Influence of bedding planes on fracture characteristics of coal under mode II loading

Due to bedding planes and natural fractures, the mechanical behavior of coal under different loading modes is usually anisotropic. The anisotropy usually influences the strength and stability of coal and rock mass in engineering. In this research, we used the asymmetric semi-circular bend (ASCB) method to carry out fracture tests of coal specimens with various bedding angles (0 degrees, 22.5 degrees, 45 degrees, 67.5 degrees and 90 degrees) between loading direction and bedding planes under mode II loading. The influence of the bedding angle on the fracture toughness and fracture pattern of coal specimens under mode II loading was investigated. Moreover, based on the acoustic emission (AE) technology and digital image correlation (DIC) technique, the influence of the bedding plane on the failure process of coal specimens was studied. The research results indicate that the effect of the bedding plane is significant. The apparent mode II fracture toughness decreases and then gradually increases with the increase of bedding angle, picturing a check mark shape. The minimal value was obtained at the bedding angle of 22.5 degrees. The obtained fracture patterns can be classified into fracture growth along the bedding and across the bedding. The influence of the bedding plane on the crack propagation is the most obvious at the bedding angle of 22.5 degrees and 45 degrees, where the cracks mainly grow along the bedding planes. The failure process of ASCB coal specimens can be divided into the compaction stage, the stable crack propagation stage, the rapid crack propagation stage, and the unstable crack propagation stage. The bedding angle has more influence on the crack initiation load while has less influence on the crack damage load between 80 %P-max to 90 %P-max.