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Implications of the results of the Defect Detection Trials
Implications of the results of the Defect Detection Trials R.A. Murgatroyd and H. Seed The important conclusions arising from the UKAEA Defect Detection Trials are reviewed. It is concluded that ultrasonic techniques exist with a good capability to detect and accurately size flaws in weldments typical of those in PWR pressure vessels. Techniques used for detecting and sizing near-surface flaws also proved to be effective. The importance of automated data analysis and display techniques to inspection reliability is noted. As an aid to improving reliability the small number of errors that occurred are identified and discussed. Further development requirements are considered.
Keywords: ultrasonic testing, defect detection, defect sizing, nuclear pressure vessels A number of national and international programmes have been performed to determine the capability of ultrasonic techniques by means of test plate exercises. Among these were the Defect Detection Trials (DDT) organized by the United Kingdom Atomic Energy Authority (UKAEA) in 1981. These were designed to determine, under controlled conditions, the capability of selected ultrasonic techniques to detect and size flaws with dimensions of concern in the context of a PWR pressure vessel. Eight teams from France, the F R G and the UK participated in the exercise, employing techniques based either on the alternative procedures used successfully in the PISCI exercise or on other techniques which showed high potential. The results, presented at a conference held at Risley in 19821'1, demonstrate clearly that ultrasonic techniques are available which can effectively detect and size significant flaws.
austenitic strip cladding on one surface. Each contained a central butt-weld in which a total of 44 flaws covering a range of types and sizes were deliberately introduced throughout the weldments with the orientation of the flaws lying principally in the through-thickness direction of the weldment. The other two test assemblies were intended to examine the effectiveness of ultrasonic techniques to detect and size flaws in the near-surface region adjacent to austenitic cladding, One of these was a flat plate containing 26 cladding and subcladding flaws with through-thickness dimensions in the range 1 to 30 mm. The other simulated a PWR main coolant inlet nozzle in which 20 flaws were inserted in the inner radius and corner before being clad. Strict confidentiality was applied to prevent the inspectors having detailed prior knowledge of the number, size or location of flaws within the test blocks.
The purpose of this review is to summarize the main observations and conclusions of the D D T and to identify the factors which exerted a significant influence on performance. It deals first with the requirements for good detection capability, then assesses the performance of some of the sizing techniques employed. As an aid to improving the reliability of ultrasonic inspection, the small number of errors that occurred in the D D T which led either to a flaw not being reported or to an incorrect classification are summarized. Finally, the implications of the conclusions drawn from the D D T on the further development of ultrasonic inspection are outlined.
The test assemblies were inspected by eight teams from France, the F R G and the UK using techniques and procedures based on the best current practices or on techniques of high potential under development. For the detection stage on D D T plates 1 and 2 both the G e r m a n and the UK Central Electricity Generating Board teams employed a combination of tandem and high sensitivity pulse-echo techniques together with high angle longitudinal wave twin-crystal techniques. The French teams used high sensitivity pulse-echo techniques with focussed probes, whilst UKAEA Risley Nuclear Laboratories (RNL) applied conventional pulse-echo techniques at high sensitivity combined with 70° longitudinal wave twin-crystal probes. AERE Harwell developed a system based on the time-of-flight-diffraction technique (TOFD) which was used for the first time in the D D T for all four plates.
Brief review of the DDT The principal objective of the D D T was to determine the capability of selected ultrasonic techniques to detect and correctly classify flaws of significant dimensions in the context o f a PWR pressure vessel. A detailed description of the D D T has been given previouslyt21. There were four test assemblies two of which were flat plates of dimensions 1.5 m × 1.5 m × 250 m m thick with
For the near-surface flaws in plates 3 and 4, RNL employed 70 ° longitudinal wave twin-crystals and 0° twincrystals in an automated system, whilst Babcock Power Ltd used similar techniques in a semi-automated system for the inspection of plate 3.
0308-9126/85/040195-04 $3.00 © 1985 Butterworth 8- Co (Publishers) Ltd NDT INTERNATIONAL. VOL 18. NO 4. AUGUST 1985
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All four plates were examined destructively at the Joint Research Centre at Ispra, ItalyPl.
Defect detection for plates 1 and 2 The performances of the teams taking part in the D D T have been reported previously and will not be discussed in detail here. Dealing first with the results for the two thickplate weldments, all defects were detected in inspections from the clad surfaces[41. In a few cases defects were missed in inspections from the unclad surface although they were reported from the clad side. The results demonstrate that both conventional and more advanced techniques are capable of highly reliable detection provided that due regard is paid to aspects which influence detection capability and reliability including inspection techniques, sensitivity and data analysis and display.
