Effect of Triaxial Loading and Unloading on Crack Propagation and Damage Behaviors of Sandstone: An Experimental Study

In the current study, a series of laboratory triaxial loading and unloading experiments were conducted for the purpose of comprehensively understanding the effects on sandstone of loading processes under axial stress conditions and unloading processes under confining pressure conditions. The mechanical properties and crack propagation behaviors of sandstone specimens were analyzed. In addition, a new damage variable based on crack evolution was proposed. The experimental results showed that the strain-softening modulus of the sandstone specimens under triaxial loading and unloading compression conditions decreased with the increases in the loading rate. Meanwhile, the elastic modulus was only observed to undergo small changes. This study found that with the increases in the loading rates of axial stress, the deformation and strength levels of the sandstone specimens also increased. In addition, the crack initiation stress showed a linear increasing trend with the increases in the loading rates of axial stress. The peak crack volumetric strain displayed a slight increase with the loading rate of axial stress, which indicated that the larger the loading rate of axial stress on the sandstone specimens was, the smaller crack propagation strain levels at the peak stress levels would be. The crack propagation velocities were observed to be larger when the loading rates of axial stress were higher. Also, at the same loading rate of axial stress, the crack propagation velocity corresponding to the peak stress of the volumetric cracks was the largest, followed by radial cracks. The crack propagation velocity of the axial cracks was determined to be the smallest. In this study, a new crack damage variable based on crack volumetric strain was proposed. According to the evolution of the crack damage variables of the sandstone specimens, the following four stages of the failure process were obtained: original damage stage; elastic stage; linear damage stage; and accelerated damage stage. The results obtained in this research study provided an important scientific basis for the support of future designs and safety protection measures in underground mining operations.

Automated summary

The study found that with the increase in loading rates of axial stress, the deformation, strength levels, and crack initiation stress of the sandstone specimens also increased; additionally, the initial stress and volumetric strain stress of the cracks showed a linear increasing trend with the increase in loading rates.