 Table of Contents ECNDT '98 Session: NDT of Welds | The Performance of the Time Of Flight Diffraction(TOFD) Technique in Various International Round Robin Trials and the Continuing Research Work UnderwayN. TrimbornAEA Sonomatic BV, Krombraak 15, 4906 CR Oosterhout, The Netherlands Phone: +31 (0)162 42 55 88. Fax: +31 (0) 162 42 43 43; Norbert.Trimborn@aea-technology.nl; http://www.aeat.co.uk |
| TABLE OF CONTENTS |
The major aim of Non Destructive Examination of engineering structures and systems is assurance of integrity by confirming a lack of flaws in both the construction process and during the service life of the component. When a flaw is detected it is particularly important to apportion an accurate size of the discontinuity to be able to confidently of NDT methods has been the need for more reliable, simpler, accurate and cost effective techniques in order to achieve a plant operators requirements. The developments in automated inspection technology and the growth of Fitness For Purpose (FFP) inspection have propelled emerging techniques such as the Ultrasonic Time of Flight Diffraction (TOFD) to the forefront of industry. This method uses the phenomena of ultrasonic Diffraction rather than the traditional Pulse Echo technique which uses reflective principles. In order to give the method a reputation and pedigree it has been subject to many validations over the years. Among the validations has been the performance of the TOFD method in several "Round Robin" exercises which have been held over the past 20 years. These Round Robins have been organised in many formats and by various organisations. The development of the technique in concert with the Round Robins has been in four phases:
| Phase 1 | To validate the technique as a successful defect sizing tool |
| Phase 2 | Confirming the technique as a rapid detection tool |
| Phase 3 | Comparing the technique and ascertaining its effectiveness in replacing older traditional means of inspection. |
| Phase 4 | Development of acceptance criteria plus the continuing development work on projects such as complex geometries and austenitic weld inspection. |
The first and second phase of round-robin trials saw the assessment of TOFD as an accurate Defect Sizing tool. This is summarised by the study undertaken by EPRI (Electric Power Research Institute, USA) (Ref. 2) to assess the performance of different Ultrasonic techniques and procedures of relevance to reactor vessel examination.Techniques assessed include TOFD, Backward-scattering Tip-Diffraction and Conventional dB drop techniques. Mock-ups contained defects of two major classes; Cracks under the cladded surface and embedded defects in welds. Approximately 200 measurements were made.
The conclusions of the study were,
Sizing techniques based on detecting and measuring tip-diffracted signals from the defect edges were much more accurate than amplitude-based sizing techniques. TOFD techniques were the most accurate for measuring the through-thickness dimension of defects."
The inefficiency of amplitude based techniques as against TOFD's performance is evident even after incorporating beam-spread corrections as depicted in figure 1. These included techniques incorporating the 20% and 50% DAC. thresholds.
 Fig.1: Summary of sizing measurements of the EPRI sizing capability assessment of Ultrasonic techniques and procedures for reactor vessel examination. |
The reaffirmation of TOFD as one of the most powerful defect sizing tools was established through the PISC (Programme for Inspection of Steel Components) Programme. The PISC programme, since 1974 has been evaluating the overall effectiveness of Non-Destructive techniques and procedures used on structural components through a series of round robin trials under the direct supervision of the Joint Research Centre of the Commission of the European Committee has been executed in three phases. On PISC I and II (3) thick walled clad seam welds with a series of flaws were examined by a number of teams using conventional ASME XI based procedures and some alternative procedures incorporating advanced techniques like TOFD.
The extended summary of PISC II stated that:
TOFD alone is found in this work capable of doing most of the work and that is clear that a combination of techniques involving TOFD must reach high performances.
During the PISC exercise one team used TOFD alone with 100 % success in detecting and sizing a large population of flaws (18) in a 1.5m length. The actual scanning time for this exercise was one hour. PISC III which is looking into the reliability aspect of inspection and the importance of performance demonstration has confirmed the findings of PISC II through on-site trials (3).
This phase has seen a round robin trial in the recently completed Dutch Welding Institute (NIL) of thin plates (4). The project carried out an evaluation of the reliability of mechanised ultrasonic inspection in comparison with standard NON-Destructive inspection techniques like radiography and manual Ultrasonic Testing (UT) for the detection of defects in welds in steel plate in the wall thickness range of 6-15mm.
The project used various techniques and procedures which satisfied the Dutch "Rules for Pressure Vessels" (RTOD). The evaluated techniques were TOFD (3 systems); mechanised ultrasonic Pulse Echo (PE, 4 systems); manual UT (4 operators); standard (0°) radiography; gamma radiography and double exposure weld bevel radiography as well as a few non-commercial techniques.
The performance of TOFD was very reliable and it was evident that AEA Sonomatic's TOFD and PE systems (TOFD I and PE I) had the best combination of high Probability of Detection (POD) and low False Call Rate (FRC) thereby having the highest reliability as against other similar systems. This indicates the importance of experience that is reflected in AEA Sonomatic's procedures for inspection.
 Fig.2: Results of NIL Thin Plate Project |
The results of the trials on the 21 test samples containing a total of 244 artificial, yet highly realistic defects is summarised in figure 2.
The recommendation of the project stated that if both high detection reliability as good sizing and localisation capabilities are important, mechanised ultrasonic testing techniques (TOFD or PE) are to be preferred above manual UT and standard radiography.
The increased application of TOFD as an alternative inspection technique and other advantages like fingerprinting have made it a very popular tool of inspection. The existing British Standard BS 7706 and the draft CEN Standard act as baselines for the general application of TOFD. However for the complete utilisation of the technique's potential, an acceptance criteria is very essential. This in essence is at the moment being framed in the NIL acceptance criteria project and a draft document of the same should be available by the end of this year. This should propel increased utilisation of the technique's other advantages like being fast and non-intrusive making it a total inspection solution provider.
At the time of this paper being prepared development work continues into several aspects of TOFD. Examination of geometrically complex components has also been done as part of the PISC programme but more work needs to be done in the positioning of flaws as part of a fast detection process.
In addition to this the examination of austenitic weldments is problematical due to the nature of its structure. Work also needs to be done to that end.
The results of the Round Robin tests over the years has clearly demonstrated the efficiency of the Time of Flight Diffraction Technique in defect detection, sizing and case of use. With this knowledge it is now time to introduce the technique into International codes such as ASME and API. This must be taken with the consultation between these bodies, end users, equipment manufacturers and Statutory Authorities.
Use of the method in the inspection of plant at both the pre and in service inspection periods has shown that major cost savings are achievable throughout the life of the plant. The source data can also be used to augment life extension arguments due to the inherent quality of the technique.
As a method to replace radiography in site work the improvements in the safety aspect is obvious but considerable cost savings can be made due to the inherent versatility and accuracy of the method.
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