A Study on Crack Damage Stress Thresholds of Different Rock Types Based on Uniaxial Compression Tests

When rock samples are loaded until macroscopic fractures develop, the failure process can be divided into several stages based on axial and lateral strain responses or the acoustic emission sequence during uniaxial compression tests. Several stress thresholds may be identified: the crack closure stress σ cc, crack initiation stress σ ci, crack damage stress σ cd, and uniaxial compressive strength σ ucs; these may be used as a warning indicator for rock rupture. We investigated the crack damage stress σ cd, its threshold, and a possible relationship between σ cd and the uniaxial compressive strength. The σ cd of different rock types were compiled from previous studies based on uniaxial compression tests. The results showed that the overall averages and standard deviations of σ cd /σ ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.11), ~0.85 (±0.11), and ~0.73 (±0.18), respectively. There were no significant differences in σ cd /σ ucs between the different rock types, except that the sedimentary rock had a slightly larger standard deviation attributed to the variation of porosity in the samples, while the metamorphic rock had higher average σ cd /σ ucs resulting from the small statistical sample size. By excluding the higher-porosity (>10 %) rock samples, the averages and standard deviations of σ cd /σ ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.09), ~0.85 (±0.09), and ~0.78 (±0.11), respectively. The results imply that the rock origin process (i.e., igneous, metamorphic, and sedimentary) has a minimal effect on σ cd /σ ucs. The ratio σ cd/σ ucs could be an essential intrinsic property for low-porosity rocks, which could be used in rock engineering for predicting the failure process.