An experimental study of fracture mechanism and morphology of granite specimens under various dynamic loading rates

Numerous studies have shown that dynamic fracture toughness (DFT) of rock is dependent on loading rate. This paper quantitatively studied the effect of loading rate on DFT from perspective of mesoscopic fracture morphology. First, notched semi-circle bend (NSCB) and short-core-in-compression (SCC) samples of granite were prepared for dynamic mode I and mode II fracture tests. Then, the DFT values of NSCB and SCC specimens at various loading rates were calculated and analyzed. After that, mesoscopic morphologies of failure surfaces of NSCB and SCC specimens under various loading rates were obtained by scanning electron microscope (SEM). The fracture morphology features of specimens were quantitatively characterized by a method combining deep learning and SEM images. The analytical results suggested that as loading rates rose, the increase in the percentage of mesoscopic fracture morphology caused by shear stress (MFM-S) on the fractured surface was the primary reason for the increase of DFT. When dynamic loading varied from 40 to 120 GPa m^1/2 s⁻¹, there was a linear relationship between the DFT and the proportion of MFM-S. Additionally, with an increasing loading rate, the proportion of MFM-S on the fracture surface of NSCB specimens varied slightly more, which can explain why the dependence of DFT on loading rate in NSCB specimens was somewhat more obvious than in SCC specimens.