Damage Analysis Of Sandstone Under Graded Cyclic Loading And Unloading

In order to explore the damage evolution characteristics of sandstone materials under the condition of graded cyclic loading and unloading,this paper analyzes the rock sample damage and failure instability process by establishing the damage statistical constitutive model and from the perspective of energy dissipation.Firstly,based on the Weibull probability distribution function and the theory of rock strain strength,the expression of damage variables was defined through the theory of damage mechanics,and a damage constitutive model that can describe both the cyclic loading stage and unloading stage of rock was established.Then,compared with the experimental data in the two stages of cyclic loading and unloading of sandstone,the rationality of the constitutive model is verified,the physical significance of the parameters is clear,and the parameter sensitivity analysis is carried out.By analyzing the energy dissipation evolution of sandstone during cyclic loading and unloading,the damage process and mechanical deformation characteristics of sandstone are illustrated.In order to accurately calculate the energy parameters of each stage,the influence of residual deformation and hysteretic characteristics is considered,and the form of hysteretic ring is analyzed.The energy conversion process is described by energy storage rate and energy dissipation rate,and the relationship between cycle times,energy storage rate,energy dissipation rate and residual strain is analyzed.Finally,the advantages and disadvantages of the two methods are discussed.The research results show that:First,the rock damage constitutive equation established under cyclic loading and unloading has a high fitting degree with the test stress-strain curve,which proves its rationality and applicability.Th e s tress-strain curve and damage value obtained by the damage constitutive model can correctly reflect the evolution process of rock damage and describe each stage of rock failure.Parameter sensitiv ity has different effects on the constitutive model.Second,the damage evolution and hysteresis effect of the hysteresis loop curve of each stage of cyclic loading and unloading were analyzed in detail.In the process of cyclic loading and unloading,the energy dissipation rate of rock samples has a positive correlation with residual strain,which indicates that the residual strain has a certain impact on the energy dissipation rate,and the influence of residual strain cannot be ignored in describing the damage evolution process of rock samples.When cyclic loading and unloading is close to failure,all energy parameters and residual strain will mutate when accelerated damage is close to failure,which indicates that the rock sample is in the stage of failure and instability occurs.Third,the hysteresis effect has a significant effect on the formation of sandstone hysteresis ring shape("pointed leaf' shape).The strain phase is equal or lagging behind the stress phase,in which the strain phase is ahead of the stress phase.Because of the uncompactness of rock samples,plastic deformation will occur rapidly in the process of continuous loading,and the damage degree will increase continuously,and the deformation rate will also be accelerated.Therefore,the main reason why the strain phase is ahead of the stress phase is the uncompactness of the rock sample,and the fundamental reason is the generation of plastic deformation.The strain phase lags behind the stress phase in the loading and unloading phase,which is the effect of the obstructed damping force,but is mainly due to the first trip.The cyclic loading makes the sandstone highly compacted with local plastic deformation,which has a great influence on the situation that the strain phase in the subsequent cycle lags behind the stress phase.Figure[29]Table[3]Reference[52].