The impact of normal stress, along with the material composition and shear velocity, on both the steady-state shear resistance and viscosity of rapid dry granular flows

Research on granular material flows has gained significance due to their critical role in various industrial applications and processes occurring on planetary surfaces. However, experimental studies examining granular flows under high-stress conditions where significant grain breakage occurs, as seen in phenomena like rock avalanches, fault ruptures, and post-impact crater formations are relatively scarce.   This study presents findings from high-speed rotary shear experiments conducted on eight types of crushable granular materials and non-crushable materials, exploring different shear velocities and normal stress levels. We analyzed variations in shear resistance and viscosity during the experiments. After undergoing large strains, both shear resistance and viscosity stabilized, exhibiting independence from normal stress and material composition, but showing dependence on shear velocity. Our investigation identified two distinct behaviors: the strain-hardening regime and the strain-weakening regime. For crushable materials, there was a general trend towards velocity hardening at shear velocities below 0.1 m/s. However, a notable power-law weakening in steady-state shear resistance was observed with increasing velocity for shear rates exceedingly approximately 0.1 m/s, signaling potential material instability. Similarly, non-crushable glass beads displayed a comparable response. In the strain-weakening regime, all crushable materials adhered to a common set of power-law relationships, while non-crushable materials followed a different set. The transition from the strain-hardening regime to the strain-weakening regime can elucidate the onset of rock avalanches following prolonged creep deformation. Additionally, the pronounced weakening observed at higher velocities accounts for the enhanced fluidity and hypermobility characteristic of large geophysical grain flows. Under conditions of high-speed shear, the steady state of granular flow demonstrated that normal stress and material composition do not influence shear resistance and viscosity. However, the rate of weakening and the  slip weakening distance are affected by these factors and are correlated with WEIBULL modulus.