Mechanical Properties And Damage Evolution Of White Sandstone Under Combined High Temperature-cyclic Loading

The deformation and damage characteristics of rocks during underground coal gasification are fundamental mechanical problems encountered in coal bed gasification production.In order to study the mechanical properties of rocks under the combined effect of high temperature and high stress as well as the characteristics of deformation and damage processes,the mechanical properties of thermally damaged rocks and the damage intrinsic model need to be investigated.In this paper,the physical and mechanical properties of thermally damaged white sandstone are firstly investigated by uniaxial compression test,and then the damage intrinsic model based on Mohr-Coulomb strength criterion under the combined action of high temperature and cyclic loading is established by using damage mechanics theory,and the rationality of this intrinsic model is verified by graded loading-unloading test.The relationship between the microscopic damage evolution characteristics and the macroscopic mechanical effects of thermally damaged white sandstone under the effect of graded addition-exit was investigated;finally,the fracture expansion pattern and penetration mechanism of white sandstone under different temperatures were analyzed.The results of the study showed that:(1)The physical and mechanical properties of white sandstone at different temperatures were investigated by conducting uniaxial compression tests on thermally damaged white sandstone to obtain each parameter of the test.It is found that:temperature will cause cracks in the inner layer of sandstone to form microcracks due to anisotropy and mismatch of thermal expansion and water loss,increase porosity,decrease uniaxial compressive strength,decrease elastic modulus,decrease Poisson’s ratio,and deteriorate the mechanical properties of sandstone;higher temperature will lead to increase sliding resistance between adjacent soil layers,increase shear strength,increase internal friction angle,and increase cohesion.(2)The damage intrinsic model of white sandstone under coupled high-temperature-cyclic loading established in this paper can more accurately describe the stress-strain relationship of thermally damaged white sandstone under cyclic loading-unloading.Due to the inherent nonlinear characteristics of sandstone,hysteresis loops occur during cyclic loading and unloading,and new residual strains are generated with each loading and unloading.The higher the temperature,the lower the strength of the sandstone,the lower the peak stress and the higher the peak strain.(3)The total damage variables of sandstone with the development of strain curve can be generally divided into four stages,which include damage weakening stage,plastic deformation stage,strain softening stage and residual deformation stage.The damage intrinsic evolution curve reveals the microscopic damage evolution characteristics and macroscopic mechanical effects of white sandstone,and the damage mechanism of white sandstone under the coupled macroscopic high-temperature-cyclic loading is revealed from the microscopic damage evolution.(4)In the study of fracture extension patterns and penetration mechanisms of rocks at different temperatures,four types of fractures leading to the overall fracture of white sandstone were found through the analysis of fracture extension patterns of rocks:open fracture,secondary coplanar fracture,secondary inclined fracture and oblique fracture,and there is also a "wedge" damage mode.Figure [37] Table [3] Reference [86]...