Mechanical behavior and failure mechanism of composite layered rocks under dynamic tensile loading

Abstract

To bolster the safety and efficacy of blasting and impact drilling for underground works in composite rock strata, the dynamic tension of composite rocks requires thorough examination. Given the challenges of composite rock sampling, cement mortars with varying mix ratios are utilized to create composite rock-like test blocks. These blocks are composed of two different strength materials, further processed into differently sized experimental samples as required. Using a split Hopkinson pressure bar (SHPB) test device, we conducted dynamic Brazilian splitting tests at varying incident angles α (the angle between the incident wave direction and the bedding plane) and strain rates on these composite rock-like samples. The modes of crack propagation in some samples were recorded with a high-speed camera. Laboratory tests and PFC2D simulation were used to analyze the dynamic tensile mechanical properties and failure mechanisms of composite rock samples. Findings indicate the dynamic tensile strength of a composite rock sample is governed by the bedding plane inclination angle. As the incident angle rises from 0° to 90°, the dynamic tensile strength of the composite rock sample first decreases, then increases, consistently reaching a minimum value at α = 30°. The damage model of composite rock samples is also influenced by the dip angle of the bedding planes. At α = 0°, the composite rock sample exhibits through-cracks along the incident wave direction and splitting tensile damage is observed. As α increases, the failure mode gradually transitions from splitting tensile damage to a combination of tensile and shear slip damage along the bedding. Tensile damage always manifests in materials of lower strength. When α surpasses 45°, the failure mode reverts from combined damage to splitting tensile damage. Cross-sectional tensile stress clouds generated by PFC software demonstrate that the tensile stress contours at the specimen's center are elliptically distributed, with the ellipse's long axis nearly parallel to the bedding plane.