Mechanical Behaviour

The mechanical behaviour of rocks depends on the pressure, temperature and strain-rate. At rapid strain-rates and low temperatures and pressures, most rocks behave in an elastic manner. A spring is a simple mechanical analaogue. All strain is recovered when the load is removed. The slope of the stress v strain curve is the elastic modulus of the rock.

At lower strain-rates and elevated temperatures and pressures, rocks behave in a plastic manner, showing large deformations at stresses beyond a critical stress level (called the yield stress. Strain is not recoverable, deformation is permanent. A block sliding on a plane is a simple mechanical analogue.

Sometimes the deformation of rocks becomes similar to that of a flowing fluid. The rate of deformation or strain-rate is controlled by the applied stress. Such behaviour is called viscous. The slope of the stress v strain-rate curve is the fluid viscosity. A dashpot is a simple mechanical analogue. Strain is not recovered. Viscous elements are introduced into models of rock behaviour to incorporate time-dependency.

The elasto-plastic model of material behaviour is often used to charcterize rocks. It combines elastic and plastic aspects of stress-strain behaviour. A block on a plane attached to a spring is the mechanical analogue. Strain is only recovered for the elastic part of the deformation.