The characterization of deformation stage of metals using acoustic emission combined with nonlinear ultrasonics

This paper introduces a new acoustic emission (AE) characteristic to determine the deformation stage of metals under quasi-static loading. The AE characteristic is extracted from the derivative of the cumulative AE energy with respect to time. The reproducibility of the developed feature is demonstrated on a set of A572 Grade 50 steel coupons loaded up to different levels of plastic deformation, and then evaluated using the AE data recorded from aluminum 1100. To address the prior load-dependence of AE data, nonlinear ultrasonics is studied to quantify the presence of plastic deformation as a way to determine the initiation strain during quasi-static load testing typically used for evaluating metallic structures with the AE method. The sensitivity of nonlinear ultrasonics is compared with the linear ultrasonics showing the higher sensitivity of third harmonic nonlinearity coefficient to plastic deformation. The microstructural changes affecting ultrasonics are identified with metallographic characterizations.