Inspection techniques One of the most important requirements for effective detection is to select and optimize the inspection techniques. Pulse-echo and tandem are widely used in pressure vessel inspections and the effectiveness of these techniques was studied in the DDT. The tandem technique was developedl~l for flaws oriented in the through-wall direction and since many of the flaws in the plates had this orientation the technique was applied by both German and CEGB teams in the DDT with very good results. Pulse-echo techniques based on both focussed and divergent beams were used under conditions where flaws were generally not well oriented for specular reflection, and it was necessary to ensure that the search sensitivity was adequate to ensure the detection of all the flaws. Both the Germanl61 and the CEGB resultsM specified a reporting threshold for the tandem technique equivalent to a 3 mm flat-bottom hole standard and achieved a good detection capability. At this sensitivity there is a high signal-to-noise ratio for the flaws for which it is designed. In the case of pulse-echo techniques, high search sensitivity is only possible with good quality steels and weldments in which the general ultrasonic background noise level is low. By specification of the quality of DDT plates and adequate control of the welding process during fabrication it was possible to gather pulse-echo data at high sensitivity, in some instances at approximately 5% I)AC. At this sensitivity, satellite indications in the vicinity of the intended flaws were often detected and, whilst generally not significantly influencing the detection stage, they sometimes increased the difficulty of interpretation. This is a factor which at present appears to be an inherent problem in test-block exercises. However, a similar situation can arise in actual vessel inspection due to the presence of small inclusions in the weld. The influence of search sensitivity on detection rate with pulse-echo techniques has been studied for the RNL resultslSI by analysing the stored data at selected amplitude thresholds. This work concluded that at 50% DAC using 0°, 45 ° and 60° shear waves together with 70° shear waves for the near-surface region, 5 of the 44 significant flaws were not detected from the clad surfaces and 10 were not detected from the unclad surfaces. All the flaws were detected at 10% and 20% DAC but in the inspection at 20% DAC from the unclad surfaces one major flaw was recorded only by the 70° shear wave probe.
196
Thus, on the basis of the DDT data, for effective detection with pulse-echo techniques a search sensitivity of between 10% and20% DAC appears to be necessary. This highlights one of the main reasons for the difference in performance of the techniques used in PISC I and the DDT where in the former the reporting threshold was stipulated to be 50% DAC. An analysis of the effectiveness of the tandem and pulseecho techniques has been madelgl using the CEGB data for plates 1 and 2. For the type of flaw included in these plates the 45 ° tandem technique proved most effective for detection, with 60° pulse-echo performing better than the 45 ° technique. Tandem generally gave the largest signals, but, for defects close to the cladding. 70° compressionwave beams from the clad surface and 45 ° pulse-echo scanning from the unclad surface for the 'corner effect' gave the best detection results. A novel technique first used for detection in the D D T was TOFD D°l. This is based on the detection and accurate time measurement of edge diffracted waves, with the detection and sizing inspections being performed in a single operation. Sensitivity is less important to this technique which relies on accurate time measurement. However, the technique is required to operate at high sensitivity for planar reflectors oriented in the 'through-thickness' direction since in these cases it is normally detecting diffracted signals. TemplelUl has shown that the diffracted signals from edges of elliptical flaws are sufficient to give good detection capability provided the background noise level is low. The technique performed extremely well in the trials. The conclusion reached is that to achieve a good detection capability the inspection techniques must be based on sound physical principles coupled with an understanding of the general parameters of the types of flaw that must be detected, with the magnitude of the ultrasonic signals being carefully considered when specifying the procedures. The reliability of the inspection is improved by including sufficient techniques to give an adequate level of redundancy in the inspection data.
Data analysis and display The DDT inspections emphasized that aspects of the design of an inspection system which contribute to good detection capability, such as high sensitivity, multiprobe techniques etc, result in a substantial amount of data being generated. Many of the teams applied automated data processing methods and graphics display systems to facilitate analysis. In the case of the Harwell system the data were gathered in a single pass inspection; other systems operated on essentially a two-stage approach in which the detection data were gathered and assessed to identify flawed volumes, and then these volumes were sized` Both approaches achieved good results, although over different timescales, and it is considered that the automated data analysis and display techniques contributed significantly to the reliability of the inspections. In practice, on a reactor pressure vessel it would not be expected that such a large quantity of data would require second-stage analysis and the benefits of a combined search and size system are less obvious. However, there is merit in a system which is capable of reliably discriminating between significant and trivial signals at an early stage, and this is an important development requirement.
