Mechanical Properties And Fatigue Constitutive Model Of Argillaceous Quartz Siltstone Under Cyclic Loading

In the construction and operation stages of large-scale and super-large geotechnical engineering,the surrounding rock is often subjected to cyclic loads,such as frequent disturbance loads in the tunnel blasting and excavation stage,reciprocating vehicle loads in the road and bridge operation stage,periodic operation loads in the reservoir dam storage and drainage process,and gas storage injection and production process.Rock mechanical behavior is closely related to the loading path.The mechanical properties of rock under cyclic loading are significantly different from those under conventional loading.In-depth study of rock deformation characteristics,mechanical properties and macroscopicmesoscopic response mechanisms during cyclic loading has important significance for guiding engineering design and safe construction,and improving the accuracy of engineering disaster prediction.This work is based on the National Natural Science Foundation of China"Dynamic Fatigue Properties and Energy Dissipation Mechanism of Rock(mass)with High Stress and High Energy in Deep Mining"(Grant No.51974014),taking argillaceous quartz siltstone as the research object,using macroscopic rock mechanics test,mesoscopic scanning test,mechanism analysis and theoretical derivation,a quantitative calculation method of characteristic stress based on the energy evolution law of rock during stepwise cyclic loading was proposed.The deformation and mechanical characteristics of argillaceous quartz siltstone,the spatial-temporal evolution law of acoustic emission(AE)and the macroscopicmesoscopic fracture mechanism under different upper limit loads and cycle times were studied.A fatigue constitutive model with the dynamic phase difference of rock stress-strain was established.The main research results obtained were as follows:(1)By analyzing the energy evolution law of the stepwise cyclic loading test,and considering the influence of rock stress-strain asynchronous effect on energy calculation results,the elastic energy was separated into particle elastic energy and crack elastic energy.A new method for calculating characteristic stress(closure stress σcc,crack initiation stress σci and damage stress σcd)of rock was proposed based on the particle elastic energy ratio,crack elastic energy ratio and dissipation energy increment.The accuracy of the method was verified using the dilatancy point and AE impact number during stepwise cyclic loading.(2)The upper limit loads(Fmax)were set at different stress intervals,and cyclicto-monotonic loading tests and fatigue failure tests under different upper limit loads were carried out.The deformation evolution law of residual strain,hysteresis loop,monotonic loading strain ratio,and fatigue limit strain ratio of argillaceous quartz siltstone were analyzed.The change characteristic of mechanical parameters such as peak strength,fatigue life,elastic modulus,and lateral-axial strain ratio were obtained.The upper limit load threshold for weakening-strengthening of argillaceous quartz siltstone under cyclic loading was determined.According to the variation law of AE RA/AF and the critical slowing characteristics of AE parameters,the precursors of rock fatigue failure was accurately identified.The correlation between macroscopic mechanical behavior and meso-structural characteristics of argillaceous quartz siltstone was established.The strength variation characteristics and mechanical response mechanism of argillaceous quartz siltstone under cyclic loading were revealed.(3)Cyclic-to-monotonic loading tests with different cycle times were carried out in the σci and σcd(Fmax=45 kN)interval and σcd and fatigue strength interval(Fmax=55 kN),respectively,and the evolution law of peak strength,dilatancy point stress,hysteresis loop,strain and compliance of argillaceous quartz siltstone with the number of cycles were obtained.It was found that under the upper limit load of 45kN and 55kN,the specimen was weakened and strengthened after low and high cycles,respectively.The ratio of dilatancy point stress at the last cycle to peak strength of different specimens followed a logarithmic function distribution,and a formula was proposed to predict the uniaxial peak strength of rock after near fatigue strength cyclic loading.The AE timing parameter,peak frequency,b-value and AE spatial-temporal evolution law were analyzed.It was pointed out that the failure mode of specimens changed from relatively simple tension failure under uniaxial compression test to through shear failure under cyclic-to-monotonic loading tests and to tension shear conjugate failure mode for those specimens which experience cyclic loading with a large number of cycles.(4)The micro structure morphology of argillaceous quartz siltstone after failure under different loading conditions were obtained by SEM,and the particle contact mode and micro fracture characteristics of the specimens before and after failure were compared.The pore fractal dimension,fracture spatial configuration characteristics and fracture scale distribution of argillaceous quartz siltstone were obtained by X-ray CT scanning.Based on the evolution law of deformation,mechanical parameters and AE singles of argillaceous quartz siltstone,the weak structure fracture effect and the compaction effect under cyclic loading on the rock were proposed,which could reveal the weakening-strengthening mechanism of argillaceous quartz siltstone under different upper limit loads and cycle times.Under cyclic loading,the increase of crack scale uniformity,the decrease of fracture degree and the increase of effective bearing area are the reasons for rock strengthening,while the crack development tends to localize,and the increase of fracture scale under cyclic loading leads to the weakening of rock.(5)Based on the cross-correlation function method,the phase difference between the loading and unloading stages of rock under different upper limit loads and cycle times was quantitatively calculated.It was revealed that the rock stressstrain asynchronous phenomenon was the result of the interaction between the sliding frictional resistance of particles and the accumulation of plastic deformation.Based on the stress-strain curves of rock under rheological loading and cyclic loading,rheological deformation and fatigue deformation characteristics,and longterm strength and fatigue strength,the rationality of using rheological model to describe fatigue deformation of rock was illustrated.Newtonian dashpot in classical Burgers model was replaced by Abel dashpots of different orders,and a variable viscosity coefficient Abel dashpot with stress switch was connected in series to establish a fatigue constitutive model of rock under cyclic loading.Where,the cyclic load(strain)was decomposed into static load(strain)and alternating load(strain).The stress-strain dynamic phase difference was introduced to modify the fatigue constitutive model.The critical damage threshold and instability criterion for fatigue failure were proposed,and the fatigue life prediction equation was established.Based on the fatigue test data of granite and argillaceous quartz siltstone under different cyclic upper limit loadings,parameter identification and applicability verification of the fatigue constitutive model were carried out.The results showed that the model gives a precise description of the whole process of deceleration,constant velocity and accelerated fatigue deformation of rock and the stress-strain asynchronous phenomenon under cyclic loading.