The effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow of carbonate rocks

Triaxial compression tests were conducted on cold-pressed calcite, aragonite and limestone aggregates and on Solnhofen limestone specimens to study the effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow. The tests were pel-formed at confining pressures up to 195 MPa and at strain rates between 5 . 10(-4) s(-1) and 5 . 10(-6) s(-1). Axial as well as volumetric strain were measured. Samples were produced by cold-pressing powders of crushed calcite and aragonite crystals and of crushed Solnhofen limestone. Sample porosity ranged between 5 and 25% and the average grain size varied between 5 and 400 mu m. For both the cold-pressed aggregates and the intact limestone specimens, the confining pressure at the transition from localized brittle failure to non-localized cataclastic flow decreases with increasing porosity and grain size. The transition is characterized by a zero work-hardening coefficient, by dilation for low porosity and compaction for high porosity rocks, by a constant ratio between axial stress and confining pressure, and by decreasing yield strength for increasing confining pressure. The experimental results disagree with the critical state concept over most of the porosity range investigated, and indicate non-associated material behaviour. These properties of the brittle-ductile transition are addressed on the basis of continuum mechanics or by models suggested for granular materials. The problems discussed and the results obtained are of fundamental interest to rock deformation and structural geology.