Simulating acoustic emission: The noise of collapsing domains

Abstract

Microstructural changes during mechanical shear of a ferroelastic or martensitic material and their signature in acoustic emission (AE) spectroscopy during strain-induced yield and detwinning are investigated by computer simulation. Complex domain patterns are generated during the main yield event, which leads to large displacements of surface atoms and emission of acoustic waves. Loading beyond the yield point leads, eventually, to a simplification of the domain patterns by local movements of needle domains, the nucleation and movement of kinks in domain walls, and the collapse of domains spanning the entire sample (from surface to surface). These microstructural changes lead to much weaker acoustic emissions than those near the yield point. Nucleation/collapse during a yield event involves an energy drop of some 3.7 meV/atom; the collapse of spanning domains releases 0.56 meV/atom, a kink crashing into the surface changes the energy by 0.017 meV/atom, and the collapsing vertical needle changes the energy by 0.017 meV/atom. All these energy bursts can, in principle, be seen by AE. The large energy spread means that AE spectroscopy measures a mixture of events whereby weak and strong signals may signify smaller and bigger events of the same kind or different microstructural changes with intrinsically different signal strengths. In order to disentangle the various contributions, other observables are needed, such as the time-dependent strain matrix of the deformed sample.