Size effect of mode II fracture characteristics of a typical sandstone using the short beam compression specimens

Shear-sliding mode fracture, i.e., mode II fracture, is a very common rupture mode of rocks. The size effect on the mode II fracture of rocks is critical yet remains underexplored. This study investigates the size effect on the mode II fracture behavior of sandstone using short beam compression specimens with four different lengths. Uniaxial compression tests combined with digital image correlation (DIC) were employed to characterize the mode II fracture toughness (KIIC and KIIC0; the former is defined as the experimentally measured mode II fracture toughness, and the latter as the actual mode II fracture toughness without the confinement effect) and fracture process zone (FPZ) evolution. Results revealed that KIIC increased significantly with specimen size, where KIIC increased by approximately 97% as the specimen length expanded from 10 to 50 mm. The FPZ length measured via DIC increased from 1.77 to 5.25 mm as the specimen size increased, while the normalized FPZ length decreased from 0.89 to 0.53, indicating reduced relative influence of the FPZ on fracture behavior for larger specimens. The analysis of strain evolution during crack propagation revealed that smaller specimens formed a continuous high strain band at an earlier loading stage compared to larger specimens. Additionally, comparative analysis showed FPZ lengths obtained by DIC were larger than the finite element model and stress intensity factor method predictions due to inelastic damage evolution and stress redistribution in the FPZ during loading. These findings are essential in evaluating the safety and stability of rock engineering structures.