Dynamic Damage and Failure of Layered Sandstone with Pre-cracked Hole Under Combined Cyclic Impact and Static Loads

The bedding structure and in situ stress state both play important roles in the design and stability analysis of bedding rock masses. The purpose of this study is to investigate the influence of bedding angles and pre-static loads on dynamic mechanical properties and damage mechanisms of layered sandstone under cyclic impact loads. A series of cyclic impact tests with a constant amplitude were performed on sandstone specimens with different bedding angles under three pre-static loads. Two-dimensional digital image correlation technique was used to investigate the damage evolution and failure modes of the specimens. The results show that the number of resisting cyclic impacts and the total dissipated energy of the sandstone decline first and then increase with the rising of the bedding angles. When the pre-static load increases from 15 to 30 MPa, the total dissipated energy of specimens with different bedding angles under cyclic impacts reduces by 61.4-76.4%; as the pre-static load increases to 45 MPa, the reduction rate becomes 78.1-89.5%. Based on the surface strain field obtained by the digital image correlation technique, a damage factor is used to describe the damage evolution of the sandstone under cyclic impacts. The damage factor and impact time curves can be divided into three stages: the initial damage stage, damage accumulation stage and damage aggravation stage. As the pre-static load increases from 15 to 30 MPa and 45 MPa, the damage threshold, which is the plateau of the damage accumulation stage, decreases from 0.3 to 0.2 and 0.1, respectively.