A rate- and state-dependent ductile flow law of polycrystalline halite under large shear strain and implications for transition to brittle deformation

We have conducted double-shear biaxial deformation experiments in layers of NaCl within its fully-plastic (FP) regime up to large shear strains (γ < 50) with velocity steps. From this, we have empirically formulated a rate- and state-dependent flow law which explains the transient mechanical behavior. The steady state flow stress in the FP regime can be explained by a power-law with a stress exponent ~8.5 and an activation enthalpy of ~1.3 eV, with the instantaneous response having a higher stress exponent (13 ± 8), although there is data scatter. The transition to brittle regime is associated with weakening from the ductile flow law. In FP regime, the mechanical response is characterized by a monotonic decay to a new steady state while in the transitional regime, by a peak-decay behavior. The transient flow law obtained here is of considerable importance in the study of the brittle-ductile transition in rocks.