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

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. fracture. Numerical results show that the maximum shear stress tau(max) at the crack tip is much larger than the maximum tensile stress sigma(1) and the ratio of tau(max)/sigma(1) 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 sigma(1) and tau(max) are increased as the load increases, but the slope of tau(max) is larger than that of sigma(1) and the value of tau(max) is always larger than that of sigma(1). Therefore, tau(max) reaches its limited value at peak load before sigma(1) 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.