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Hengxing Lan

and 2 more

Abstract: Strength criteria for intact rock is essential for the safe design of many engineering structures. These criteria have been derived mainly from tests in the compressive stress region, and only indirect tensile tests are commonly found in the literature to complement them. Very few results have been published for confined, direct tensile tests on intact rock. No appropriate criteria are available for addressing the issue on tensile strength of intact at the current stage. The failure process in the tensile region and associated mechanism are lack of understanding. Here we present the results of direct triaxial tensile tests on Longmaxi Shales under varying confining stresses. These and the results from previous tests in marble and sandstone are used to increase our understanding of the processes leading to tensile failure of intact rock and develop strength criteria in the tensile region. The confined direct tensile tests and the Nano-scale SEM imaging prove that the phenomenon of “tension cut-off” at low confining stress and the positive correlations between confining stress and tensile strength above the confining stress threshold for brittle rocks occur also in more ductile rocks like shales. Such findings are consistent with the concept that tensile failure processes for intact rock are universal. Our results demonstrate that friction processes still have a significant role on intact rock strength in the tensile region. The role of frictional processes leading to confined tensile failure and transitioning to a purely tensile mode have been interpreted. Moreover, the tensile strength of shale behaves in a similar fashion as for brittle rocks, proving that tensile failure processes for intact rock are universal. Further, strength criteria are presented here which consider the frictional processes leading to failure under confined, direct tension tests and validated against published tensile strength data. Introduction