Nonlinear Acoustics for Microstructural Evolution

Abstract

Acoustic nonlinearity holds the potential of becoming the primary means of characterizing microstructural evolution caused by, for instance, fatigue of metals, because it is capable of probing the processes of dislocation movement (Hikata et al. (1966); Hikata and Elbaum (1966); Cantrell and Yost (1994)) and crack nucleation and growth (Buck et al. (1978); Richardson (1979); Morris et al. (1979); Nagy (1998)). The sensitivity to microstructural attributes during the incubation period of aging is often higher than that of the linear properties (velocity and attenuation) . However, there are several other nonlinearity sources being related to mechanical contact between material’s surface and the transducers. The EMAT has then been expected for the nonlinearity measurement to isolate material’s nonlinearity , but it has failed to excite high enough vibration amplitude in the material for inducing higher harmonics. Electromagnetic acoustic resonance (EMAR) again has overcome this problem by superimposing many ultrasonic waves coherently, and as shown later, three methods for nonlinear acoustics have been emerged.