Crack initiation stress of thermally damaged rock under uniaxial compression

Crack initiation stress (CI) is an important index of rock and the index of thermally damaged rock under uniaxial compression is examined in this study. Methods for determining CI are first reviewed, and the CI values of two thermally damaged rocks are then identified using several methods including crack volumetric strain (CVS) method, lateral strain response (LSR) method, and cumulative acoustic emission hit (CAEH) method. The results show that CI estimated from the CVS method and the CAEH method is comparable with each other. On the other hand, it is found that CI determined using the LSR method is generally lower than these determined by the other two methods. Because the lateral strain of a thermally damaged rock is small (sometimes positive in the initial stage), the LSR method is not applicable for determining CI of thermally damaged rocks. A modified LSR (MLSR) method, which excludes the initial collapse deformation stage, is proposed, and the new method truly reflects new crack initiation. By comparing the determined CI with other methods, it is found that the proposed MLSR method is capable of determining CI of thermally damaged rock. Variation of CI with the degree of thermal damage is also studied. It is found that CI generally decreases as the number in the cyclic treatment increases, while the difference between CI to uniaxial compressive strength (UCS) ratio and crack damage stress (CD) to UCS ratio decreases with increasing thermal damage in the rock. The results indicate that unstable cracking propagation is easy to occur as the thermal damage in the rock increases.