The influence of sharp edges in corrosion profiles on the reflection of guided waves

The capacity of guided ultrasonic waves to size real corrosion defects remains challenging due to the complexity of the profiles encountered in practice, defects with large sudden changes in depth over a small circumferential region being particularly problematic. The objective of this paper is to diagnose whether the defect profile changes sharply by studying the frequency dependence of the reflection from the defect by separating the low and high spatial frequency components of defect profiles and analyzing their effect on the reflection coefficient (RC) spectrum. It has been found that the low pass spatial frequencies give significant reflection only at low frequencies; similarly, high spatial frequencies result in significant reflection only at high frequencies. For a defect with a sharp change in depth within a small region otherwise surrounded by general corrosion, the reflection from the high spatial frequencies contributes significantly to the maximum peak of the RC. In contrast, in the case of a more gradual corrosion patch, its high spatial frequencies make only a very modest contribution to the maximum peak RC. Also, the RC calculated by superposition of the low and high spatial frequency content of a defect gives a good approximation to the RC from the full profile. A method to diagnose the presence of sharp edges in corrosion patches based on the analysis of the RC spectrum is proposed, and the limitations of the procedure are defined. The main FE results are validated experimentally. (C) 2012 Elsevier Ltd. All rights reserved.