Damage characteristics and energy evolution of sandstone with defect of different geometric profiles under heterogeneous loads

To study the influence of non-uniform stress on rock damage, failure and the evolution law of damage zones, the mechanical response characteristics and energy evolution law of sandstone with defects under non-uniform stress were studied. The results show that the stress-strain curve stage in the low-stress area is highly consistent with the overall curve of the specimen, and the residual stress fluctuation characteristics of sandstone are highly consistent with the curves in the high and low stress areas. The higher the overall bearing capacity of sandstone, the greater the peak difference between stress areas. It is also found that in the high-stress area, circular defect sandstone shows low stress and high deformation at the peak, while trapezoidal and rectangular defect sandstone shows high stress and low deformation at the peak. In the low-stress area, the initial deformation field is prone to intersect, and trapezoidal defect sandstone has the largest radial deformation, circular defect sandstone has the largest axial deformation, and rectangular defect sandstone has the smallest. In the high-stress area, the strain curve contracts inward, while it expands outward in the low-stress zone. The acoustic emission ring count of sandstone with rectangular and circular defects shows a first increasing, then decreasing, and final increasing trend, while that of trapezoidal defect sandstone shows a continuous increase and sudden increases at multiple points, showing an overall dense-sparse-dense change pattern. In the initial stage, sandstone with rectangular defect generates more shear cracks, while sandstone with circular and trapezoidal defects generates fewer cracks. With the increase of stress, the number of shear cracks in the three defect shapes of specimens shows an increasing trend, and the failure mode of sandstone is mainly shear failure. In the high-stress area, there are more shear cracks and they are more likely to form coalesced cracks, while in the low-stress area, there are fewer cracks and they are mostly tensile cracks. The efficiency proportion in the high-stress area is higher than that in the low-stress area, and the proportion of elastic deformation energy in the low-stress area is higher than that in the high-stress area, especially in the specimen with circular defect shape. Energy dissipation in high-stress regions shows a steady increase, while growth in low-stress regions slows. The dissipation energy accumulation curve in high-stress zone shows a sequential trend of acceleration, uniform speed and deceleration trend, while the dissipation energy of the low-stress zone shows a sequential trend of acceleration, deceleration and acceleration trend.