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

Abstract

[1] Triaxial tests on gypsum polycrystal samples are performed at confining pressure (P c ) ranging from 2 to 95 MPa and temperatures up to 70°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°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°C may arise from the thermal activation of mineral kinking.

Fibrous gypsum veins as diffuse features and within fault zones: the case study of the Pisco Basin (Ica desert, southern Peru)

“…In agreement with our observations on gypsum veins, recent laboratory deformation experiments performed on natural samples of gypsum (Zucali et al 2010;Brantut et al 2011) suggest that, at shallow crustal levels, gypsum can accommodate strain by both brittle and plastic deformation mechanisms and that these can occur also in concert (Zucali et al 2010).…”

“…Being soft and characterized by low density and viscosity, gypsum tends to undergo plastic deformation at relatively low effective pressures (Brantut et al 2011) and flow without rupture. In addition, gypsum easily undergoes diagenetic modifications (e.g.…”

Long-period seismicity in the shallow volcanic edifice formed from slow-rupture earthquakes

“…Consistent with our numerical seismicity results (Fig. 4a), laboratory experiments in gypsum at the brittle-ductile transition show swarms of low frequency acoustic emissions associated with the dynamic propagation of shear bands 41 , showing that brittle-ductile materials can support both dynamic rupture and low frequency seismicity. …”

Strength Failure and Crack Evolution Behavior of Rock Materials Containing Pre-existing Fissures

“…AEs are transient elastic waves generated by the rapid release of energy within a material, such as the strain energy released during micro-crack propagation. Monitoring AE during deformation has become an increasingly important diagnostic tool in material science which has provided a wealth of information regarding the failure process in brittle materials (Fortin et al 2006;Lockner et al 1991;Benson et al 2007;Brantut et al 2011) and can be used in failure forecasting modeling (Bell et al 2011a, b). AE counts were recorded by a DS2 fullinformation AE measuring system via two AE sensors that were attached to the back face of the specimen using hot bar as a coupling agent and fixed in place using tape, a similar method to that of .…”