NDT INTERNATIONAL. AUGUST 1985
Defect sizing for plates 1 a n d 2
Sizing results for plates 3 and 4
The seven teams participating in the exercise on plates l and 2 employed a variety of sizing techniques including focussed probes, dynamic tandem, time-of-flight, holography, diffracted edge waves and wide beam pulse-echo processed by the synthetic aperture focussing technique (SAFT). From the D D T results it is concluded that those teams using diffracted edge waves either with a single probe or in TOFD produced the most accurate sizing results. An important development arising from the D D T is the improvement in the lateral resolution of the TOFD technique achieved by SAFT processing the raw data.
The sizing results for plate 3 were, in general good, with the techniques based on the two-probe TOFD techniques achieving high accuracy of the order o f + 2 mm. One of the teams using TOFD achieved similar accuracies on plate 4, but the other team's data were subject to a small systematic error due principally to the complex geometry of the nozzle inner radius. This caused an oversizing error of up to 4 mm and also led to failure to detect the lower edge of the deepest flaw because the depth searched was reduced by 4 mm, as a result of which this flaw was considerably undersized. It is recommended that in the design of an inspection system for this type of complex geometry, techniques should be included to ensure that the full depth range of a flaw is covered and that the possibility of large flaws is not overlooked.
It is clear, however, that errors can occur in interpreting edge wave signals due to small satellite flaws, changes in flaw transmission coefficient, or geometrical and mode converted signals. For reliable interpretation of sizing data, therefore, it is prudent to employ additional, diverse techniques. Methods based on tandem or obscuration techniques can often confirm that a large reflector is present rather than a cluster of small flaws. No teams used fully automated analytical procedures, although automation played a major role in processing. analysing and displaying the data. Decisions were often made by groups of specialists using advanced data display techniques based on time-series A-scans, and B and Cscans. These allowed the data to be analysed on an integrated basis which not only highlighted the flaws but also revealed the operation of other mechanisms such as mode conversions and ultrasonic scattering. Interpretation of the data was greatly facilitated by these methods of analysis, and it is concluded that the automated data display systems contributed significantly to the reliability of flaw sizing,
Defect detection for plates 3 and 4 Fewer techniques were employed for this second phase of the D D T involving near-surface defects in test assemblies 3 and 4. Plate 3 containing 26 intentional flaws was inspected by three teams using, respectively, semi-manual and automated pulse-echo, and the TOFD technique. The semi-manual techniques applied by Babcock Power based on 0 ° and 45 ° shear wave pulse-echo and 70 ° longitudinal wave twin-crystals with computerized data recording performed welL detecting all the planar flaws. A slag line and an area of cracking in the austenitic cladding were not reported although the latter was observed below reporting level. RNL applied 0 ° pulse-echo and the 70 ° longitudinal wave twin-crystal technique in an automated system and detected all the flaws. The system also showed potential for characterizing the morphology of the flaws1121 although the sizing capability of these techniques in the through-thickness direction was considered to be limited. The Harwell system applying automated TOFD techniques was not designed to inspect the first 5 m m below the cladding and special dispensation was given by the managers of the DDT. Within this specification the team reported all the flaws. Inspection of plate 4 was performed with both the RNL and Harwell systems. All flaws were detected although due to a clerical error Harwell did not report one flaw. The conclusion drawn from the detection phase of these inspections is that the combination of the 0° pulse-echo and 70 ° longitudinal wave twin-crystal techniques gives an excellent detection capability. The automated scanning and data recording system appears to have a somewhat better capability than the semi-manual system,
NDT INTERNATIONAL. AUGUST 1985
Control of error Several errors occurred in the D D T which led either to a defect not being reported or to an incorrect classification. It is not the intention to explore these aspects in detail but it is important to identify potential sources of error if the high capability achieved in the DDT is to be transferred reliably into routine inspections. Errors arising from equipment design and operation were few and only isolated cases were reported where the design of the equipment proved inadequate for the requirement Nevertheless, there is a clear need for the specification of equipment and procedures to consider in detail relevant aspects of geometry, inspection zones, flaw parameters and data handling to achieve highly reliable inspection. These factors are normally considered carefully during the planning of an inspection, and studies are proceeding in the UK to prepare guidelines which will contribute to the assurance of reliability. A few errors occurred at the detection stage when indications were not observed due to additional signals from nearby flaws or because of high background noise levels. These occurred during inspections from the unclad surfaces for flaws near the clad surface with a large defect in mid-plate. These flaws were detected during inspection from the clad surface. This type of error may be explained as an effect of the high defect population in the D D T plates, but nevertheless it is analagous to the inspection of components of complex geometry where geometrical features may produce large signals which interfere with those from a flaw. The control of this type of error is not easy but is assisted by good data display methods, with the ability to display and analyse individual sets of data from each technique employed. In this analysis, the combination of information from a diversity of techniques would also be valuable. In one instance a flaw was not reported due to clerical error. This demonstrates that reliability can be affected at all stages of an inspection and emphasizes the need for the application of reliability assurance procedures in all areas. For a small number of flaws, errors arose at the stage of interpreting and assessing sizing information. In part this was due to the high sensitivity employed for sizing and the presence of small satellite indications adjacent to the intended flaws. It is considered that this type of error can be controlled by employing sizing techniques which are sensitive to different physical properties of a flaw. The main requirement in selecting the various sizing
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techniques is to ensure that significant through-wall defects are not classed as acceptable arrays of small flaws. The assessment and analysis of the data in the D D T was in general performed by experienced and well-qualified scientists. This implies that for reliable inspection in practical situations not only is training on the application of equipment and procedures required, but that it is important that the inspection team has had experience in interpreting data from complex, ~difficulf flaws.
