Experimental study on the failure and dynamic tensile strength of layered sandstone under impact loads

Due to the anisotropy of layered sandstone,the deformation characteristic of layered sandstone with different bedding angle under impact loads is investigated. The standard sandstone samples with weak layer were fabricated,the dynamic Brazilian disc test of sandstone with different bedding angles is conducted using the split Hopkinson pressure bar (SHPB),and the evolution of strain field of specimen during fracturing is obtained combined with digital image correlation method. From the failure of sandstone,it shows that the angle between bedding plane and loading direction significantly affects both the deformation and the failure of layered sandstone. When the weak plane is parallel to the loading direction,a strain concentration zone is first generated near to the loading end,and rapidly propagate along the weak plane under impact loads,which eventually presents an cambered shape crack. When the weak plane is perpen- dicular to the loading direction,multiple strain concentration areas are first formed at the middle of the disc specimen, which indicates that micro-cracks first initiate on the middle of the specimen,then grow continuously and coalesce,and eventually form a macro crack parallel to the loading direction. When the loading direction is at 45° to the weak plane, the significant mixed tensile and shear strain concentration is formed along the weak plane at the loading end of the specimen,and induces micro-cracks initiation from the loading end and propagates along the weak plane due to a rela- tively lower tensile stress and shear stress of layered media compared with the rock media. Under the same loading rate,the strength of specimen with vertical weak plane is highest,while the strength of specimen with parallel weak plane is lowest,and the strength of specimen with oblique weak plane falls between the strength of specimen with hori- zontal weak plane and the strength of specimen with vertical weak plane. The dynamic tensile strength of layered sand- stone increases linearly with the loading rate,which shows less sensitive to the loading rate compared with the sand- stone without weak plane. Besides,the failure strain of layered sandstone presents significant change with the increase of bedding angle,the crack strain of sandstone with oblique weak plane is higher compared with the sandstone with vertical weak plane.