Investigating the crack initiation and propagation mechanism in brittle rocks using grain-based finite-discrete element method

Fracturing process and possible factors influencing crack initiation, propagation and coalescence of granitic rocks are investigated using a grain-based finite-discrete element method (GB-FDEM). In contrast to conventional methods, the GB-FDEM used herein consists of dual-scale contact models ensuring grain breakage, and pre-processing scheme for reproducing the realistic micro heterogeneity of rocks. A standardised calibration procedure is proposed after an analysis of uncertain parameters. An optimized model is built according to calibration results of benchmark experiments including uniaxial compression test, confined compression test and Brazilian disc test in laboratory. The compression testing under different end friction, slenderness, loading rate and confining stress are systematically performed to investigate the external influences on rock fracturing. Boundary constraint is revealed in association with the macro failure pattern, in which the shearing slide leads to slight extension on ends and causes limited influence on the stress distribution in the far field. Effects on crack stresses are consistent with that of uniaxial compressive strength when the end friction takes effect. Slenderness affects the stress distribution and in turn changes the fracture pattern of rocks. Slight influence on the stress level of crack initiation and crack damage can be caused by the change of height-to-width ratio. Loading rate dramatically increase the rock strength based on two underlying mechanisms that the increase in overall number of micro cracks and the transition from intergranular fracturing to transgranular fracturing. These effects on crack initiation and crack damage stresses are inconsistent because the responses of normalized crack initiation and crack damage stresses subject to strain rate have inverse responses. The proportion of different micro cracks is shown to characterize the unchanged micro fracturing when the confining stress is increased. The enhancement of crack initiation and the change of the pattern of crack coalescence are attributed as the mechanism of confinement effect.