Crack length directivity effects on guided-wave acoustic emission: Numerical investigation of radiation patterns

This paper investigates the effect of a finite-sized crack surface on acoustic emission (AE) generated at the crack tip in a thin metallic plate using a 3D time-domain finite element model. Directivity effects on the first arrivals are of particular focus, and the AE is interpreted in terms of a guided mode decomposition. Crack lengths from 0-10 plate thicknesses are studied, in addition to a semi-infinite crack surface, for frequency content in the neighborhood of 0.1-0.4 MHz-mm. Far-field radiation patterns for the fundamental symmetric (S₀) and shear horizontal (SH₀) guided modes are measured as a function of crack length. The results show an increase in radiation behind the crack tip for both modes and across all considered crack lengths. The surface wave traveling along the crack length appears to be one of the main drivers behind this increase, due to mode conversion after reaching the opposite end of the crack. However, a similar increase is also observed for the semi-infinite crack case, in which there is no mode conversion (the opposite end of the crack is never reached). A radiation offset (RO) metric is introduced to capture this behavior. Parametric studies of the RO across center frequency and bandwidth are presented. Findings suggest that this metric may be of use for AE-based crack length estimation.