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 sigma (cc), crack initiation stress sigma (ci), crack damage stress sigma (cd), and uniaxial compressive strength sigma (ucs); these may be used as a warning indicator for rock rupture. We investigated the crack damage stress sigma (cd), its threshold, and a possible relationship between sigma (cd) and the uniaxial compressive strength. The sigma (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 sigma (cd) /sigma (ucs) for igneous, metamorphic, and sedimentary rocks were similar to 0.78 (+/- 0.11), similar to 0.85 (+/- 0.11), and similar to 0.73 (+/- 0.18), respectively. There were no significant differences in sigma (cd) /sigma (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 sigma (cd) /sigma (ucs) resulting from the small statistical sample size. By excluding the higher-porosity (> 10 %) rock samples, the averages and standard deviations of sigma (cd) /sigma (ucs) for igneous, metamorphic, and sedimentary rocks were similar to 0.78 (+/- 0.09), similar to 0.85 (+/- 0.09), and similar to 0.78 (+/- 0.11), respectively. The results imply that the rock origin process (i.e., igneous, metamorphic, and sedimentary) has a minimal effect on sigma (cd) /sigma (ucs). The ratio sigma (cd)/sigma (ucs) could be an essential intrinsic property for low-porosity rocks, which could be used in rock engineering for predicting the failure process.