Crack initiation stress of brittle rock with different porosities

Accurately determining the crack initiation stress (CI) is of great importance to evaluate the stability of deep underground openings and the permeability of host rock in the near field. For this paper, uniaxial compression tests and increasing amplitude cyclic stressing tests were performed on four kinds of rocks with different porosities. The stress-strain data and acoustic emission (AE) data of these tests are analyzed to evaluate the applicability and accuracy of the existing methods for determining the CI. The results demonstrate that for marble and Beishan granite with low porosity, the lateral strain (LS) method is the simplest and most direct method to obtain the CI_LS. For high porosity sandstone, the result of lateral strain response (LSR) method is significantly smaller than that obtained by AE. The cumulative AE hits (CAEH) method heavily depends on the “S-shaped” characteristic of the cumulative AE hits curve, which limits its scope of application. The cumulative AE hits curve slope (CAHS) method, which determines the key inflection point of the cumulative AE hits curve by slope variation to determine the CI_CAHS, possesses the widest applicability and highest accuracy. The CI/UCS ratios of rocks with similar grain sizes decreases with increasing porosity. For Beishan granite, the average CI_CAHS is less than that of CI_LS. They are 0.43 UCS and 0.53 UCS, respectively. According to the residual strain data from the cyclic loading tests, the two crack initiation thresholds of Beishan granite can be clearly identified. These two crack initiation thresholds represent different crack initiation sequences in grain boundaries and different minerals.