Future developments The D D T results show that defect detection and sizing by ultrasonics can be very effective with good procedures. Some development of detection procedures is required to improve discrimination between significant and trivial signals and to reduce operator involvement in these decisions. It is considered that an important requirement is to improve the reliability of data interpretation and flaw classification. It is clear that techniques exist which are capable of accurately sizing most flaws but that in some instances, for example in the case of complex flaws. interpretation can be difficult Studies are required to reduce the possibility of error at the flaw sentencing stage and these should include a detailed study of the interaction of ultrasound with relevant flaw types to give reliable characterization of flaws. There is a requirement to develop systems which will apply these methods on real plant. The developments will need to produce rugged manipulators, which can move probes with the precision required for the application of techniques such as SAFT and to permit the correlation of results from diverse techniques. Also, the data processing and display equipment must be reliable for field applications. As the systems and techniques become available in industry there must be suitable training for inspection teams, which will allow the expertise gained in laboratory studies on complex, 'difficulf flaws to be transferred.
Authors The authors are at UKAEA Risley Nuclear Power Development Laboratories, Risley, Warrington WAg 6AT, UK
References I
The UKAEA De~,ct Dewction Trialx Bir~hwood Warring/on, UK (7-8 October 1982) 2 Watkins, B., Ervine, R.W. and Cowburn, ICJ. "The UKAEA l)cfect Detection Trials" Brit d NDT, 25 No 4 (July 1983) pp 179-185 3 Crutzen, S. et al "Destructive examination of test plates I and 2 of the Defect Detection Trials', Brit J NI)T, 25 No 4 (July 1983) pp 193-194 4 Watkins, B. 'The UKAEA Defect l)clection Trial programmes and tl sun]mary of the inspection rcsuhs', lnt A),mp NDE Reliabilio~ Portland. Oregon, USA (June 19-21 1983) chapter 7.1 5 Wustenberg, H. and Mundry, E. 'Limiting inlluences on the reliability of uhrasonic in-service inspection methods'. Cot!f Periodic Inspection of Pressurised Component,,~ I Mcch E., London (4-6 June 1974) C112/74 6 Barbian, O.A. et al 'A second view of the German results obtained in the Defect Detection Trials'. Brit J NDT. 26 No 2 (Feb. 1984) pp 92-96 7 Bowker, K.J. et al "CGB inspection of plates I and 2 in the UKAEA Defect Detection Trials'. Brit .1 NDT. 25 No 5 (Sept. 83) pp 249-255 8 Murgatroyd, R.A. "Automated defect location and sizing by advanced ultrasonic techniques'. 5th In/ Conf Quantitative NDE in the Nuch, ar Industry: San Diego, CA. USA. (1(I-13 May 1982) 9 Langston, D.B. and Wilson, R. 'The UKAEA l)cfcct Detection Trials: The application of some results to model validation and assessment of technique capability" CEGB Report TPRD/B/()465/R84 1{I Gardner, W.E. and Hudson, J.A. "Ultrasonic inspection of thick section pressure vessel steel by the time-otZflight diffraction method' 5th lnt ('onf Quantitative NDE in the Nuclear lndust~', San Diego. CA, USA 1111-13 May t982) 11 Temple, J.A.G. 'The amplitude of ultrasonic time-of flight signals compared with those from a reference rctlcctor', lnt J Press Vess and Piping, 16 11984) pp 145-159 12 Rogerson, A. et al qnspection of Defect Detection Trials plate 3 by the Materials Physics Department, RNU, Brit J NDT. 26 No I (Jan. 1984) pp 20-26
Paper received 20 May 1985
198
NDT INTERNATIONAL. AUGUST 1985
Keywords: ultrasonic testing, defect detection, defect sizing, nuclear pressure vessels A number of national and international programmes have been performed to determine the capability of ultrasonic techniques by means of test plate exercises. Among these were the Defect Detection Trials (DDT) organized by the United Kingdom Atomic Energy Authority (UKAEA) in 1981. These were designed to determine, under controlled conditions, the capability of selected ultrasonic techniques to detect and size flaws with dimensions of concern in the context of a PWR pressure vessel. Eight teams from France, the F R G and the UK participated in the exercise, employing techniques based either on the alternative procedures used successfully in the PISCI exercise or on other techniques which showed high potential. The results, presented at a conference held at Risley in 19821'1, demonstrate clearly that ultrasonic techniques are available which can effectively detect and size significant flaws.
