A VARIABLE VELOCITY APPROACH TO LOCATE FATIGUE-INDUCED MICROCRACKS OCCURRED IN STRUCTURES WITH MULTIPLE MATERIAL LAYERS

摘要

In most AE source location applications, a constant signal velocity (or global velocity) is assumed in the Geiger's algorithm, which uses derivative information to represent the propagation of acoustic emission (AE) signals and is widely used to locate crack sources. However, the actual signal velocities vary notably from the constant velocity when the testing structures are composed of multiple material layers. This velocity variation is known to induce substantial location error and divergence in the computation. The present work proposed a variable velocity approach, which assigns a specific bulk velocity to the AE signal waves received at each sensor. In this approach, first an initial signal velocity is assumed for each sensor based on its relative location to a guessed microcrack; this initial velocity is then adjusted in the following computation according to the residual of arrival time. The performance of variable velocity approach in terms of convergence in computation was discussed. Using this approach, we studied fatigue-induced microcracks cumulated in a cemented total hip arthroplasty (THA) specimen. It was found that the static location error was reduced from 7.1 mm to 4.7 mm in pencil-lead break (PLB) tests. In the fatigue experiments performed on the specimen, we found this approach reduced computational divergence with respect to using the constant velocity approach, and achieved higher accuracy in microcrack source locations. The results indicated that the proposed variable velocity approach has significant advantages over the constant velocity approach when a specimen is composed of multiple material layers.