Study On Sandstone Deformation Parameters And Damage Evolution Under Multi-level Cyclic Loading And Unloading With Variable Lower Limit

The engineering disasters in rock engineering are mostly caused by fatigue.On the one hand,fatigue is often complex and unpredictable,on the other hand,people's cognition of fatigue still needs to be further deepened,and the corresponding theoretical research also needs to be further strengthened.In this paper,some conditions in rock engineering are abstracted as the multi-level cyclic loading and unloading process with variable lower limit.The mechanical parameters and damage evolution analysis are mainly carried out for the factors such as lower limit of stress,confining pressure,amplitude and frequency.Based on the evolution of mechanical parameters such as elastic modulus,strain and damage,combined with the micro fracture characteristics of SEM scanning,the mechanical response behavior of sandstone during multi-level cyclic loading and unloading with variable lower limit is studied.The results show that:(1)Compared with multi-level cyclic loading and unloading process with constant lower stress limit,the process with variable lower limit will produce hardening effect,but it will show brittle fracture characteristics in failure and reduce the threshold of each stage.(2)The main influence of confining pressure in the process of multi-level cyclic loading and unloading with variable lower limit process is concentrated in the plastic stage,which affects the threshold value of the third stage and improves the strength of the rock,so that the length of the shear plane decreases with the increase of confining pressure,and the fracture position shifts from the whole to the center.(3)The effect of amplitude on the process mainly lies in the competition of hardening effect and softening effect.The increase of stress amplitude will contain important points,change the degradation rate of mechanical properties caused by softening mechanism,and then change the deformation evolution characteristics of sandstone.(4)The frequency mainly affects the deformation parameters by changing the strain rate and the number of cycles.Generally,shortening the time to increase the frequency will harden the rock,but when the number of cycles increases,there will be compaction effect and over compaction effect,which will increase the cumulative amount of degradation and soften the rock to a certain extent.(5)Through the axial strain,the damage development can be well characterized.Based on this,the fatigue life prediction equation is put forward,which can predict the fatigue life better.(6)According to the fracture characteristics in SEM,the fatigue pattern at the constant lower limit is usually fluvial,while that at the variable lower limit is shell.Confining pressure mainly changes the size of dimple,and then changes the plasticity.With the increase of the amplitude,the fracture characteristics gradually develop from step to shell to plane.The increase of frequency will increase the proportion of transgranular fracture.