Experimental study on time-dependent stress and strain of in-plane shear (Mode Ⅱ) fracture process of rock

摘要

Shear-box test with strain measurement was used to study time-dependent stress and strain of in-plane shear (Mode Ⅱ)fracture process of rock and to reveal the mechanism of Mode Ⅱ fracture. Numerical results show that the maximum shear stress τmax at the crack tip is much larger than the maximum tensile stress σ1 and the ratio of τmax/σ1 is about 5, which favors Mode Ⅱ fracture of rock. Test results indicate that the strain-time curve comprises three stages: the linear deformation stage, the micro-cracking stage and the macroscopic crack propagation. The strain in the direction of the original notch plane is negative, due to restraining effect of compressive loading applied to the original notch plane. Both σ1 and τmax are increased as the load increases, but the slope of τmax is larger than that of σ1 and the value of τmax is always larger than that of σ1. Therefore, τmax reaches its limited value at peak load before σ1 and results in Mode Ⅱ fracture of rock. Shear-box (i.e. compression-shear) test becomes a potential standard method for achieving the true Mode Ⅱ fracture and determining Mode Ⅱ fracture toughness of rock.