austenitic strip cladding on one surface. Each contained a central butt-weld in which a total of 44 flaws covering a range of types and sizes were deliberately introduced throughout the weldments with the orientation of the flaws lying principally in the through-thickness direction of the weldment. The other two test assemblies were intended to examine the effectiveness of ultrasonic techniques to detect and size flaws in the near-surface region adjacent to austenitic cladding, One of these was a flat plate containing 26 cladding and subcladding flaws with through-thickness dimensions in the range 1 to 30 mm. The other simulated a PWR main coolant inlet nozzle in which 20 flaws were inserted in the inner radius and corner before being clad. Strict confidentiality was applied to prevent the inspectors having detailed prior knowledge of the number, size or location of flaws within the test blocks.
The purpose of this review is to summarize the main observations and conclusions of the D D T and to identify the factors which exerted a significant influence on performance. It deals first with the requirements for good detection capability, then assesses the performance of some of the sizing techniques employed. As an aid to improving the reliability of ultrasonic inspection, the small number of errors that occurred in the D D T which led either to a flaw not being reported or to an incorrect classification are summarized. Finally, the implications of the conclusions drawn from the D D T on the further development of ultrasonic inspection are outlined.
The test assemblies were inspected by eight teams from France, the F R G and the UK using techniques and procedures based on the best current practices or on techniques of high potential under development. For the detection stage on D D T plates 1 and 2 both the G e r m a n and the UK Central Electricity Generating Board teams employed a combination of tandem and high sensitivity pulse-echo techniques together with high angle longitudinal wave twin-crystal techniques. The French teams used high sensitivity pulse-echo techniques with focussed probes, whilst UKAEA Risley Nuclear Laboratories (RNL) applied conventional pulse-echo techniques at high sensitivity combined with 70° longitudinal wave twin-crystal probes. AERE Harwell developed a system based on the time-of-flight-diffraction technique (TOFD) which was used for the first time in the D D T for all four plates.
Brief review of the DDT The principal objective of the D D T was to determine the capability of selected ultrasonic techniques to detect and correctly classify flaws of significant dimensions in the context o f a PWR pressure vessel. A detailed description of the D D T has been given previouslyt21. There were four test assemblies two of which were flat plates of dimensions 1.5 m × 1.5 m × 250 m m thick with
For the near-surface flaws in plates 3 and 4, RNL employed 70 ° longitudinal wave twin-crystals and 0° twincrystals in an automated system, whilst Babcock Power Ltd used similar techniques in a semi-automated system for the inspection of plate 3.
0308-9126/85/040195-04 $3.00 © 1985 Butterworth 8- Co (Publishers) Ltd NDT INTERNATIONAL. VOL 18. NO 4. AUGUST 1985
195
All four plates were examined destructively at the Joint Research Centre at Ispra, ItalyPl.
Defect detection for plates 1 and 2 The performances of the teams taking part in the D D T have been reported previously and will not be discussed in detail here. Dealing first with the results for the two thickplate weldments, all defects were detected in inspections from the clad surfaces[41. In a few cases defects were missed in inspections from the unclad surface although they were reported from the clad side. The results demonstrate that both conventional and more advanced techniques are capable of highly reliable detection provided that due regard is paid to aspects which influence detection capability and reliability including inspection techniques, sensitivity and data analysis and display.
