Image Analysis of Shear Banding

Plane strain compression tests were performed on Berea sandstone to investigate the mechanisms involved in the development and propagation of a shear band. Thin sections for optical microscopy and digital photographs were prepared by injecting blue-colored epoxy into failed specimens, where deformation localized and softening occurred. The digital image contained three parameters, R, B, and G, that defined the color of each pixel. The intensity of the R channel consistently differentiated the blue (epoxy-filled) pore space from the white grains composing the matrix. The areas of increased porosity did not extend beyond the tip, which was determined by the last observable intragranular microcrack. The porosity change that corresponded to the shear band was observed in areas with high densities of microcracking. Therefore, the localized porosityincrease was related to the evolution of microcracks within the shear band itself.