Crack coalescence in a rock-like material containing two cracks

This paper investigates the pattern of crack coalescence and strength of a sandstone-like material containing two parallel inclined frictional cracks under uniaxial compression, with changing values of inclination of preexisting cracks α, bridge angle β (inclination between the inner tips of the two preexisting cracks), and the frictional coefficient μ on the surfaces of the preexisting cracks. Three main modes of crack coalescence are observed: the shear (S) mode (shear cracking between the two preexisting cracks); the mixed shear/tensile (M) mode (propagation of both wing and shear cracks within the bridge area); and the wing tensile (W) mode (coalescence of wing cracks from the tips of the preexisting cracks). The M-mode and W-mode of crack coalescence can further be divided into two and six types, respectively. Simple regime classifications of coalescence in the α-β space are proposed for different values of μ (=0.6, 0.7 and 0.9). In general, the S-mode mainly occurs when α = β or when β < β* (α, μ) = a - bα, with both a and b depending on μ; the M-mode dominates when βL>β>β* (α, μ) (where βL≈82.5°); and the W-mode is only observed when β>βL. However, more experiments are still required to refine the classification. The observed peak strength, in general, increases with μ. Our results show that the peak strength predicted by the Ashby and Hallam (1986) model basically agrees with experiments. A minimum occurs at about α = 65° when the peak strength is plotted against α. For α>45°, the peak strength is essentially independent of the bridge angle β.