Mechanical characteristics and energy dissipation characteristics of sandstone under triaxial stress conditions

In order to obtain the rock characteristics of mechanical properties and energy evolution under loading condition,the conventional triaxial compression tests are conducted under different confining pressures. Based on the energy balance theory,the energy conversion law of sandstone during loading under different confining pressures is analyzed. The relationship between characteristic stresses,crack evolution and energy dissipation of sandstone are discussed under different confining pressures. The results show that the crack closure stress,the crack initiation stress,the dilatation stress and the peak stress all increase linearly with the increase of confining pressure during the conventional triaxial compression loading of sandstone specimen. The crack initiation stress and the dilatation stress can better describe the stable state of rock. The crack initiation stress can be regarded as the initial stress for the occurrence of new micro fracture,while the dilatation stress can be regarded as the sign of entering the plastic yield state. The energy characteristics have a good correspondence with the stress-strain curve and the characteristic stresses under different confining pressures. In the stage of the micro-crack compressed,the rate of energy conversion is very low. In the stages of elastic deformation and micro-crack steady growth,most of the strain energy converted from work done of external forces is stored as elastic strain energy,and less dissipation of energy is caused by internal damage and plastic deformation of rock. In the stage of crack unsteady growth after volume dilatation stress,the dissipated energy increases significantly due to the internal damage and plastic deformation of rock. Near the peak stress,the accumulated elastic strain energy is rapidly converted into the dissipated energy for rock failure. The dissipated energy ratio (U^d/U) shows the rule of increasing-decreasing-increasing again with the increase of the axial strain. The turning points caused by the variation tendency of dissipated energy ratio correspond to crack closure stress and volume dilatation stress. Dissipated energy increases gradually with the accumulation of axial crack strain. Before the volume dilatation stress,the dissipated energy increases slowly at the low level with the increase of axial crack strain. After the volume dilatation stress,the energy dissipation increase linearly with the increase of axial crack strain caused by the unstable propagation and transfixion of cracks in rock. In the post-peak stage,although the stress decreases,the energy dissipation remains at a high level resulting from the crack continuing to expand and sliding. The energy evolution can reflect the damage state of rock in the process of deformation and failure,and it is very important to describe the mechanical properties and damage characteristics of rock from energy theory for understanding the damage evolution of rock.