Inspection techniques One of the most important requirements for effective detection is to select and optimize the inspection techniques. Pulse-echo and tandem are widely used in pressure vessel inspections and the effectiveness of these techniques was studied in the DDT. The tandem technique was developedl~l for flaws oriented in the through-wall direction and since many of the flaws in the plates had this orientation the technique was applied by both German and CEGB teams in the DDT with very good results. Pulse-echo techniques based on both focussed and divergent beams were used under conditions where flaws were generally not well oriented for specular reflection, and it was necessary to ensure that the search sensitivity was adequate to ensure the detection of all the flaws. Both the Germanl61 and the CEGB resultsM specified a reporting threshold for the tandem technique equivalent to a 3 mm flat-bottom hole standard and achieved a good detection capability. At this sensitivity there is a high signal-to-noise ratio for the flaws for which it is designed. In the case of pulse-echo techniques, high search sensitivity is only possible with good quality steels and weldments in which the general ultrasonic background noise level is low. By specification of the quality of DDT plates and adequate control of the welding process during fabrication it was possible to gather pulse-echo data at high sensitivity, in some instances at approximately 5% I)AC. At this sensitivity, satellite indications in the vicinity of the intended flaws were often detected and, whilst generally not significantly influencing the detection stage, they sometimes increased the difficulty of interpretation. This is a factor which at present appears to be an inherent problem in test-block exercises. However, a similar situation can arise in actual vessel inspection due to the presence of small inclusions in the weld. The influence of search sensitivity on detection rate with pulse-echo techniques has been studied for the RNL resultslSI by analysing the stored data at selected amplitude thresholds. This work concluded that at 50% DAC using 0°, 45 ° and 60° shear waves together with 70° shear waves for the near-surface region, 5 of the 44 significant flaws were not detected from the clad surfaces and 10 were not detected from the unclad surfaces. All the flaws were detected at 10% and 20% DAC but in the inspection at 20% DAC from the unclad surfaces one major flaw was recorded only by the 70° shear wave probe.
196
Thus, on the basis of the DDT data, for effective detection with pulse-echo techniques a search sensitivity of between 10% and20% DAC appears to be necessary. This highlights one of the main reasons for the difference in performance of the techniques used in PISC I and the DDT where in the former the reporting threshold was stipulated to be 50% DAC. An analysis of the effectiveness of the tandem and pulseecho techniques has been madelgl using the CEGB data for plates 1 and 2. For the type of flaw included in these plates the 45 ° tandem technique proved most effective for detection, with 60° pulse-echo performing better than the 45 ° technique. Tandem generally gave the largest signals, but, for defects close to the cladding. 70° compressionwave beams from the clad surface and 45 ° pulse-echo scanning from the unclad surface for the 'corner effect' gave the best detection results. A novel technique first used for detection in the D D T was TOFD D°l. This is based on the detection and accurate time measurement of edge diffracted waves, with the detection and sizing inspections being performed in a single operation. Sensitivity is less important to this technique which relies on accurate time measurement. However, the technique is required to operate at high sensitivity for planar reflectors oriented in the 'through-thickness' direction since in these cases it is normally detecting diffracted signals. TemplelUl has shown that the diffracted signals from edges of elliptical flaws are sufficient to give good detection capability provided the background noise level is low. The technique performed extremely well in the trials. The conclusion reached is that to achieve a good detection capability the inspection techniques must be based on sound physical principles coupled with an understanding of the general parameters of the types of flaw that must be detected, with the magnitude of the ultrasonic signals being carefully considered when specifying the procedures. The reliability of the inspection is improved by including sufficient techniques to give an adequate level of redundancy in the inspection data.
Data analysis and display The DDT inspections emphasized that aspects of the design of an inspection system which contribute to good detection capability, such as high sensitivity, multiprobe techniques etc, result in a substantial amount of data being generated. Many of the teams applied automated data processing methods and graphics display systems to facilitate analysis. In the case of the Harwell system the data were gathered in a single pass inspection; other systems operated on essentially a two-stage approach in which the detection data were gathered and assessed to identify flawed volumes, and then these volumes were sized` Both approaches achieved good results, although over different timescales, and it is considered that the automated data analysis and display techniques contributed significantly to the reliability of the inspections. In practice, on a reactor pressure vessel it would not be expected that such a large quantity of data would require second-stage analysis and the benefits of a combined search and size system are less obvious. However, there is merit in a system which is capable of reliably discriminating between significant and trivial signals at an early stage, and this is an important development requirement.
