Investigation on dynamic anisotropy of bedded shale under SHPB impact compression

• Clarify five failure patterns of shales under impact compression. • Reveal the mechanical mechanisms underlying dynamic anisotropy of bedded shale. • Elucidate fundamental laws affecting micro-cracks propagation and fracture morphology. • Summarize the influence of bedding inclination and strain rate effect. Revealing the fracture characteristics of shales under impact is significant for the exploitation and utilization of geological energy. In this study, the progressive failure process of bedded shale was tested by the dynamic Split Hopkinson Pressure Bar system and measured by the high-speed camera. Furthermore, their dynamic mechanical properties and fracture features under impact compression were comprehensively investigated. The results show that according to the triggering mechanisms of micro-cracks and mechanical factors governing fracture morphology, the failure patterns of shales can be categorized into five types: splitting failure along bedding plane, slip failure along bedding plane, mixed shear cracks between layers, tensile-shear mixed cracks, and tensile crack along bedding plane. Meanwhile, when the bedding angle θ = 0°, the sample experiences splitting failure, with acoustic emissions concentrated predominantly at horizontal bedding planes. When θ = 30° and 60°, the sample experiences shear failure characterized by shear cracks, mixed shear cracks and multiple composite tensile-shear cracks. When θ = 90°, the sample exhibits tensile and splitting cracks. With increasing strain rate, the fracture pattern of the 60° sample becomes dominated by slip and tensile-shear mixed cracks. With increasing peak stress intensity, the fracture modes of the 30° and 60° samples gradually transition from center-oriented failure towards shear failure along bedding planes. Additionally, for the 90° specimen, as the shock wave intensifies, secondary splitting cracks emerge in matrix, indicative of a fracture pattern across bedding planes.