Experimental study on progressive failure process and crack evolution of confined red sandstone under impact loading

Rock failure is a progressive process, initiating at the micro-scale and gradually evolving into macroscopic damage over time. While advances have been made in understanding the progressive failure mechanisms of rock under quasi-static loading conditions, the characteristics of progressive failure induced by impact loading remain poorly understood. This study investigated the dynamic failure characteristics of red sandstone through conventional triaxial compression tests using a split Hopkinson pressure bar (SHPB) system under varying confining pressures. The experimental setup incorporated a transparent confining chamber to facilitate high-speed 3D digital image correlation (DIC) measurements, enabling precise quantification of strain evolution during impact loading. Based on the crack/volumetric strain feature, dilatancy pattern, strain field evolution and the specimen morphology after impact, the stress thresholds of progressive failure and crack evolution were characterized and compared to those in quasi-static condition. Key findings demonstrate that the normalized crack initiation stress threshold σ ci / σ peak exhibited a linear increase with confining pressure under dynamic loading (10.6-25.7%), less than half the corresponding values observed under quasi-static conditions. The normalized crack damage stress threshold ( σ cd / σ peak ) demonstrated pronounced pressure sensitivity, increasing from 22.5% to 90.8% as confining pressure rose from 1.0 to 11.0 MPa. Elevated confining pressures effectively inhibited rock dilatancy, with complete suppression of volumetric expansion occurring at 11.0 MPa confining pressure under impact loading conditions. The dominant failure mode of red sandstone under impact transitioned from composite shear-tension failure to singular fracture mode with increasing confinement. • 3D strain evolution of confined rock under impact was monitored by DIC with a transparent chamber • Dynamic crack initiation stress was 10.6∼25.7% peak stress with increasing confinement which is less than half of that in quasi-static cases • Dynamic crack damage stress varied from 22.5% to 90.8% peak stress with the transition of dynamic fracture pattern • Impact-induced dilation was fully suppressed at confinement of 11 MPa • Confined rock after impact contained combined shear cracks, axial and lateral tensile cracks