Inter-granular damage in experimentally deformed rock-salt investigated by X-ray micro-tomography: confining pressure and brine effects

We have investigated the mechanical behaviour of synthetic rock-salt during compression tests using in situ X-ray micro computed tomography (XR-μCT). Samples were deformed under controlled strain rate in an X-ray semi-transparent triaxial cell adapted to X-ray tomography. We were interested in the development of micro damage in relation to the stress state and the presence or absence of brine. For that purpose, we produced by powder compaction in œdometric cells two types of materials with different amounts of water. One is considered “laboratory dry” and the other is considered “water saturated”. Both materials exhibit similar mean grain size (c.a. 200 μm). Both types of samples were deformed at uniaxial and triaxial conditions (15 MPa confining pressure), at room temperature and constant displacement rate (1 μm/s). The results show that the dominant damage mechanisms correspond to the progressive development of diffuse inter-granular micro-cracking. This can be explained by the strong anisotropy in the crystal plasticity of NaCl, which results in local plastic strain incompatibilities between neighboring grains in the polycrystalline rock-salt. Conversely, the development of micro-crack damage is strongly reduced by both confining pressure and presence of brine. Natural Leine rock salt samples bearing inter-crystalline brine and tested under similar conditions show similar trends as regards inter-granular micro-damage. We suggest that the development of inter-granular micro-cracking is suppressed or retarded in the presence of brine thanks to dissolution-precipitation processes, which in addition to the well-known healing effect may also reduce the local plastic strain incompatibilities.