Shock wave fracturing is a novel technology in coalbed methane (CBM) exploitation. Understanding dynamic damage and failure mechanisms of rock with different grain sizes is of vital importance to shock wave fracturing. In this study, we investigate the influence of grain size on the dynamic compressive strength and failure modes of sandstones under strain rates ranging from 20 to 100 s(-1) and the corresponding mesoscopic responses. The Split Hopkinson Pressure Bar (SHPB) results show that finer-grained samples are mildly fragmented with a moderate strength increase (longer microcracks with lower density) at lower strain rates ((epsilon) over dot = 20-60 s(-1)) but intensively fragmented with a steeper strength increase (shorter microcracks with higher density) at higher strain rates ((epsilon) over dot = 60-100 s(-1)). However, the trend for coarse-grained samples is the opposite. Microcracking characteristics of the tested samples were then obtained using optical microscopy. The microcrack density and the microcrack length may be used to explain the strain-rate dependence of the dynamic compressive strength and fracturing modes of the samples, respectively. These results may be helpful to build a multiscale framework to analyze the natured mechanism of crack production in CBM exploitation. Article Highlights The influence of grain size on the dynamic compressive strength and failure modes of sandstones werediscussed under medium strain rates and the corresponding mesoscopic responses; Finer-grained samples are mildly fragmented with a moderate strength increase at lower strain rates butintensively fragmented with a steeper strength increase at higher strain rates compared with coarser-grained ones; The microcrack density and the microcrack length may be used to explain the strain-rate dependence of thedynamic compressive strength and fracturing modes of the samples, respectively.

This work is published on GEOMECHANICS AND GEOPHYSICS FOR GEO-ENERGY AND GEO-RESOURCES 7.3(2021):1-15.