Damage and rupture dynamics at the brittle-ductile transition: The case of gypsum

Triaxial tests on gypsum polycrystal samples are performed at confining pressure (P-c) ranging from 2 to 95 MPa and temperatures up to 70 degrees C. During the tests, stress, strain, elastic wave velocities, and acoustic emissions are recorded. At P-c <= 10 MPa, the macroscopic behavior is brittle, and above 20 MPa the macroscopic behavior becomes ductile. Ductile deformation is cataclastic, as shown by the continuous decrease of elastic wave velocities interpreted in terms of microcrack accumulation. Surprisingly, ductile deformation and strain hardening are also accompanied by small stress drops from 0.5 to 6 MPa in amplitude. Microstructural observations of the deformed samples suggest that each stress drop corresponds to the generation of a single shear band, formed by microcracks and kinked grains. At room temperature, the stress drops are not correlated to acoustic emssions (AEs). At 70 degrees C, the stress drops are larger and systematically associated with a low-frequency AE (LFAE). Rupture velocities can be inferred from the LFAE high-frequency content and range from 50 to 200 m s(-1). The LFAE amplitude also increases with increasing rupture speed and is not correlated with the amplitude of the macroscopic stress drops. LFAEs are thus attributed to dynamic propagation of shear bands. In Volterra gypsum, the result of the competition between microcracking and plasticity is counterintuitive: Dynamic instalibilities at 70 degrees C may arise from the thermal activation of mineral kinking.