Anisotropic crack damage and stress-memory effects in rocks under triaxial loading

Abstract

We present and analyse the results from tests performed on sandstone cubes in which the bulk acoustic emission (AE) and the ultrasonic P and S wave velocity changes in the three principal directions were monitored and measured simultaneously with the stress and strain as samples were deformed in three orthogonal directions independently. This combination of measurements has enabled us to directly link AE to the formation of new crack damage. From uniaxial cyclic stressing experiments the existence of the Kaiser stress-memory effect was confirmed based on the AE data and the velocity data from directions normal to the uniaxial stress direction. Furthermore, the effect was found to occur in each principal stress direction independently, regardless of whether or not stresses were applied along orthogonal axes. This suggests that the crack damage formed during stressing is highly anisotropic, with the new microcracks formed during each stress cycle having minor axes parallel to the minor principal compressive stress direction. The observed effects have been modelled in terms of the elastic closure of preexisting cracks and the formation of new highly-oriented dilatant cracks. The dilatant cracks produce AE. By modelling these two separate processes we have demonstrated a clear quantitative correlation between the AE output and the new dilatant crack damage formed during deviatoric stressing. We believe our results confirm the potential of the use of the Kaiser effect to determine crustal stresses from measurements on oriented core plugs taken from borehole cores.