Mixed mode I/II fracture behavior and surface morphology of hard rock under dynamic loading

To investigate the dynamic mixed mode I/II fracture properties of hard rock, split-Hopkinson pressure bar (SHPB) was utilized to conduct dynamic fracture tests on cracked straight through Brazilian disc (CSTBD) diorite specimens with different mode mixities. The dynamic fracture load and fracture toughness for different crack angles or mode mixities were determined. The dynamic fracture process and crack mouth opening displacement (CMOD) of CSTBD specimens were monitored by high-speed camera and digital image correlation (DIC) tech-nique, and the fractal and morphology characteristics of fracture surfaces were further analyzed using laser 3D scanner and scanning electron microscope (SEM). The test results indicate that the dynamic CMOD behavior is interrelated closely to the force acting on the crack surface, and the allowable CMOD corresponding to the dynamic peak load decreases with increasing crack angle. In addition, the stress-based and strain-based fracture criteria were introduced to analyze the mixed mode fracture mechanism of diorite. It is found that the GMTS criterion with small fracture process zone (FPZ) size and the EMTSN criterion have the best prediction effect for fracture toughness and crack initiation angle respectively. Observation results show that the macro fracture trajectories of primary cracks and secondary cracks are controlled by the prefabricated crack angle, while the variation in mode mixity has little impact on the meso-roughness and micro-morphology of the fracture surface. The fracture surface of CSTBD diorite specimen under dynamic load exhibits the SEM morphology features of mixed tensile-shear cracks.

Automated summary

The dynamic mixed mode I/II fracture properties of hard rock were investigated through split-Hopkinson pressure bar tests on cracked straight through Brazilian disc (CSTBD) diorite specimens. The fracture load and toughness for different crack angles or mode mixities were determined. The dynamic fracture process and crack mouth opening displacement (CMOD) were monitored, and the fractal and morphology characteristics of fracture surfaces were analyzed. The study found that the CMOD behavior is closely related to the force acting on the crack surface, and the allowable CMOD decreases with increasing crack angle. Stress-based and strain-based fracture criteria were introduced, and the GMTS criterion and EMTSN criterion showed the best prediction effect for fracture toughness and crack initiation angle respectively. Observation results showed that the macro fracture trajectories were controlled by the prefabricated crack angle, while the mode mixity had little impact on the meso-roughness and micro-morphology of the fracture surface. The fracture surface of CSTBD diorite specimen exhibited mixed tensile-shear cracks under dynamic load.