5 AE and Failure Characteristic
Figure 10 illustrates the evolution of AE account and accumulative AE counts of granite specimens under cyclic loading-unloading compression, AE and AAE represent as AE account and accumulative AE counts, respectively. On the whole, AAE shows a ladder-like growth with duration time. At room temperature, weak AE events are monitored at the initial compaction and elastic stage. When the loading near the peak strength, accumulative AE hits thrived increases. After the peak strength, axial stress drop quickly, and violent AE events are monitored. However, the AE events decreases obviously when the loading enter to the residual strength stage, and few micro-crack initiated due to the friction between the micro-crack under cyclic loading process. Compared with Figs. 4 and 7, it can be seen that the evolution of AAE is similar to that of axial plastic strain and dissipated energy, whereas there is also difference. This may be because dissipated energy result in not only the plastic strain but also crack propagation. Under uniaxial compression, the coupling is insufficient between AE probe and specimen, and the AAE is lesser than that under triaxial compression.
When T = 600°C, more thermal cracks are induced in the specimen, and lead to the carrying capacity loss under uniaxial compression. Under cyclic loading process, displacement occurred between grains with less friction, and less AE events are monitored, as shown in Fig. 10a. Confining pressure closed the thermal induced crack and increases friction between grains, therefore more AE events are monitored once applied loading. Compared with Figs. 10b and 10d, after thermal treatment, violent AE events is lesser than that at room temperature, whereas AAE is more than that at room temperature. It means that the micro-crack is easier to initiate and dispersed distribute in the specimen, and the loading process shows plastic characteristic whenT = 600°C.
Figures 11 depict the comparison of the ultimate failure model of granite after thermal treatment between triaxial monotonic and cyclic loading compression, it is clear that loading path has slight effect on the ultimate failure modes. Under uniaxial compression, the granite specimens under monotonic and cyclic loading failed with axial splitting. Under triaxial compression, the distribution of cracks under cyclic loading is more complicated than that under monotonic loading, even though the specimen failed with shear cracks. On the one hand, more energy dissipated in the granite specimens under cyclic loading, which result in more micro-cracks initiation. Therefore, the potential position to form macro-cracks in the specimen increases. On the other hand, the shear cracks propagated quickly under monotonic loading after the peak strength. However, the shear cracks propagated step by step under cyclic loading. The shear cracks near stop propagated under unloading process, and then it may not propagate in the original direction under loading process. Therefore, the distribution of cracks in the specimens under cyclic loading is more complicated than that under monotonic loading. This phenomenon is also obtained by Yang et al. (2015).