Study of the deformation characteristics and fracture criterion of the mixed mode fracture toughness of gypsum interlayers from Yunying salt cavern under a confining pressure

Based on the project background of salt caverns used for oil and gas storage in laminated salt rock, this study examined the combined mode fracture toughness, the fracture pattern, the morphological evolution of fracture surfaces, and the fracture criterion of gypsum interlayers under confining pressure. Series of laboratory tests, including fracture strength tests and scanning electron microscopy tests, were performed at four confining pressures. The results indicate that the failure load of gypsum increased with the increase of confining pressure. By contrast, the failure load of specimens first decreased and then increased with the increase of pre-existing flaw inclination angle. As the confining pressure increased, the residual energy gradually decreased and the brittleness index gradually increased. Although increasing the confining pressure (i.e., 1-7 MPa) shifted the behaviour of a specimen from brittle to ductile, the major failure mechanism still fell within the brittle failure regime. For the case where the confining pressures are 1 MPa and 3 MPa, cleavage fractures characterized by cleavage steps, river lines, and tearing ridges were observed on the surface, indicating that typical brittle failure occurred in the specimens. However, typical ductile failure occurred in the specimens subjected to confining pressures of 5 MPa and 7 MPa. Subsequently, the size of the fracture process zone at varying confining pressures was obtained using a modified maximum tangential stress criterion to optimally fit the experimental results. The results of this study can be used to evaluate the safety of salt caverns in bedded salt formations.