Dynamic Mode II Fracture Toughness of Rocks Subjected to Various In Situ Stress Conditions

The shear failure of deep rocks under both a dynamic disturbance and an in situ stress (e.g., preload and confining pressure) is common in deep underground engineering. Thus, it is important to quantify the dynamic mode II fracture toughness K-IIC of deep rock considering the in situ stress state. Recently, the punch-through shear (PTS) method and the short core in compression (SCC) method have been successfully adopted to measure the dynamic K-IIC of rocks. However, the applicability of these two methods to determine the dynamic fracture toughness K-IIC of rocks under preload has not been verified. In this study, the PTS and SCC methods were applied to experimentally measure the dynamic K-IIC of rocks subjected to different preload levels. Further, the dynamic K-IIC of rocks under various confining pressures was numerically obtained using the SCC method and finite element analysis, because it is difficult to exert the confining pressure on the SCC specimen. The results indicate that the dynamic K-IIC of rocks increases with the confining pressure and the loading rate but decreases with the preload, and the total K-IIC of rocks derived from the PTS/SCC specimens are almost consistent under the same loading rate regardless of the magnitude of the preload exerted on the PTS/SCC specimens. Another important observation is that the dynamic K-IIC of rocks under confining pressures derived from the PTS method is remarkably different from that obtained from the SCC method. Theoretical analysis was conducted to quantitatively rationalize this discrepancy using the difference of the normal stress state and the stress intensity factor in these two methods. An empirical formula was proposed to predict the effect of the loading rate, the confining pressure and the specimen geometry on the dynamic K-IIC and to establish the relationship between the dynamic K-IIC of rocks measured from the PTS specimen and the dynamic K-IIC from the SCC specimen.

Automated summary

In this study, the PTS and SCC methods were used to measure the dynamic K-IIC of rocks subjected to different preload levels, and the dynamic K-IIC of rocks under various confining pressures was obtained using the SCC method and finite element analysis. The results reveal that the dynamic K-IIC of rocks increases with the confining pressure and loading rate, but decreases with the preload. Furthermore, the dynamic K-IIC derived from the PTS method differs significantly from that obtained from the SCC method, and an empirical formula is proposed to predict and establish the relationship between the dynamic K-IIC measured from the PTS and SCC specimens.