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Unlike metals, rocks show bi-modularity (different Young’s moduli and Poisson’s ratios in compression and tension). Displacements monitored during the Brazilian test are used in this study to obtain the Young’s modulus and Poisson’s ratio in tension. New equations for the displacements in a Brazilian test are derived considering the bi-modularity in the stress–strain relations. The digital image correlation technique was used to monitor the displacements of the Brazilian disk flat surface. To validate the Young’s modulus and Poisson’s ratio obtained from the Brazilian test, the results were compared with the values from the direct tension tests. The results obtained from the Brazilian test were repetitive and within 3.5% of the value obtained from the direct tension test for the rock tested. [ABSTRACT FROM AUTHOR]

The flat-jointed (FJ) contact model implemented in the particle flow code resolves the long-standing issue of the code producing unrealistically low ratios of uniaxial compressive strength (UCS) to direct tensile strength. Despite the improvements introduced with the FJ model, an FJ sample calibrated to the UCS still produces a crack initiation stress much lower than the expected value of approximately 0.4 UCS, as commonly observed in laboratory compression tests for most rocks. In this study, we investigated the role of initial crack volume on the crack initiation stress of a rock. We found that the microcracks among the mineral grains and their associated void space played a major role in controlling the stress magnitude associated with crack initiation. We also observed that the closure of these cracks between mineral grains was the primary driver of the crack closure phase in the compression test, and the elastic bimodularity was due to the crack density. We developed a methodology to incorporate the initial microcracks using the FJ contact model to match the crack closure stress and bimodularity of a laboratory rock sample. By introducing microcracks and voids among the mineral grains, we automatically capture the crack initiation stress of Lac du Bonnet granite measured at approximately 40% of the UCS. [ABSTRACT FROM AUTHOR]