An evaluation of the performance of the TOFD technique as a means of sizing flaws, with particular reference to flaws with curved profiles

Simulated TOFD data have been employed as a means of evaluating the ability of the TOFD technique to size flaws in both the through-thickness and length dimensions. The interest in the evaluation arose as a result of an earlier study which suggested that the fundamental precision ofthe technique may be greater than is currently assumed. The current assumptions are based on the results of blind trials and it may be that the difference between the expected and achieved precisions is simply a reflection of the wide natural variability of flaws. On the other hand it may be that the procedures currently employed to estimate the flaw size are insufficient or that the trials themselves incorporate other sources of error. These possibilities were of sufficient interest to warrant a preliminary study of the sizing potential of the technique. The results of the study indicate that the potential of the technique for determining the through-thickness extent of flaws is greater than has been assumed. The results from this trial are consistent with a precision of 0.25% of the specimen thickness, whereas the current assumption is a precision in the region of 2% of specimen thickness. An allowance must be made for the use of simulations rather than real data, but it is difficult to envisage this bridging the gap between these figures. An alternative explanation that should be considered is whether the confirmatory (destructive or other NDT) examinations associated with blind trials are contributing to the apparent errors. Ideally, these should not be producing errors greater than 0.25% of specimen thickness but it is difficult to believe that this could be achieved in practice. As regards flaw length, the results from this study highlight a distinction between flaws with a substantially flat profile (broadly parallel to the surface) and flaws with a continuously curving profile. The former group would include many common flaws such as slag lines and most lack of fusion defects. The latter would include most surface-breaking cracks. Generally, the estimation of flaw length using TOFD is based on assumptions which are correct only for the first class offlaws. For flaws with curved profiles the estimate of length can be quite seriously in error. The results of blind trials, where some attempt has been made to estimate properly the flaw length, tend to suggest an error of about 7 mm to 10 mm. However, it now appears probable that this is a compromise between data for flat profile and curved profile flaws. The former are sized in length to a precision of a few millimetres, whereas the latter may be in error by well over 10 mm. A number of procedures for flaw length estimation have been evaluated in this study and it is shown that the optimum approach is to identify the flaw type and employ a different sizing procedure for each. This would be expected to give an overall precision of better than 4 mm. One empirical procedure appears to provide acceptable length estimates for both types of flaw and the mean error using this would be in the region of 6.5 mm. At first sight the use ofa single procedure appears to have some advantages. However, the identification of flaw profiles is straightforward from TOFD B-scans, so the use of the twin procedures appears to be the optimum under most conditions that can be envisaged. This identification could be made automatic tofit in with modern trends in data analysis.