New approaches to model-based ultrasonic flaw sizing

Abstract

Ultrasonic equivalent flaw sizing methods have been recently used to size a flaw in a material by obtaining a best-fit simple equivalent shape that matches the ultrasonic scattering data. However, current ultrasonic equivalent flaw sizing methods have a number of important limitations: (1) they are both iterative and highly nonlinear in nature, and (2) they require the availability of flaw classification information. Here, a series of approaches are outlined that address both of the above problems. Both numerical and experimental results that validate these new approaches are also given. First, when the flaw shape is determined in terms of a best-fit equivalent ellipsoid, a new linear least-squares/eigenvalue method is described that can replace existing nonliner routines and provide a computationally fast and robust sizing procedure. Second, it is shown that if the shape is determined instead in terms of an expansion in spherical harmonics, the sizing problem again reduces to a simpler linear least-squares/differentiation process. Third, a way to do equivalent flaw sizing even when flaw classification information is not present is demonstrated. When such classification information is available, however, one can see how it can be used to improve sizing estimates for cracks, by the elimination of systematic measurement errors due to finite transducer bandwidth effects.