NDT INTERNATIONAL. AUGUST 1985
Defect sizing for plates 1 a n d 2
Sizing results for plates 3 and 4
The seven teams participating in the exercise on plates l and 2 employed a variety of sizing techniques including focussed probes, dynamic tandem, time-of-flight, holography, diffracted edge waves and wide beam pulse-echo processed by the synthetic aperture focussing technique (SAFT). From the D D T results it is concluded that those teams using diffracted edge waves either with a single probe or in TOFD produced the most accurate sizing results. An important development arising from the D D T is the improvement in the lateral resolution of the TOFD technique achieved by SAFT processing the raw data.
The sizing results for plate 3 were, in general good, with the techniques based on the two-probe TOFD techniques achieving high accuracy of the order o f + 2 mm. One of the teams using TOFD achieved similar accuracies on plate 4, but the other team's data were subject to a small systematic error due principally to the complex geometry of the nozzle inner radius. This caused an oversizing error of up to 4 mm and also led to failure to detect the lower edge of the deepest flaw because the depth searched was reduced by 4 mm, as a result of which this flaw was considerably undersized. It is recommended that in the design of an inspection system for this type of complex geometry, techniques should be included to ensure that the full depth range of a flaw is covered and that the possibility of large flaws is not overlooked.
It is clear, however, that errors can occur in interpreting edge wave signals due to small satellite flaws, changes in flaw transmission coefficient, or geometrical and mode converted signals. For reliable interpretation of sizing data, therefore, it is prudent to employ additional, diverse techniques. Methods based on tandem or obscuration techniques can often confirm that a large reflector is present rather than a cluster of small flaws. No teams used fully automated analytical procedures, although automation played a major role in processing. analysing and displaying the data. Decisions were often made by groups of specialists using advanced data display techniques based on time-series A-scans, and B and Cscans. These allowed the data to be analysed on an integrated basis which not only highlighted the flaws but also revealed the operation of other mechanisms such as mode conversions and ultrasonic scattering. Interpretation of the data was greatly facilitated by these methods of analysis, and it is concluded that the automated data display systems contributed significantly to the reliability of flaw sizing,
Defect detection for plates 3 and 4 Fewer techniques were employed for this second phase of the D D T involving near-surface defects in test assemblies 3 and 4. Plate 3 containing 26 intentional flaws was inspected by three teams using, respectively, semi-manual and automated pulse-echo, and the TOFD technique. The semi-manual techniques applied by Babcock Power based on 0 ° and 45 ° shear wave pulse-echo and 70 ° longitudinal wave twin-crystals with computerized data recording performed welL detecting all the planar flaws. A slag line and an area of cracking in the austenitic cladding were not reported although the latter was observed below reporting level. RNL applied 0 ° pulse-echo and the 70 ° longitudinal wave twin-crystal technique in an automated system and detected all the flaws. The system also showed potential for characterizing the morphology of the flaws1121 although the sizing capability of these techniques in the through-thickness direction was considered to be limited. The Harwell system applying automated TOFD techniques was not designed to inspect the first 5 m m below the cladding and special dispensation was given by the managers of the DDT. Within this specification the team reported all the flaws. Inspection of plate 4 was performed with both the RNL and Harwell systems. All flaws were detected although due to a clerical error Harwell did not report one flaw. The conclusion drawn from the detection phase of these inspections is that the combination of the 0° pulse-echo and 70 ° longitudinal wave twin-crystal techniques gives an excellent detection capability. The automated scanning and data recording system appears to have a somewhat better capability than the semi-manual system,
NDT INTERNATIONAL. AUGUST 1985
Control of error Several errors occurred in the D D T which led either to a defect not being reported or to an incorrect classification. It is not the intention to explore these aspects in detail but it is important to identify potential sources of error if the high capability achieved in the DDT is to be transferred reliably into routine inspections. Errors arising from equipment design and operation were few and only isolated cases were reported where the design of the equipment proved inadequate for the requirement Nevertheless, there is a clear need for the specification of equipment and procedures to consider in detail relevant aspects of geometry, inspection zones, flaw parameters and data handling to achieve highly reliable inspection. These factors are normally considered carefully during the planning of an inspection, and studies are proceeding in the UK to prepare guidelines which will contribute to the assurance of reliability. A few errors occurred at the detection stage when indications were not observed due to additional signals from nearby flaws or because of high background noise levels. These occurred during inspections from the unclad surfaces for flaws near the clad surface with a large defect in mid-plate. These flaws were detected during inspection from the clad surface. This type of error may be explained as an effect of the high defect population in the D D T plates, but nevertheless it is analagous to the inspection of components of complex geometry where geometrical features may produce large signals which interfere with those from a flaw. The control of this type of error is not easy but is assisted by good data display methods, with the ability to display and analyse individual sets of data from each technique employed. In this analysis, the combination of information from a diversity of techniques would also be valuable. In one instance a flaw was not reported due to clerical error. This demonstrates that reliability can be affected at all stages of an inspection and emphasizes the need for the application of reliability assurance procedures in all areas. For a small number of flaws, errors arose at the stage of interpreting and assessing sizing information. In part this was due to the high sensitivity employed for sizing and the presence of small satellite indications adjacent to the intended flaws. It is considered that this type of error can be controlled by employing sizing techniques which are sensitive to different physical properties of a flaw. The main requirement in selecting the various sizing
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techniques is to ensure that significant through-wall defects are not classed as acceptable arrays of small flaws. The assessment and analysis of the data in the D D T was in general performed by experienced and well-qualified scientists. This implies that for reliable inspection in practical situations not only is training on the application of equipment and procedures required, but that it is important that the inspection team has had experience in interpreting data from complex, ~difficulf flaws.
