Experimental Study of Shear and Compaction Band Formation in Berea Sandstone

Many field, experimental, and theoretical studies have contributed greatly to our understanding of the occurrence and formation of deformation bands in porous granular materials, but questions remain regarding the mechanics of strain localization, and how the orientation, thickness and internal strain (shear relative to volume change) of deformation bands is influenced by loading history and evolving rock properties. Here we report on triaxial rock deformation experiments using a non-traditional sample geometry to investigate band formation across the brittle-ductile transition. Five-cm diameter cylinders of Berea sandstone were machined with a circular (8.77 cm radius) notch to form a dog-bone sample geometry. In triaxial compression, the sample geometry obviates end-effects without creating heterogeneous stress gradients that can influence localization. Samples were instrumented to measure local strains in the neck region and acoustic emissions (AE), and then shortened to failure at confining pressures of 50 to 250 MPa. Deformation bands formed at all conditions, and photo mosaics of the outer sample surface were used to determine the thickness and orientation of the bands. Band thickness increases from several to tens of mm thickness and the angle between the bands with the shortening axis changes from 35 to 80 degrees, as confining pressure increases from 50 to 250 MPa, respectively. Mechanical data, including local strain measurements through yield, were used to test theoretical models for the onset of localization and formation of deformation bands as an instability in the constitutive description of homogeneous deformation. Generally, theoretical predictions compare favorably with the observed onset of localization determined by marked changes in the AE rate, and are consistent with more compactive bands at higher mean stress. Predictions of changes in band orientation with mean stress are largely consistent with observed trends, but deviate from the observed orientation by as much as ten degrees.