Mixed mode dynamic fracture toughness of Crystalline and Granular rocks: Implications for off-fault damage styles

Coseismic fracture and fragmentation of fault core and damage zone rocks is controlled by the micromechanics of dynamic fracture propagation; however, the near-tip character of cracks under impulsive, mixed mode loading conditions expected during earthquake rupture are as of yet poorly constrained. Here, we report on the dynamic, mixed mode fracture toughness of crystalline (Westerly Granite) and granular (Berea Sandstone) materials determined using a notched semi-circular bend specimen and a split-Hopkinson pressure bar apparatus. During experiments, crack histories are tracked using high speed photography and crack-tip velocity fields are measured using digital image correlation (DIC). Dynamic mixed mode I-II initiation toughness ( and ), propagation toughness ( and ) and energy release rate are determined by comparing DIC-derived velocity fields to the theoretical asymptotic solutions for the moving semi-infinite crack via multivariable linear regression. During initial work to implement this method in our lab, we show that the variation of mixed mode contributions to the K-field are much larger than regression error, and the relationship between the total energy release rate and relative contribution fom Mode II loading are fundamentally different in crystalline and granular rocks. For example, with increasing Mode II loading for inclined cracks, the fracture energy release rate increases in crystalline rocks (granite), but it decreases in granular rocks (sandstone). This hints at a fundamentally-different behavior of damage production depending on rock microstructure and may explain why pulverized rocks are commonly recognized principally in crystalline rocks.