Rock Fracture Criteria

When stresses in the Earth’s crust exceed the crustal strength, the rock fractures and fails (Brace 1964; Hoek 1968; Obert 1972). A fracture criterion describes the conditions for which failure occurs in a material. In principle, one distinguishes between phenomenological and mechanistic failure theories.

Phenomenological theories

(Coulomb, Coulomb-Mohr, Mohr or Hoek-Brown) quantify the spatial orientation of fracture planes with respect to the stress field producing the discontinuities.

Mechanistic theories

(Griffith, McClintock & Walsh, fracture mechanics models) start from the premise that fracture initiates from existing flaws acting as stress concentrators through which the brittle fracture process in rock is controlled. Both theories are used to determine the stress state in the Earth’s crust and to evaluate the stresses for some of the stress measurement techniques.