An extension of the Griffith theory of fracture to rocks

The Griffith fracture has recently been extended by McClintock and Walsh to include the closing of Griffith cracks during compression with the development of frictional forces along crack surfaces. This modified Griffith theory can be used to predict failure of any brittle material, such as rock, for a general stress state. The predicted failure condition is shown to be of the form ±τ = 2K − μσ in the Mohr diagram (in the region of compression), where K is the tensile or one-eighth of the compressive strength of the material and μ is the coefficient of sliding friction at crack surfaces. For a coefficient of friction of 0.8 to 1.0 this failure condition is nearly identical with the empirical Coulomb failure law observed for rocks (in the region of compression). This suggests that the Griffith mechanism of crack growth plays an important part in the fracture of rocks at low confining pressures.