Experimental And Simulation Study On Damage Characteristics Of Sandstone Under Cyclic Loading And Unloading

Cyclic load is one of the most common types of dynamic load.In the study of rock damage under complex loads,it is necessary to study the defommation and failure of rock under different cyclic load conditions.Under the action of cyclic loading,the internal of rock is damaged and permanent cracks are produced.After repeated cyclic loading,the cracks continue to expand until the rock is completely broken.At present,the development level of geotechnical engineering is constantly improving,and the requirements for long-term stability of rock mass are also constantly improving.According to the engineering practice and related experimental results,even when the stress level is lower than the static compressive strength,the rock structure will still be damaged under the cyclic load.Due to the great discreteness of rock,the fatigue failure criterion of metal material can not be applied to rock material very well.Therefore,it is necessary to put forward the fatigue failure criterion of rock,study its damage process under cyclic load,and summarize its failure evolution rule.In the aspect of numerical analysis,the whole stress-strain curve of sandstone is obtained through the uniaxial compression experiment in laboratory.At the same time,the cyclic loading and unloading tests of sine wave and triangle wave are carried out to study the damage and failure characteristics of sandstone under different loading conditions.The experimental results verify the rationality of the numerical model established by PFC software,adjust the loading and unloading parameters and carry out the numerical simulation of cyclic loading,analyze the damage characteristics of sandstone under loading conditions,and finally compare the damage characteristics of sandstone under other loading waveforms.The main work and achievements of this paper include:(1)Based on the experimental results,the micro parameters of rock static mechanical characteristics are calibrated.By comparing the experimental curves and numerical analysis results,it can be proved that the established rock static mechanical model can basically reflect the static mechanical properties of sandstone.After that.the cyclic loading and unloading experiments of sine wave and triangle wave are carried out respectively.According to the results,the stress-strain curve of rock under cyclic load can be divided into three stages:initial deformation stage,isokinetic deformation stage and accelerated failure stage.Each stage is accompanied by the accumulation of rock damage stress variables.When the damage accumulates to the maximum strain value of sandstone,the rock sample will be damaged.(2)Based on the analysis of uniaxial compression test results,the basic mechanical parameters of sandstone are obtained.Through the sensitivity analysis of different micro parameters in the static mechanical model of sandstone,the influence of each parameter on the change of macro mechanical characteristics is studied.According to the numerical simulation results,it can be seen that the loading rate of the loading plate,particle size,particle elastic modulus and other particle micro parameters have little influence on the macro mechanical parameters of the model;the parallel bonding elastic modulus,the ratio of normal to tangential stiffness,tensile strength and shear strength and other parallel bonding parameters have great influence on the macro mechanical parameters of the model.Based on the influence law of the meso parameters of the model,the reference for the damage model of cyclic loading and unloading is provided.(3)In the study of cyclic loading and unloading damage,PSC stress corrosion model is introduced into PFC3D numerical simulation software.Through the numerical simulation of sandstone under different cyclic loading and unloading conditions,the influence of loading conditions such as upper limit stress ratio,lower limit stress ratio,stress amplitude and unloading frequency on rock life is analyzed.Through the comparison of simulation results,it can be seen that the higher the upper limit stress ratio is,the faster the loading frequency is,the faster the axial stress accumulation of rock is,and the shorter the damage life of rock is;at the same time,two kinds of waveform experiments are carried out,the rock sample of sine wave is destroyed first,and the influence of sine wave under the same loading and unloading frequency on the damage life of rock is greater than that of triangular wave stress,when the peak load,that is,the upper limit stress is fixed,With the increase of loading frequency,the average rate increases and the life of rock decreases.(4)On the basis of the numerical model of sandstone,the loading mode of different waveform loading is introduced,and the damage characteristics of sandstone under different waveform loading conditions are analyzed.The results show that the more cyclic loading times are inserted in the creep process,the lower the damage life of the rock is,and the more obvious influence of the fatigue cycle times than the creep time is,which is the main factor affecting the damage life of the rock.Compared with the effect of the stable loading time on the rock life,the longer the stable loading time is,the lower the rock life is.The results show that the effect of trapezoid wave is greater than sine wave and sine wave is greater than triangle wave.