7 Conclusions
In this work, triaxial cyclic loading unloading compression test was adopted to study the mechanical behavior of granite specimens after thermal treatment by experiment and numerical simulation. The stress-strain curves, elastic modulus, Poisson’s ratio, plastic strain, energy and AE characteristic was analyzed, and the effect of temperature and confining on the damage evolution was investigated. The following conclusion are obtained:
(1) On the one hand, the strain energy release gradually after several cycles under cyclic loading compression, and the damage degree is lesser than that under monotonic loading. On the other hand, the failure modes under triaxial cyclic loading are more complicated than that under triaxial monotonic loading, which make the shear plane rougher. Therefore, the residual strength is higher than that under monotonic loading.
(2) After 600°C thermal treatment, the bearing structure of specimen is destroyed by high temperature, micro-crack continuously increases under unloading and loading process. Therefore, the peak strength of specimen under uniaxial cyclic loading is lesser than that under monotonic loading.
(3) Elastic modulus first increases and enter a stable phase, then decreases before becoming constant again with cycle number. Applied confining pressure make the increasing stage shorter, and high temperature treatment make the elastic modulus more sensitive to confining pressure. 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. However, the initial constant stage disappear when T = 600°C.
(4) Axial plastic strain non-linearly increases with the maximum axial strain on a concave type, whereas the non-linear characteristic decreases when T = 600°C. It means that damage occurred once applied compression, and it can be verified by AE characteristic. Radial strain even appear compressive characteristic in the initial stage, which indicates that the grain adjust under triaxial cyclic loading-unloading compression. And, the compressive characteristic is more obvious with increasing temperature and decreasing confining pressure.
(5) The radial input and elastic strain energy is relatively small compared with axial input energy, axial elastic strain energy and dissipated energy. When T = 25 and 300°C, axial input energy mainly transfer as axial strain energy at initial loading stage, whereas axial strain energy and dissipated energy are near equal when T = 600°C. Confining pressure increases the carrying capacity of rock, and need more dissipated energy to destroy the rock. Therefore, when the specimen failed, the dissipated energy increases with the increase of confining pressure. Confining pressure also restrain the crack initiation and propagation, therefore the variation of dissipated energy of specimen with confining is irregular before failure.
These observations indicate that changes in mechanical of granite are impacted significantly by thermal treatment and these effects must be accommodated in performance assessment.