Micromechanics of the brittle to plastic transition in Carrara marble

Abstract

Samples of Carrara marble were deformed at room temperature to varying strains at confining pressures spanning the range in mechanical behavior from brittle to plastic. Volumetric strain was measured during the experiments, and the stress‐induced microstructure was characterized quantitatively using optical and transmission electron microscopy. The range of confining pressure over which transitional (or semibrittle) deformation occurs is 30–300 MPa. The macroscopic initial yield stress is constant for confining pressures greater than 85 MPa, whereas the differential stress at the onset of dilatancy increases with pressure up to 300 MPa. The dilatancy coefficient decreases rapidly with increasing pressure up to 100 MPa, and then asymptotically approaches zero for pressures up to 300 MPa. The work hardening coefficient increases with pressure up to 450 MPa; the pressure sensitivity is greatest for pressures up to 100 MPa. Active deformation mechanisms include microcracking, twinning, and dislocation glide. Transmission electron microscopy observations indicate that dislocation glide occurs, at least on a local scale, in samples deformed in the semibrittle field at pressures as low as 50 MPa and applied differential stress well below the critical resolved shear stress for glide on the easiest slip system. Cracks and voids frequently nucleate at sites of stress concentration at twin boundaries, at twin terminations, and at the intersection of twin lamallae. Geometries suggestive of crack tip shielding by dislocations are also observed. Stereological measurements indicate that at constant strain in the semibrittle field, the stress‐induced crack density and anisotropy decrease with increasing pressure. Crack density and anisotropy in samples deformed to strains of 3–5% in the semibrittle field at pressures up to 120 MPa are comparable to those in the prefailure brittle sample, although an analysis of the energetics of deformation suggests that the ratio of brittle energy dissipation to total energy dissipation is at least 60% lower. We also detect a qualitative difference in the characteristic length of the cracks in the brittle and semibrittle fields. The mean dislocation density at constant differential stress increases significantly for samples deformed at pressures of 230 MPa and greater. Our results suggest that although semibrittle flow occurs over a wide range of pressure, the most marked changes in strain partitioning, and hence the style of deformation occur over a small range in pressure.