Brittle and ductile behaviour

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Rock failure can either be referred to as brittle, ductile, or at the brittle-ductile transition.

Brittle failure
is said to occur when the ability of the to rock resist load decreases with increasing deformation. Brittle failure is associated with materials that undergo little to no permanent deformation before failure and, depending om the test conditions, may occur suddenly and catastrophically. See brittleness.
Ductile failure
is said to occur when the material can sustain permanent deformation without losing it's ability to resist loading (without failing). Ductility increases with increased confining pressure and temperature, and is common in weathered rocks, heavily jointed rock masses and some weak rocks such as evaporites.
Brittle-ductile transition
As the confining pressure is increased a rock specimen will tend to exhibit more ductile behavior. Which of these two general modes of behaviour occurs depends on the relative stiffness of the specimen under loading.
Fracture initiation
A rock material contains a large number of randomly oriented zones of potential failure along grain boundaries. For a grain boundary, high tensile stress accumulates at an sufficiently oriented grain boundary with the external stress field, and it is assumed that fracture initiates from the boundary of an open flaw when the tensile stress on this boundary exceed the local tensile strength of the material.

See also