Influence of time on the strength, deformation and fracture properties of a lower devonian sandstone

Abstract

A study of the influence of strain rate on the strength, deformation and fracture properties of a Lower Devonian sandstone is presented. Strain rates were varied between 2·5 x 10 −3/ sec to 2·5 x 10 −9/ sec A new criterion, based on the incremental Poisson's ratio, capable of predicting both the long-term strength of a material and also able to establish whether a material under load is safe from long-term failure is suggested. This criterion is based on short-term creep tests and is substantiated by the constant strain-rate strength results. Comparison of strength results obtained at different rates of loading and rates of straining showed than for similar loading times to failure the constant rates of loading gave slightly higher strength values. Modes of rupture were found to be independent of both loading methods but dependent upon time taken to reach strength failure. The widely held belief that the onset of dilation represents the critical stress in a material and that thus unstable fracture propagation commences at this stress level was found not to be true. A definition of critical stress based upon the incremental Poisson's ratio is suggested.