3.2 Evolution of Elastic Modulus and Poisson’s ratio of thermal treated granite with Cyclic Number
Figure 2 shows the variation of elastic modulus (E ) with cycle number, Young’s modulus represents the slop of stress-strain curve in linear segment under loading stage (30%-70% of the peak strength in each cyclic loading). It is clear that elastic modulus first increases and enter a stable phase, then decreases before becoming constant again with cycle number. In the early loading stage, pre-existing fissures closed and result in the increase of elastic modulus. After that, the loading enter to the elastic stage, few crack initiate in this loading stage, and the elastic modulus remains near constant. With the increase of cycle number, more micro-cracks were induced by compression, and macro-crack formed gradually, which result in the decrease of elastic modulus. However, after the loading enter to the residual strength stage, and the elastic modulus reach to a plateau again.
Fissures pre-closed when applied confining pressure partially, and make the increasing stage of elastic modulus shorter than that under uniaxial compression. It is also clear that elastic modulus increases with the increase of confining pressure at the same cycle number, this phenomenon is more obvious in the granite specimens treated by 600°C. After high temperature treatment, more cracks are induced, the potential for cracks to close is great, and make the elastic modulus more sensitive to confining pressure. Confining pressure also restrain crack propagation under loading process, therefore the elastic deformation stage is more obvious with the increase of confining pressure, and the decreasing rate of elastic modulus with cycle number decreases in the macro-crack formed stage. Confining pressure also increases the contact area of the macro-crack, and result in the increase of elastic modulus in the residual strength stage.
Figure 3 depicts Poisson’s ratio of granite after thermal treatment under different confining pressure. Poisson’s ratio represents the ratio between radial and axial strain in the elastic stage, it may be larger than 0.5 after the macro-cracks formed (not in the elastic deformation stage). Poisson’s ratio remains near constant in the initial stage, then increases rapidly, and decreases slightly before entering a stable phase again with cycle number on the whole. The slight decreases of Poisson’s ratio are also obtained by Xiong et al (2019). It indicates that there is few crack induced by loading in the initial stage, and the radial strain is not evident. After that, more vertical micro-cracks are formed, and it re-open in the elastic stage, which result in the expansion in radial direction. However, slippage occurred along macro-crack, and lead to the re-open for vertical micro-cracks decreases, when the loading to the residual strength stage. Therefore, Poisson’s ratio slightly decreases before entering a stable phase again. Poisson’s ratio of specimen under uniaxial compression is lesser than that under triaxial compression at initial loading stage when T = 25°C and 300°C, as shown in Figs. 3a-b. It can be explained as that pre-existing fissure closed when applied confining pressure, and result in the decrease of potential for grain to adjustment under loading process, therefore the specimen is easier to expansion in radial direction. The splitting tensile crack is easier to initiate in specimen under uniaxial compression, and it re-open under loading process, which make the increasing rate of Poisson’s ratio higher than that under triaxial compression. Confining pressure can restrain micro-cracks re-open, and Poisson’s ratio decreases with increasing confining pressure on the whole.
When T = 600°C, more thermal cracks are induced in specimen, which lead to the increases of Poisson’s ratio with cycle number once applied compression as shown in Figs. 3c. Lacking confining pressure, Poisson’s ratio of specimen under uniaxial compression is larger than that under triaxial compression. After macro-cracks formed, the decreasing rate of Poisson’s ratio with cycle number is larger than that when = 25°C and 300°C, it indicates that there are more vertical cracks in specimen when T = 600°C.