Heavy Industries) on an off-shore oil rig structure in which TOFD was used. Originally the requirements of API RP2X were required (Recommended Practice for ultrasonic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Ultrasonic Technicians). The technique used combined both TOFD and pulse-echo ultrasonics and did NOT use a raster scan to provide full volume coverage by the pulse-echo probes. Instead, the pulse-echo was used only for the inner and outer edges while the middle volume was inspected ONLY using TOFD. After verification of the 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 welds from 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 and other 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 manually welded circumferential seams. However, due to the higher probability of off-angle defects in the SMAW process as compared to the mechanised GMAW process, the new specification issued now requires 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 Standard E-1961. This application with TOFD combines the rapid detection/evaluation abilities associated with multiple pulse-echo probe arrays used in a linear scan with the characterisation capabilities of TOFD to aid in elimination of false calls. The pulse-echo probes required in the system also ensure that the poor coverage that can occur in the near zone (upper 3-5mm) and mismatched back-wall regions using only TOFD, are adequately inspected.
In another project submerged-arc-welded seams were being inspected using the standard code requirements found in CSA Z-245 (essentially the same as ASTM E-273 and API 5L). During routine sectioning on a lot, some welds were found to have shrinkage cracks. The ultrasonic technique being used could not reliably detect these defects so a technique was developed to add a TOFD configuration into the standard set-up. In this case the shortcomings of pulse-echo are overcome by TOFD.
|Fig:TOFD Results on Chevron Qualification Weld 48mm Thick||Fig:Defects Placed in the Chevron Qualification Weld|
|Fig:Example of the "required" display for GMAW welds (includes TOFD and pulse-echo information)|
|Fig : Shrink Cracks in SAW Weld|
TOFD significantly improved detection over the code required technique
|Number of Pipe Seams tested||Number of Indications detected using Mechanised OD UT (>80% reference)||Number of Indications detected using TOFD|
|Fig : TOFD indication of a shrink crack|
These three examples are typical of the application of TOFD to solve problems that are not adequately addressed by the existing Codes and Standards.
What all these examples have in common is a linear scan. A raster scan is the traditional way of carrying out a manual inspection. This is done by moving the probe perpendicular to the weld axis allowing the volume to be covered.
|Fig : Raster scan volume coverage of a weld using forward and backwards motion.|
When mechanised this process moves the probe in a fixture with a series of motions similar to the manual 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 scan parallel to the weld and the raster step may not be required if multiple probes are used or if the probes used provide the coverage required (e.g. TOFD and limited pulse-echo coverage).
|Fig : Linear scan for increased data collection speed|
In addition to the insistence of various industries to push ahead with the advantages of TOFD there are also efforts being made to get Codes and Standards changed or made to recognise and incorporate TOFD more fully.
In November 1996 one of the authors submitted a formal Technical Inquiry to the ASME Section V committee concerning TOFD. The Question was stated in the required format. This included a background comment, the question, then a proposed answer and rationale:
Does this imply that Article 5 paragraph T542.4.3 allows non-amplitude based techniques, such as Time 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 the basic calibration block and the technique is demonstrated to provide the volume coverage required by the 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:
Are the requirements of Article 4 paragraph T-424.1 and Article 5 paragraph T-523.1 met by techniques employing a single pass; such as Time of Flight Diffraction, multiprobe arrays and phased arrays?
Proposed answer :
YES. Provided that the technique can be demonstrated to detect the basic calibration reflectors in the basic calibration block and the technique is demonstrated to provide the volume coverage required by the 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 a non-raster technique described in E-1961 suitable for ASME style inspections. The standard raster scan moves the probe perpendicular to the weld axis for the scan step and the small raster parallel to the weld 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 very slow process. When the main axis of data collection can be the scan parallel to the weld axis the maximum scan speed can me sustained for a long period of time and less time is wasted in the small increment step. When phased arrays or multiple probe arrays can be arranged to ensure the coverage in a 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 the signal peaked in the centre of the beam. Whereas the linear scan may result in a less than maximum amplitude 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.
If Computerized Imaging Techniques in conjunction with Time Corrected Gain can the requirement of Article 5 paragraph T-5220.127.116.11 for scanning with extra gain be met by setting appropriate thresholds?
Proposed answer :
YES. Article 5 paragraph T-518.104.22.168 also states that Evaluation will be performed with respect to primary reference level. Since the computerized image is the recording that would be used for evaluation 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 explaining how scanning at 6dB over the reference was fundamentally different from reducing the data collection/evaluation threshold of the raw data by 6dB.
