Experimental Study of the Crack Propagation and Acoustic Emission Characteristics of Red Sandstone under Cyclic Loading

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Crack propagation in sandstone was tested under uniaxial cyclic loading with three different loading rates and flaw configurations. The crack propagation, acoustic emission (AE) characteristics, and strength variation were analyzed. It was found that crack initiation and rock bridge failure cause a sharp increase in AE activity, as observed from the slope of the cumulative AE count. The energy released during the initiation of shear cracks, secondary cracks, and rock bridge failure was much greater than that released during the initiation of wing cracks. The loading rate had an interesting effect on the failure process, and the energy release under a higher loading rate was less than that under a lower loading rate before the damage stage. With the same flaw configuration, a specimen of the same group under a higher loading rate underwent more severe damage. In specimens of Groups 1 and 2, with overlapping preexisting flaws, the effects of the loading rate were limited, and the rock bridge failure pattern was basically dominated by the flaw configuration. However, in Group 3, with nonoverlapping preexisting flaws, the effect of the loading rate was more obvious, and many more cracks initiated under a higher loading rate. In this case, each crack propagated as an independent system during the initial loading stage, and the crack propagation path was random. For the same loading rate, the crack propagation can be divided into three mechanisms depending on the flaw configuration: (1) wing crack and mixed mode crack, (2) wing crack and antiwing crack, and (3) wing crack and shear crack.