Variation Law of Mode-I Dynamic Stress Intensity Factor of Rock under Unloading Condition

The variation law and influencing factors of mode-I dynamic stress intensity factors (DSIFs) at the crack tip under unloading condition were analyzed with the numerical simulation software ANSYS, and the dynamic initiation behavior of cracks in rock was studied.The results show that the greater the peak value of DSIFs, the greater the confining stress and unloading rate, the more obvious the dynamic effect of unloading, and the axial stress has no effect on the unloading dynamic effect. During transient unloading, the maximum DSIFs appears in the post-unloading stage, and there is time-lag effect. During unloading, the time-history curve of DSIFs shows attenuation effect with the release of internal strain energy. The stiffness of surrounding rock has a significant impact on DSIFs under unloading conditions. The smaller the stiffness of surrounding rock relative to rock burst, the greater the peak value of DSIFs, and the more obvious the dynamic effect. At the initial stage of high unloading rate case, the increasing rate of crack initiation toughness is higher than that of DSIFs, the crack will not initiate immediately. With the increase of unloading time, the crack will crack.