Slide 13 of 20

Depending on the relationships between stress and strain, three types of deformation can be defined.   These relationships derive from triaxial mechanical tests in which a cylinder of rock is submitted to an axial pressure imposed by a piston.  The sample may be enclosed in a pressure vessel to simulated high lithostatic stress.  The deviatoric stress responsible for deformation is the difference between the stress related to the piston and the lithostatic stress. A linear and reversible relationship between stress and strain characterises elastic deformation.  The deformation is transient.  The sample recovers its initial shape as soon as the deviatoric stress is removed. Brittle failure: the deviatoric stress reaches a treshold (the yield point) beyond which fractures develop.  The deviatoric stress drops suddently as the rock breaks down. Plastic deformation: the deviatoric stress reaches a treshold (the yield point) beyond which permanent deformation is stored in the rock sample.  Beyond the elastic yield point the stress/strain relationship is no longer linear.  Eventually brittle failure may develop.