A novel testing method for examining mode II fracture of rock and its application

Abstract

Mode II fracture often occurs in rock, which often result in violent ruptures of rock and catastrophic failure of rock engineering. However, attempts to replicate mode II crack extension on a laboratory scale generally fail due to the existence of tensile stress at the notch tip. In this study, a novel testing method named the Z-shaped centre cracked direct shear (ZCCDS) test was developed to examine the mode II fracture of rock, and a series of tests were performed on sandstone with various dislocation ratios (α1) and crack ratios (α2). The equation for calculating the mode II fracture toughness (KIIC) of rock was determined using the finite element method. The validity and reliability of this test method were verified by digital image correlation analysis and comparison with KIIC values determined by the other methods. The testing results showed that the peak shear load changes slightly with increasing α1 and decreases with increasing α2. The mode I stress intensity factor (SIF) decreases while mode II SIF increases with increasing α1 or α2. When α1 = 0.25 and α2 = 0.3, relative sliding deformation at the notch tip was captured via displacement vector analysis, corresponding to a true KIIC of 4.16 MPa∙m0.5. In addition, the effect of α1 and α2 on the failure characteristics of ZCCDS sandstone specimens is slight, and the pure mode II crack of the sandstone satisfies “self-similar extension”. The fracture surface becomes smoother with increasing contribution of the mode II component due to the wide distribution of transgranular fractures. The findings of this study can facilitate better understanding of mode II fracture behaviour of rock.