Mechanical Characteristics And Fatigue Life Analysis Of Sandstone Under Cyclic Loading

As China's national strength continues to grow,the construction of the project is gradually extended to areas with poor geological conditions.During the construction and operation of the facility,the rock and soil subjected to the load on the project will be subjected to various stresses,usually except for construction.In addition to the static load of the object,most of the facilities are subjected to cyclic loads,which will profoundly affect the stability of the rock and soil and cause damage to the engineering building and threaten personal safety.Therefore,the study of the effects of cyclic loading on rocks is particularly important.At the same time,the change of rock damage is gradual and slow,and it is difficult to be noticed on a macroscopic scale,especially in the case of hazardous engineering,which is important for the study of damage and life of rock cyclic loading.In this paper,through the similar material research on medium weathered sandstone obtained from a nuclear power project in Guangdong,it is concluded that the similar material ratio of the original rock is satisfied,and the sample made by using the similar material is subjected to different cycle conditions on the MTS815 rigid mechanical testing machine.In the mechanical test,the cycle life,mechanical characteristics and deformation law of the sample with different cycle frequency,stress amplitude and upper limit of stress are studied.The damage morphology and damage development characteristics of the rock are analyzed.The damage curve is fitted and the residual life prediction process of the rock is established.The specific research is as follows:(1)According to statistics,the average density of the original medium weathered sandstone is 2.33g/cm3,the average compressive strength is 22.36 MPa,and the average peak strain is 0.0085.Through the orthogonal experiment,the optimal similar material ratio is quartz.Sand: barite: cement: early strength agent:water to glue ratio is 2:1.4:1:0.03:0.21.(2)The cyclic load test was carried out on the sample by changing the frequency,upper limit of stress and stress amplitude.It was found that the increaseof frequency significantly reduced the cycle life of the sample,and the frequency of the fitting result was linearly and negatively correlated with the life of the sample.The life of the specimen decreases as the magnitude of the stress increases.When the upper limit of the stress does not reach the threshold value of the sample stress,the number of cycles of the sample can be increased indefinitely.Conversely,when the stress exceeds the threshold value,the number of cycles of the sample is significantly reduced due to the increase of the stress.(3)Due to the internal cracks and defects,the response time of stress and dynamic strain to dynamic loads is different,and the response time difference between dynamic stress and dynamic strain is generated.On the stress-strain curve,the hysteresis curve under cyclic loading is shown as hysteretic curve.The shape of the curve is consistent with the development trend of the axial strain curve.The shape of the hysteresis curve can be divided into four stages,namely the initial compaction phase,the stable development phase,the failure instability phase and the complete failure phase,and the specimen circumferential deformation and The amount of axial deformation rises steadily in the middle and early stages of the cycle,in which the circumferential direction changes.The shape of the axial deformation increases more.When the sample transitions from the stable development stage to the failure and instability stage,the hoop strain first increases greatly compared with the axial strain,and then the sample is destroyed,and the hoop and axial growth are abrupt.The volume of the sample is continuously reduced in the middle of the cycle.As the cycle progresses,the body strain will undergo a turning point,from the volume of the sample to the expansion,and the point at which the sample breaks the instability.The sample life is discrete under different cycling conditions,but its ultimate strain is relatively stable.Rock damage and life prediction can be performed by ultimate strain.(4)The dynamic elastic modulus of the sample decreases with the increase of the number of cycles at different frequencies,but its value keeps close to the situation.The dynamic elastic modulus of the sample is weakly controlled by the frequency,and the dynamic elastic modulus of the sample is weak.The reduction is related to the increase in stress amplitude.However,if the upper limit of stress is less than the threshold value,the dynamic elastic modulus will increase,and when the upper limit of stress is greater than the threshold value,the dynamic elastic modulus will show a downward trend.The PFC numerical simulation of the cyclic load of the sample was established,and the PFC mesoscopic parameters of the simulatedmoderately weathered sandstone were established.The differences in the direction of motion of the particles and the difference in the intensity of the particle motion led to the development of meso-fracture and the generation of fissures.(5)The development law of the specimen under cyclic loading shows three-segment S-shaped curve shape under the upper limit of low stress,frequency and stress amplitude,which correspond to the initial compaction,the initial expansion of damage,the stationary development of medium-term damage and the accelerated expansion of late damage..With the increase of stress,frequency and stress amplitude,the shape of the damage curve gradually becomes two-stage,and the boundary line of the middle and late stages gradually merges,indicating that the damage development rate increases with the increase of the cycle condition.The damage curve under different cyclic loading conditions basically conforms to the curve form of power function plus exponential function plus constant term.The correlation between fitting function and data is more than 0.9,and the fitting function can well indicate the development trend of data.(6)Under compound cyclic loading,the specimen has certain inheritance at the transition point of different cyclic conditions,and the piecewise damage fitting function is similar to the single condition cyclic load damage fitting function.Using this characteristic combined with the maximum residual strain method to find the damage can be compounded.The remaining life of the circulating sample is better predicted.