Strain Rate Effects on Characteristic Stresses and Dynamic Strength Criterion in Granite Under Triaxial Quasi-Static Compression

To investigate the effects of the strain rate and confinement on characteristic stresses and strength criterion in granite under static to quasi-static loading, triaxial compression tests were systematically conducted across strain rates of 10−6 to 10−2 s−1 and confining pressures of 0–40 MPa. Stress–strain curves, characteristic stresses, macro-fracture patterns, and dynamic strength criterion were analyzed. The experimental results indicate the following: (1) crack damage stress (σcd) and peak stress (σp) show strong linear correlations with logarithmic strain rate, while crack initiation stress (σci) exhibits weaker rate dependence; (2) linear regression establishes characteristic stress ratios σci = 0.58σp and σcd = 0.85σp; (3) macroscopic fractures transition from Y-shaped shear patterns under low confinement and strain rate conditions to X-shaped shear failures at higher confinement and strain rate; (4) the Mohr–Coulomb criterion effectively characterizes dynamic strength evolution in granite, with cohesion increasing 22% across tested strain rates while internal friction angle remains stable at around 50°; (5) variations in microcrack activity intensity during rock deformation stages result in the dynamic increase factor for characteristic stresses (CSDIF) of σci being lower than σcd and σp. More importantly, σcd and σp exhibit CSDIF reductions as confining pressure increases. This differential behavior is explained by confinement-enhanced shear fracturing dominance during crack propagation stages, combined with the lower strain rate sensitivity of shear versus tensile fracture toughness.