Grain-size dependence of acoustic emissions in oscillatory crack propagation

Acoustic emission (AE) exhibits a strong dependence on grain size during fracture propagation. However, the underlying mechanism of grain size-dependent AE remains unclear. In this study, we employ an experimentally validated peridynamics model to systematically investigate how grain size influences AE. The frequency content of AE waveforms was identified using the Boatwright spectral model, and waveform dispersion and frequency shifts with particle spacing (lattice constant in PD) were evaluated. AE energy was calculated using a local method (based on point-sensor measurements) and a global method (based on system-wide particle vibrations). Rather than directly equating particle spacing in PD with physical grain size, the particle spacing is considered as a characteristic fracture length scale that governs crack propagation dynamics. Results demonstrate that AE radiation is strongly controlled by crack velocity oscillations induced by this length scale, leading to a clear scaling relationship between characteristic length scale and AE energy. This behaviour is consistent with experimental observations in rock materials, where grain size acts as a characteristic microstructural length scale governing crack propagation and acoustic emission. This was confirmed by both the crack kinetic energy and perturbation theory. These findings provide a new perspective on the relationship between AE, crack speed and grain size.