3.3 Evolution of plastic strain of thermal treated granite specimens with axial strain
Plastic strain is the irrecoverable strain in the cyclic loading-unloading process, which can reflect the damage in specimen to some extent. Figure 4 illustrates the variation of axial plastic strain with the maximum axial stain (ε 1u) in each cyclic loading, it is clear that axial plastic strain non-linearly increases with the maximum axial strain on a concave type. Axial plastic strain increases gradually at the initial loading stage, and inflection point appeared. Finally, axial plastic strain near linearly increases with the maximum axial strain in each cycle at macro-crack forming and shear slippage stage. With increasing confining pressure, axial plastic strain decreases, which indicates that confining pressure can retrain the axial plastic strain to some extent. When T = 600°C, axial plastic strain near linear increases with the maximum axial strain in each cycle, which indicates that damage occurred once applied loading. In conclusion, the damage evolution is influenced by the initial damage state and confining pressure.
Figure 5 shows the variation of radial plastic strain with the maximum axial strain, it can be seen that the radial strain is smaller in the initial stage, and even compressive characteristic appeared, this phenomenon was also obtained by Xiong et al. (2019). The partial enlarged drawing of the radial plastic strain is shown in Fig. 5d, the compressive characteristic is more obvious with increasing temperature. It indicates that the grain adjusted under triaxial cyclic loading-unloading compression when there are micro-cracks in specimen. The compressive characteristic is more unobvious with increasing of confining pressure, it may be because micro-crack pre-closed when applied confining pressure. And then radial plastic strain shows expansion characteristic with increasing the maximum axial strain in each cycle. It can be also seen that the reversed point from compression to expansion is backward with increasing confining, it indicates that confining pressure restrain radial plastic strain.