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 alpha, bridge angle beta (inclination between the inner tips of the two preexisting cracks), and the frictional coefficient mu 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 alpha-beta spaces are proposed for different values of mu (= 0.6, 0.7 and 0.9). In general, the S-mode mainly occurs when alpha = beta or when beta < beta* (alpha, mu) = alpha - b alpha, with both alpha and b depending on mu; the M-mode dominates when beta(L) > beta > beta* (alpha, mu) (where beta(L) approximate to 82.5 degrees); and the W-mode is only observed when beta > beta(L). However, more experiments are still required to refine the classification. The observed peak strength, in general, increases with mu. Our results show that the peak strength predicted by the Ashby and Hallam (1986) model basically agrees with experiments. A minimum occurs at about alpha = 65 degrees when the peak strength is plotted against alpha. For alpha > 45 degrees, the peak strength is essentially independent of the bridge angle beta. (C) 1998 Elsevier Science Ltd.