Effects of pre-static loads on the dynamic pure mode I and mixed mode I–II fracture characteristics of rocks

Rocks in deep underground engineering are prone to encounter pre-static loads and dynamic disturbances simultaneously. Meanwhile, pure mode I and mixed mode I–II fractures are common in brittle materials including rocks. Understanding the mode I and mixed mode I–II fracture mechanisms of rocks subjected to coupled static-dynamic loads is thus imperative and beneficial to the stability of underground rock engineering structures. In this study, using the improved split-Hopkinson pressure bar (SHPB) loading system, coupled static-dynamic mode I and mixed mode I–II fracture tests are conducted on notched semi-circular bend (NSCB) granite specimens. With the aid of high-speed digital image correlation (HS-DIC), 3D scanning technology and scanning electron microscope (SEM), the effects of pre-static loads on dynamic fracture characteristics of granite are comprehensively investigated. Experimental results show that both the dynamic and total fracture toughness reveal evident rate dependence. At a fixed loading rate, the dynamic fracture toughness decreases with increasing pre-static loads, while the total fracture toughness barely changes, indicating that the enhanced rate sensitivity compensates for the weakening effect induced by the pre-static load. The progressive fracture progress shows that with the increase of pre-static loads, the crack initiation time of the specimen is shortened and the crack propagation velocity increases. 3D scanning results show that the fractal dimension of the fracture surface continues to rise as the pre-static load increases, illustrating the fracture surface becomes rougher. In addition, the connection between macroscopic fracture characteristics and microscopic fracture mechanism of granite subjected to static-dynamic loading is discussed by SEM. The quantity and size of the micro-defects inside the internal structure of rocks increase with increasing pre-static load, revealing that the pre-static load degrades rock structure and leads to a negative response to dynamic fracture toughness.