Interrelation of flow or fracture and phase transition in the deformation of carbonate rock

Abstract

Shock wave, ultrasonic, hydrostatic, and triaxial compression data for carbonate rock are considered in terms of the mechanisms of yielding. The results suggest that yielding by flow or fracture can be replaced by phase transformation for selected stress paths. For certain calcite rocks, these stress paths include uniaxial strain loading. The results further suggest that when initial yielding is by flow or fracture, behavior is strain rate dependent. This is apparently the case for Vermont marble and Blair dolomite, as comparison of the dynamic and quasi‐static stress‐strain paths and yield stress under uniaxial‐strain loading indicates. In contrast, similar comparisons for Solenhofen limestone and Oakhall limestone show little rate dependence, suggesting that the apparent initial yielding is dominated by onset of the calcite I‐II phase transition. Both static and dynamic studies show that the I‐II transition in calcite rock is largely rate independent, reversible, continuous (gradual), and shear stress sensitive. Rate independence and reversibility are apparently the consequence of a displacive and coherent phase transformation. Continuous transformation appears to be due to rock microstructure which leads to stress concentrations and hence a spread in the macroscopic stress region over which the transition occurs. Shear stress sensitivity can be due to both microscopic dependence of the transition on local shearing stresses and shear stress dependence of the microstructural stress inhomogeneities.