Brittle fracture

Failure by crack formation or propagation with limited distributed permanent deformation. Shear and opening fractures have different strength conditions; a frictional failure envelope should not be extrapolated indefinitely into tension.

Loss of load-bearing strength accompanied by relative displacement tangential to a surface, rather than primarily opening normal to it. The Mohr-Coulomb failure criterion models frictional shear failure by $|\tau |=c+\mu {\sigma }_n^{\prime }$, using cohesion, a friction coefficient and compressive effective stress normal to the potential sliding plane.

In the simplest strength-cutoff model, opening begins when the least compressive effective stress reaches minus the tensile strength: ${\sigma }_3^{\prime }=-T_0$ with compression positive. In Mohr's circle, this is contact with the vertical line ${\sigma }_n^{\prime }=-T_0$ at $\tau =0$. The criterion represents an opening mechanism and differs from frictional shear failure.