On the use of the stress relaxation testing method in studies of the mechanical behaviour of geological materials

A summary is presented of the use of stress relaxation testing in studies of the mechanical behaviour of materials with emphasis on the underlying assumptions of the method. It is shown that relaxation testing is a powerful experimental method which could profitably be used in future studies of the mechanical properties of geological materials to augment, but not to replace, the more conventional creep and constant strain rate tests. Stress relaxation data can be analysed to determine important parameters in equations describing (a) laws of flow and fracture, and (b) dislocation dynamics. The variation of the first derivative of any stress relaxation curve with stress can be used to establish the applied stress sensitivity to strain rate and the values of activation parameters such as ‘activation volume’. Such parameters may be used to constrain the inference of the dominant deformation mechanism. This account is illustrated by reference to some recent experiments on polycrystalline, synthetic galena (lead sulphide) and pre-faulted cylinders of Tennessee sandstone deformed over a wide range of environmental conditions. The accurate sensing of the second derivative of a relaxation curve is required to establish parameters relating to dislocation dynamics, e.g. partition between thermal and athermal components of applied stress, and the stress dependence of dislocation velocity. The determination of such parameters is illustrated with new experiments on single crystals of calcite.