Post-yield Strength and Dilatancy Evolution Across the Brittle-Ductile Transition in Indiana Limestone

An extensive uniaxial and triaxial compression testing programme was performed on Indiana Limestone to assess its behaviour across the brittle-ductile transition. Particular attention has been paid to the post-yield evolution of strength and dilatancy. Specimens tested at sigma (3) = 30 MPa displayed a fully ductile failure mechanism, whereas specimens tested at sigma (3) = 15 MPa and sigma (3) = 20 MPa displayed transitional mechanisms, which were neither fully brittle nor fully ductile. Based on an examination of failure localization and dilatancy characteristics, the stress at which crack volumetric strain begins to increase was found to be an indicator of individual specimen ductility. In contrast to less porous rocks, the reversal of total volumetric strain did not coincide with the onset of axial strain nonlinearity under unconfined conditions. With respect to post-yield strength, a major change in the rate of friction mobilization relative to plastic shear strain was observed across the brittle-ductile transition. The dilatancy of the specimens was also found to undergo a major change, with the plastic shear strains to mobilization of peak dilatancy in the ductile regime being approximately one order of magnitude higher than in the brittle regime.