Strain rate effect on mixed mode I/II fracture toughness of sandstone and its micromechanism

Mixed mode I/II fracture often occurs when rock with pre-existing defects is subjected to dynamic impact. However, the understanding of dynamic mixed mode I/II fracture of rock is still at its infancy. To investigate rate dependent behavior of mixed mode I/II fracture toughness, the split Hopkinson pressure bar tests were per- formed on cracked straight through Brazilian disc sandstone specimens with different loading angles and strain rates. The results show that the peak load and peak strain increase with increasing strain rates while the influence of loading angle is almost negligible. Dynamic mode I and mode II stress intensity factors (SIFs) as well as effective fracture toughness increase rapidly with the increase in strain rate before slow increase as the strain rate exceeds approximately 173 s-1. As the loading angle increases, the dynamic mode I SIFs gradually decreases, while the dynamic mode II SIFs first increases and then decreases. The dynamic fracture initiation toughness increases linearly with increasing strain rates. In addition, a novel methodology for quantitative analysis of rock microstructure was proposed to analyze quantitatively the strain rate effect on microstructure of the sandstone. It is found that with increasing strain rate, the area ratio of pores and microcracks on the fracture surface of sandstone increases, while the area ratio of transgranular fracture almost keeps constant. Pores and microcracks are firstly activated by the high-strain-rate loading, and then a complex interconnected crack system is formed, thereby enhancing the dynamic fracture toughness of sandstone. The findings of this paper could facilitate better understanding the rate effect of mixed mode I/II fracture and its micromechanism.

Automated summary

This study investigates the rate dependent behavior of mixed mode I/II fracture toughness in rock. Split Hopkinson pressure bar tests were performed on cracked sandstone specimens with different loading angles and strain rates. The results show that the peak load and peak strain increase with increasing strain rates while the influence of loading angle is almost negligible. Dynamic mode I and mode II stress intensity factors as well as effective fracture toughness increase rapidly with increasing strain rate before reaching a plateau. The findings of this study contribute to a better understanding of the rate effect of mixed mode I/II fracture and its micromechanism.