Experimental study of rock specimens under uniaxial stress path has shown that ultimate failure in rocks occurs after the onset of crack coalescence associated with dilatancy. Typically, rock dilatancy is represented by estimation of averaged specimen-scale total volumetric strains (εv). However, in previous studies, full-field εv (i.e. variations throughout a specimen) has not been rigorously analyzed. Accordingly, in this work, the full-field deformation measurement method of 2D-DIC (digital image correlation) is implemented to characterize evolution of dilatative (εv < 0) and contractive (εv > 0) strains in Stanstead granite specimens under uniaxial loads. In-sync, ultrasonic monitoring was also conducted to non-destructively monitor damage in the specimen. Results showed that full field εv strains evolve in a heterogeneous manner, with some regions showing dilation, and some showing contraction. Also, increases in ultrasonic amplitude during the initial stages of testing were shown to be linked to the occurrence of contractant strains.


Rock microstructure is typically represented by an assembly of heterogeneous mineral grains, which may vary in their mechanical (stiffness, hardness) and geometrical (size, shape) properties (Lan et al., 2010). This grain-scale heterogeneity contributes to the evolution of a heterogeneous strain/or stress field in the rock volume, causing locally extensile regions to originate even under a globally compressive stress-field (Diederichs, 2003a). Such localized strain and associated progressive damage can lead to an overall increase in the dilatancy (negative total volumetric strains, εv) of hard brittle rocks, which is consistent with the fact that crack nucleation and coalescence processes in rocks are typically followed by absolute or relative dilatancy (Cieślik, 2018; Diederichs, 1999; Shirole et al., 2019, 2020; Sinha & Walton, 2019). In previous studies, the evolution of εv (with stress) was estimated either through point-based surficial mechanical measurement systems (e.g., strain gauges) or via computation of the fluid displacements in the triaxial cells. Accordingly, the typical εv measurements are either representative of average strains acting on a small region present on the specimen surface or are an average of the strains distributed across the whole specimen.

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