Study on crack growth behaviour in rocks having pre-existing narrow flaws under biaxial compression

摘要

The analysis of crack growth on a rock with pre-existing flaws is mostly carried out in open flaw configurations whereas limited study can be found for narrow flaw configurations. Unlike open flaw, narrow flaws are initially open and subsequently gets closed during loading. It also experiences friction which needed to be accounted for. Very few studies are available, to understand the effect of friction between the flaw surfaces. Additionally, the lateral confinement due to the biaxial stress state would influence the crack initiation and their subsequent growth. In the present study, an experimental investigation is conducted to understand the crack growth behaviour of narrow flaws under biaxial stress environments. A novel biaxial compression test setup is developed using conventional triaxial cells. The tests were conducted using gypsum having uniaxial compressive strength of 15.5 MPa and Young's modulus of 1.037 GPa, with a single flaw. The specimen's size effect due to lateral confinement is investigated by varying the ratio of the specimen width to flaw length. The change in the behaviour of crack growth and its crack stresses with respect to initiation and peak level was studied for different flaw angles. Subsequently, the numerical analysis was performed using the Extended Finite Element Method (XFEM) in conjunction with the Cohesive Zone Model (CZM) systematically validated with laboratory experiments. Analysis was further extended for higher confinement levels to know the effect of crack patterns and their stress behaviour. The study observed a change in critical flaw angle at which sooner initiation happens when the confinement increases. Hence, there was a change in crack initiation stress and their corresponding peak stress behaviour with respect to flaw angles when the lateral confinement increases. Also, the crack pattern was found to be influenced by various factors such as the ratio specimen width to flaw length, frictional coefficient between the flaw surface and the confinement stress level.