Criterion for residual strength and brittle-ductile transition of brittle rock under triaxial stress conditions

The critical condition with respect to the brittle-ductile transition (BDT) is of great significance in understanding the distinction between brittle and ductile behavior for rocks. In this paper, we develop the two criteria for the residual strength and BDT point of brittle rock based on linear elastic fracture mechanics. The effect of the microcrack friction and slip friction of brittle rock on the residual strength and BDT point is analysed. Two models are derived to predict the critical confining stress and critical compressive stress at the BDT point for brittle rock, and a new brittleness index is proposed. The reliability of models is validated by using a great number of test data from previous publications. The results show that the stress of the BDT point increases with increasing microscopic friction on the microcrack, and decreases with increasing macroscopic sliding friction. The critical confining stress and critical compressive stress at the BDT point are predicted, which is qualitatively consistent with laboratory tests. The results also show that the two criterion have the capability to predict the residual strength and BDT point, which has provided useful insights into the mechanics of brittle-ductile transition in brittle rocks.