A year later a different committee (ASME Section VIII SC-VIII SG&I) was approached with a revision to a Code Case. The Code Case 2235 was titled: Use of Ultrasonic Examination in Lieu of Radiography Section VIII, Division 1 and 2. It was issued in December 1996 and then applied to materials 4 inches thick and greater using standard UT methods described in Section V. A more recent visitation of this Code Case was made to extend the thicknesses it was applicable to and to consider non-amplitude based ultrasonic techniques.
Case number 2235 has since passed the approval of the committee and is due to be incorporated into Code.
Its inquiry asked: Under what conditions and limitations may an ultrasonic examination be used in lieu of radiography, when radiography is required in accordance with Section VIII, Division 2, Table AF-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 of the 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 in accordance with ASME Section V, Article 4. This would seem to again limit the use of TOFD based on the Section V committee reply of 1997. However, in the same paragraph requiring that ultrasonic examination 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 is defined as demonstrating that all sized flaws, including the 0.06t flaws have an indicated length equal to 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 use amplitude recording levels. This has clearly opened the door for TOFD to be used on pressure vessels.
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 methods of inspecting fabricated tubular components.
They speculated in this article that TOFD would eventually replace radiography and traditional ultrasonic testing as the primary method for detecting and sentencing on tubular components. They also caution 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 would take a great deal of time and money for the validation process. As a result, industry will need to wait many 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 industry driven.
This iterative process would involve using TOFD as a screening method for manual ultrasonics. Having proven the detection capability of the calibration holes (as in a standard ASME block with side drilled holes) TOFD would be used to detect any indications with a length greater than allowed by the manual Code. Having found any such indications the manual techniques would be used to disposition the indication. Comparing TOFD results and manual UT results to excavation results would 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 metallography would 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 entitled Guidelines for Replacing NDE Techniques with One Another describes options for industries interested in using one NDE method in lieu of another. This process is much the same as Lilley suggested in 1991 that industry should do to establish a credibility and assurance of detection for TOFD 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 Probability of 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 of rules the DNV inspection team overseeing the project was able to accept the inspection technique and results submitted.
A technical hierarchy of rules exists: Codes, Standards, Specifications, Procedures and Techniques
Until recently no Code existed that recognised TOFD so its use was restricted Two documents (BSI & CEN) are well known TOFD Standards but both are Guides
A Code using TOFD specific acceptance criteria has been drafted but is as yet still in formulation stage
Presently an ASME Code Case to replace RT with UT has resulted in incorporating TOFD into pressure 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 to replace manual UT or radiography.
TOFD has been used on projects where:
|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||Guide only|
|Pr EN 583-6||Time of Flight Diffraction Technique as a method for defect detection and sizing||Guide only|
|ASME||Section V||as a CIT option in Article 4|
|ASME||Section VIII||in a Code Case in lieu of RT)|
|ASME||Section XI||(for accurate sizing)|
|KINT||norm pr 9Exxx||Draft standard submitted to CEN based on 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 method finds ALL defects. Each method has its advantages and limitations. With the regulatory bodies now gradually recognising the strengths of TOFD for detection and sizing, it is likely that the financial benefits 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 inspections are 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 its Merits, 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 still current but it is acknowledged by most as being very out of date. The Welding Institute proposed a draft revision to this (Revision #6 was in March 1995 BSI Committee WEE/46/-/13) but as yet no replacement exists. (The existing standard covers requirements for equipment, surface condition, parent metal examination, weld examination, evaluation of imperfections, test plates and presentation of results. Appendices on determination of probe characteristics, use of DGS diagram and method for setting sensitivities.)
In 1998 ASTM E-1961 became one of the first American standards to be dedicated to the application of 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 discrimination|
with focused search units
|API 5L||Steel Line Pipe|
|CSA Z-245.1||Steel Line Pipe|
|BS 3923 part2||Ultrasonic examination of welds. Automated examination of fusion welded |
butt joints in ferritic steel
|ASTM 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 - 'Automated UT and TOFD - Techniques. - Acceptance Criteria, Reliability, Cost-effectiveness, Human Factor and Qualification' (V-1142-99). At this session a German proposal for a new European standard was introduced in the Sub-commission VC discussion.
Guide draft, September 1998
This is a guide for users of automatic ultrasonic inspection systems. This guide should help in selection and application of such systems.
The guideline will give instructive and helpful information to users considering relevant requirements of clients and of existing inspection standards to
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