Crack coalescence in a rock-like material containing two cracks

Abstract 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−b α , with both a and b depending on μ ; the M-mode dominates when β L > β > β *( α , μ ) (where β L ≈82.5°); and the W-mode is only observed when β > β L . 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 .