Mode-I fracture toughness under intermediate level dynamic loading for granite and sandstone

Rock engineering deals with a material containing multiple fractures. The critical value of stress intensity factor when a pre-existing flaw begins to develop is defined as fracture toughness. Thus, fracture toughness measurement is essential to support rock engineering design. Numerous works have revealed that fracture toughness of rock under dynamic loading is significantly different from that under quasi-static loading. However, the data within intermediate level dynamic loading, which is defined as the loading ranging between quasi-static and dynamic loading, is still limited. A dedicated loading device generated by a non-explosive powder reaction was designed to determine dynamic fracture toughness under intermediate dynamic loading. Compared to a Split-Hopkinson pressure bar, which is commonly used in dynamic testing, the device adopted by this study provides a lower loading rate. A notched semi-circular bend specimen type was selected to measure mode-I fracture toughness. The specimen was made of granite and sandstone. The results of dynamic loading test were compared with quasi-static properties tested using the hydraulic servo-controlled machine. It indicates that fracture toughness rises with increasing of loading rate. The values were also plotted together with the others’ findings and showed an acceptable agreement. Failure behavior corresponds to a numerical simulation performed by Ansys Autodyn. This study contributes to the current understanding of rock dynamics, particularly in the intermediate dynamic loading range that can be useful in assessing field engineering application such as rock excavation process.