Improvement in repeatability of ultrasonic array imaging in materials with high structural noise

Developments of automated systems in recent years have improved fast non-destructive evaluation (NDE) of complex structures. This opens up the possibility of looking for small changes between repeat measurements, either to increase the detectability of small defects, or to more accurately characterise the growth of known defects. In this paper, we study the effect of positioning inaccuracy, array element variability, and couplant wave velocity variability on inspection repeatability performance. We propose a post-processing approach to compensate for the degradation in repeatability due to changes in these parameters between inspections. A two-medium, ray-based, forward model is used to quantitatively study the effect of these parameters on inspection repeatability, and the results show a good agreement with experiments. The study indicates positioning inaccuracy and couplant wave velocity variability are the dominant factors affecting inspection repeatability and that the effect of element variability is negligible. We find the repeatability is profoundly sensitive to positioning inaccuracy, which requires a method to measure and correct for the difference between nominally identical inspection positions. Modest differences in wave velocity in the couplant can be tolerated, provided that the relative difference in true couplant wave velocity and that assumed in imaging calculations is preserved. The proposed compensation approach is shown to offer significant performance advantages for baseline subtraction and early-stage crack detection.