On tensile fracture of a brittle rock

Three-point bending experiments were performed to observe mode-I fracture in a series of geometrically similar, center-notched rectangular beams composed of Sioux quartzite, a stiff, hard rock with flexural tensile strength σ_t = 32 MPa. Acoustic emission (AE) were recorded and microcrack locations were determined to track crack growth. Digital image correlation (DIC) was used to augment the acoustic emission data. Detailed observations indicate that (i) subsize specimens of Sioux quartzite follow a size effect law, and (ii) linear elastic fracture mechanics (LEFM) can be used to explain crack growth for large specimens of Sioux quartzite, with fracture toughness K_Ic = 2.2 MPa m. In particular, crack lengths based on compliance from LEFM are in good agreement with those obtained from AE and DIC suggesting a small process zone for specimens with beam depth >20 times the characteristic length = (K_Ic/σ_t)². Further, crack velocities predicted from LEFM matched those estimated from AE locations of crack tip position with time. For sufficiently sized specimens, these observations indicate that LEFM can be used to describe fracture in this stiff, hard rock.