An acoustic emission study of damage development and stress-memory effects in sandstone

Abstract

As part of a study of damage development and stress-memory effects, we have carried out a series of laboratory acoustic emission (AE) experiments on dry Darley Dale sandstone under triaxial compressive loads. In particular, we studied the shape of damage surfaces in stress space. Like the yield surface in the theory of plasticity, a damage surface is defined as the locus of points in 3D stress space beyond which additional permanent damage develops. The shape of a damage surface thus provides a means of characterising the state of damage corresponding to the state of stress to which the material has previously been subjected. In our experiments, which were limited to the cross-section of stress space where σ 2 = σ 3 = P c (P c being the confining pressure), the points of the damage surface were indicated by the onset of AE. The damage surface in (P c, σ 1)-space corresponding to an initial stress state (P c i, σ 1 i ) is almost linear in the region where P c > P c i . In the region where P c < P c i the gradient of the damage surface becomes clearly larger. We compare the experimental results with predictions based on a 2D model of secondary microcrack (wing-crack) formation from frictional surfaces. According to this model, the damage surface is predicted to consist of two linear parts connected by a vertex (or “knee”) located at the point (P c i, σ 1 i ). The comparison shows that the model, although relatively simple, explains many features of the experimentally observed damage surfaces. The results also have implications in terms of a stress-memory effect. If the initially applied stress state (P c i, σ 1 i ) is not known, then it might in principle be derived from the position of the vertex in an experimentally measured damaged surface.