Future developments The D D T results show that defect detection and sizing by ultrasonics can be very effective with good procedures. Some development of detection procedures is required to improve discrimination between significant and trivial signals and to reduce operator involvement in these decisions. It is considered that an important requirement is to improve the reliability of data interpretation and flaw classification. It is clear that techniques exist which are capable of accurately sizing most flaws but that in some instances, for example in the case of complex flaws. interpretation can be difficult Studies are required to reduce the possibility of error at the flaw sentencing stage and these should include a detailed study of the interaction of ultrasound with relevant flaw types to give reliable characterization of flaws. There is a requirement to develop systems which will apply these methods on real plant. The developments will need to produce rugged manipulators, which can move probes with the precision required for the application of techniques such as SAFT and to permit the correlation of results from diverse techniques. Also, the data processing and display equipment must be reliable for field applications. As the systems and techniques become available in industry there must be suitable training for inspection teams, which will allow the expertise gained in laboratory studies on complex, 'difficulf flaws to be transferred.
Authors The authors are at UKAEA Risley Nuclear Power Development Laboratories, Risley, Warrington WAg 6AT, UK
References I
The UKAEA De~,ct Dewction Trialx Bir~hwood Warring/on, UK (7-8 October 1982) 2 Watkins, B., Ervine, R.W. and Cowburn, ICJ. "The UKAEA l)cfect Detection Trials" Brit d NDT, 25 No 4 (July 1983) pp 179-185 3 Crutzen, S. et al "Destructive examination of test plates I and 2 of the Defect Detection Trials', Brit J NI)T, 25 No 4 (July 1983) pp 193-194 4 Watkins, B. 'The UKAEA Defect l)clection Trial programmes and tl sun]mary of the inspection rcsuhs', lnt A),mp NDE Reliabilio~ Portland. Oregon, USA (June 19-21 1983) chapter 7.1 5 Wustenberg, H. and Mundry, E. 'Limiting inlluences on the reliability of uhrasonic in-service inspection methods'. Cot!f Periodic Inspection of Pressurised Component,,~ I Mcch E., London (4-6 June 1974) C112/74 6 Barbian, O.A. et al 'A second view of the German results obtained in the Defect Detection Trials'. Brit J NDT. 26 No 2 (Feb. 1984) pp 92-96 7 Bowker, K.J. et al "CGB inspection of plates I and 2 in the UKAEA Defect Detection Trials'. Brit .1 NDT. 25 No 5 (Sept. 83) pp 249-255 8 Murgatroyd, R.A. "Automated defect location and sizing by advanced ultrasonic techniques'. 5th In/ Conf Quantitative NDE in the Nuch, ar Industry: San Diego, CA. USA. (1(I-13 May 1982) 9 Langston, D.B. and Wilson, R. 'The UKAEA l)cfcct Detection Trials: The application of some results to model validation and assessment of technique capability" CEGB Report TPRD/B/()465/R84 1{I Gardner, W.E. and Hudson, J.A. "Ultrasonic inspection of thick section pressure vessel steel by the time-otZflight diffraction method' 5th lnt ('onf Quantitative NDE in the Nuclear lndust~', San Diego. CA, USA 1111-13 May t982) 11 Temple, J.A.G. 'The amplitude of ultrasonic time-of flight signals compared with those from a reference rctlcctor', lnt J Press Vess and Piping, 16 11984) pp 145-159 12 Rogerson, A. et al qnspection of Defect Detection Trials plate 3 by the Materials Physics Department, RNU, Brit J NDT. 26 No I (Jan. 1984) pp 20-26
Paper received 20 May 1985
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NDT INTERNATIONAL. AUGUST 1985