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 porosity increase was related to the evolution of microcracks within the shear band itself.
A suite of plane strain compression tests were performed on Berea sandstone with the University of Minnesota Plane-Strain Apparatus. Control of the tests was maintained through peak stress and into the strainsoftening regime, where some tests were halted. Thinsection microscopy and imaging observations corresponding to the localization of deformation in plane-strain compression are reported. From laboratory compression tests on rock, it is observed that at some point the uniform deformation pattern changes and further deformation is localized in a narrow region called the shear band. Depending on the initial porosity and stress state, the inception of a shear band may occur through different mechanisms. Besuelle et al.  performed tests on Vosges sandstone with a porosity of 22%. Localization began by the formation of a dilatant or compactive band, depending on the mean stress. The localized volume change was then followed by a secondary through going feature that was characterized by cataclastic deformation. Other authors have observed that shear banding was preceded by isolated clusters of Hertzian fracture that were formed near peak stress. The clusters of Hertzian fractures coalesced to form a through going shear band . Small scale experiments indicate that the formation of a shear band is related to the confining pressure and amount of axial displacement [1, 3, 4]. Tests performed at progressively higher confining pressures show that the inclination of the shear band decreases, suggesting that the failure criterion is nonlinear. Furthermore, field observations have documented that the number of shear bands may be related to the amount of shear displacement .
2. EXPERIMENTAL METHODS
A plane strain device (Fig. 1), of the type first suggested by Vardoulakis and Goldscheider , was developed at the University of Minnesota . The apparatus allows the shear band to propagate in an unrestricted manner by attaching the upper platen to a low-friction linear bearing. Thus, the device incorporates the advantages of a direct shear test, where measurement of stress, displacement, and dilatancy of a shear band can be achieved, and a constitutive plane-strain compression test, where the two-dimensional material behavior can be evaluated. A prismatic specimen with dimensions of 100 × 28 × 75 mm (l × w × h) is wedged within a stiff biaxial frame; this thick-walled cylinder restrains the displacement of the specimen to very small values so as to simulate the plane-strain condition.