The effect of normal compressive stresses on the fracture behavior of mode II crack under compression and shear loads

For the purpose of investigating the influence of normal compressive stresses on the fracture behavior in model II cracks, this study used a single cleavage (SC) specimen, which was made of the sandstone. Shear experiments under different normal compressive stresses were conducted by using a rock deformation test system. So as to study the mechanism of crack propagation under different compressive stresses, the fractured surfaces of the SC specimens after tests were scanned by using an electron microscope. The results show that the normal compressive stresses have a confining function on crack initiations and expansions. With the increase of the normal compressive stresses, the crack initiation angles decrease, whereas the critical shear loads increase. The critical stress intensity factors (SIFs) in mode II increase with the increase of the normal compressive stresses, and their relationship can be expressed by a straight line. This shows that under shear stress and normal compressive stress, the second fracture toughness is not a constant, but increases with the increase of normal compressive stress.