A series of unique laboratory uniaxial compression tests, referred to as incremental damage tests, were performed to measure the degree of stress-induced brittle fracture damage as a function of the applied load. Strain gauge and acoustic emission measurements were combined with elements of monotonic and cyclic loading in an attempt to isolate several stages of brittle fracture development and to quantify the corresponding permanent strain damage. This paper reports the findings for two sets of these tests performed on samples of Lac du Bonnet granite using two distinct load paths. Results show that measured increases in permanent axial strain correspond to crack development stages that significantly influence the axial component of deformation (i.e. crack closure, crack coalescence and crack damage). Similarly, increases in permanent lateral strain coincide with the crack initiation, secondary cracking and crack coalescence thresholds, crack development thresholds marking the initiation and/or opening of cracks. Results also suggest that in tests where elements of time are considered in the loading process, a significant percentage of brittle fracture damage can occur during periods of constant load. In these tests, measured creep strains were attributed to the continued initiation, propagation and coalescence of cracks due to the slow dissipation of induced strain energy levels.
Failure mechanisms in highly stressed rock are largely controlled by the progressive development of stress-induced brittle microfractures. Numerous laboratory-based studies have been conducted to better understand the nature of these processes, particularly with respect to identifying the different stages of crack development leading to intact rock failure (e.g. Bieniawski, 1967; Martin and Chandler, 1994; Eberhardt et al., 1998). These stages generally include, with increasing compressive load, crack closure, crack initiation, stable crack propagation, crack coalescence and unstable crack propagation.
