Failure mechanisms in coal: Dependence on strain rate and microstructure

The brittle coal failure behavior under various axial strain rates from 10^-3 to 10^-2-s^-1 is experimentally and numerically studied. The numerical microscale finite difference model is built on the accurate X-ray microcomputed tomography images, which provides a ground-breaking and bottom-up approach to investigate the effects of microstructure on coal failure under various strain rates. Experimentally, prior to loading, the coal sample is scanned, and the three-dimensional coal structure model is constructed. The microheterogeneous structures are incorporated in the model, which facilitates the deformation and failure mechanism analysis under different loading conditions. The results reveal that the microheterogeneous structures significantly affect the evolution of stress concentrations and deformation behaviors in the sample. The coal tends to fail in the shear mode before the peak strength, since the shear zone is created with high displacements. However, tensile failure ultimately controls the failure process after the peak strength. Notably, the strain rate dependence of coal strength is observed, and an empirical relationship is proposed to describe the dynamic strength of the coal under various loading strain rates. Importantly, the coal strengthens with an increase in strain rate. For brittle material, such as coal, the strength and failure mechanism are strain rate and microstructure dependent. The strain rate-dependent coal strength index (n) is found to be a dynamic parameter in the range of strain rate from 10^-3 to 10^-2-s^-1, and this finding may extend the concept of strain rate dependence over a broader range of loading conditions.