| In the process of coal mining,the physical and mechanical properties of rocks are often changed due to cyclic loading.The energy evolution and damage evolution law of rocks under cyclic loading and unloading not only play a key role in predicting the deformation of surrounding rock,but also play a guiding role in supporting surrounding rock.Therefore,it is of great theoretical significance and engineering application value to study the physical and mechanical properties,pore structure changes,energy evolution and damage mechanism of rock under cyclic loading and unloading.The ZTCR-2000 rock triaxial test system of weak cementation of Jurassic sandstone samples the uniaxial compression test and uniaxial cyclic unloading test,analyzes the sandstone under uniaxial compression and cyclic load and the mechanics properties,energy evolution and the damage evolution rule,and combined the technology of nuclear magnetic resonance(NMR)sandstone under cyclic loading is analyzed when the internal changes of the pore structure.The main research conclusions are as follows:(1)By scanning electron microscopy,the main mineral components of sandstone are feldspar,quartz,calcite,and other large grain minerals,followed by clay minerals such as kaolinite and illite.In addition,there are some compounds containing small amounts of calcium,iron and so on.Large mineral particles,such as quartz,calcite and feldspar,form the main skeleton of the structure through mutual stacking,which is then cemented and filled by clay minerals such as illite and kaolinite to form the whole structure.(2)Uniaxial compression tests were carried out on sandstone samples to obtain the basic mechanical parameters such as compressive strength,elastic modulus and Poisson's ratio.The stress-strain curve and the final failure mode of sandstone during uniaxial compression are obtained,and the failure process of sandstone under uniaxial compression is analyzed.The loading mode and loading rate of cyclic loading and unloading are determined.(3)From the perspective of energy analysis,it can be seen that the total strain energy,elastic strain energy and dissipated energy of sandstone specimen all increase with the increase of the number of cycles in the process of cyclic loading and unloading,and the fitting formulas of the three kinds of energy and number of cycles in the process of cyclic loading and unloading are obtained.In addition,at the same cycle number,the three energy values of the dry sample are all higher than that of the saturated sample.(4)In the process of cyclic loading and unloading,the energy storage rate is the smallest,the dissipation rate is the largest and the deformation is large in the compaction stage.At the stage of high compaction of rock sample,the growth rate of deformation slows down,and the energy storage capacity and dissipation capacity are stable.When the rock sample is close to failure,it is in the state of accelerated damage,and the energy parameters and residual strain will mutate,and finally fracture and instability will occur.(5)After each cycle of loading and unloading,the pore size distribution and porosity of sandstone under cyclic loading were measured by NMR equipment.The variation rule of pore structure of sandstone was analyzed by T2 spectrum curve and peak area curve,and the relationship between this change and the change of physical and mechanical properties was discussed.(6)By introducing the characteristic stress ratio coefficient,the ratios of crack closure stress,crack initiation stress,dilatation stress and peak stress of dry and saturated sandstone samples are 0.19,0.35,0.76 and 0.21,0.30,0.72,respectively.(7)The damage values of sandstone samples under cyclic loading and unloading were obtained based on the dissipated energy method.The damage values of sandstone D increased with the increase of the number of cycles.Figure [43] Table [16] Reference [76]... |
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