Experimental Study of Crack Propagation in Single Crystal Halite (Rock Salt) Using Digital Image Correlation Techniques

Cavities in deposits of halite below the earth’s surface have been used by the energy industry to store nuclear waste and petroleum due to its impermeable and self-healing material characteristics. During excavation and service, stresses applied on the cavity walls by surrounding material can cause fracturing that can damage boreholes and cause contaminants to leech out of the cavity. Halite single crystals are homogenous, anisotropic crystalline materials that exhibit different compressive strength characteristics when stresses are applied to different crystallographic orientations. Crystallographic orientations parallel to [1 0 0], 19º to (1 0 0) in (0 1 0), and 30º to (1 0 0) in (0 1 0) were evaluated for crack propagation and fracture toughness. Specimens were fabricated from a singular sample of halite and included a crack-initiation slot that measured 33.3% of the total sample length, and a speckle pattern was applied to the front face of each sample. Crack propagation was measured from the end of the crack initiation slot to the edge of the sample. All experiments were conducted using a constant strain rate of 0.01 mm/min at ambient room temperature. A high-speed optical camera captured images of the sample and 2-dimensional digital image correlation (DIC) techniques were used to evaluate stress concentrations at the crack-tip, crack growth velocity, and local stress distributions. The results indicate that the highest crack growth velocity occurs during crack initiation when the critical resolved shear stress is reached, and the local stress distributions showed that the stress concentrations occur at the crack tip.