Study On The Fatigue Damage Characteristics Of Sandstone And Stability Of Rock Slope Under Cyclic Dynamic Loading

It has been decades since the fatigue characteristics of materials were studied,and abundant results have been obtained.Compared with other materials,the research on fatigue characteristics of rock is still lagging behind.The fatigue characteristics and mechanism of rock materials under cyclic dynamic loading are not clear enough,and the theoretical system needs to be improved.Based on the project of National Natral science foundation of China,By means of laboratory uniaxial test,cyclic dynamic test,nuclear magnetic resonance?NMR?,numerical calculation and theoretical derivation,the damage mechanics characteristics,cumulative deformation law,meso-damage evolution,energy dissipation,constitutive model and long-term stability of rock slope under repeated micro-earthquakes are studied in depth and systematically.The main achievements are as follows:?1?The mechanical properties of sandstone under different loading rates show that the characteristic stresses of sandstone,including initiation stress,dilatancy stress,peak strength and elastic modulus,increase with the increase of strain rate.The cracking stress,dilatancy stress and modulus of elasticity are linearly related to the peak strength.There is a linear relationship between initiation stress and peak strength ratio,elastic modulus and logarithm of strain rate.The results show that with the increase of strain rate,the time of rock entering plastic stage is delayed,the proportion of plastic section in the total stress-strain curve decreases,and the proportion of elastic order increases,that is,the elastic properties of rock are more obvious.A uniaxial strength prediction model based on wave velocity-rebound synthesis method is established.The prediction accuracy of sandstone uniaxial strength under three non-destructive estimation methods,i.e.wave velocity,rebound and wave velocity-rebound synthesis,is compared and analyzed.The results show that the estimation accuracy of wave velocity-rebound synthesis method is much higher than that of single factor method.?2?The fatigue damage test of Sandstone on reservoir bank slope was carried out,and the relationship between loading frequency,stress level and fatigue life of sandstone was obtained.The test results show that the higher the frequency,the higher the fatigue life of rock is;The logarithm of fatigue life is linear with the frequency while the logarithm of fatigue life is lonear with the upper limit stress in the range of 1³Hz.And a fatigue life prediction model based on the curve is established.Comparing the cyclic dynamic and static test curves,it is found that the horizontal line of ultimate deformation stress under cyclic dynamic loads is close to the intersection of the horizontal line of ultimate deformation stress with the drop section of uniaxial compression stress-strain curve.In the range of 1³Hz,the ultimate axial deformation of sandstone is basically the same,which indicates that the fatigue failure limit deformation is little affected by the loading frequency.A multi-factor fatigue life prediction model considering frequency and loading stress level was established by introducing a correction factor considering the effect of strain rate on stress level.?3?Nuclear magnetic resonance?NMR?tests were carried out on sandstone specimens with different loading times.The results of porosity,T₂ spectrum and NMR imaging under different working conditions were obtained,and the results were in good agreement.The fracture development law of sandstone in initial deformation stage,low-speed deformation stage and accelerated deformation stage under different cycles is revealed.?4?The micro-evolution law and energy dissipation process of sandstone under cyclic dynamic loading were studied by using particle flow sequence.The results of cyclic dynamic loading and static loading were compared,and the mechanism of fatigue micro-damage and energy dissipation was revealed.Under cyclic dynamic loading,the energy absorbed by rock samples before failure is mainly dissipated and stored in the form of sliding friction energy and elastic strain energy.The increase of contact opportunities between particles in the sample results in a large number of internal cracks.Finally,by comparing the results of NMR and numerical experiments,it is found that the meso-damage evolution laws of the two methods are identical.The numerical calculation further explains the cause of the uniform distribution of the pore in NMR imaging.?5?Based on the cumulative deformation characteristics of sandstone under cyclic dynamic loading,a non-linear rheological constitutive model based on fractional derivative of variable order is constructed,which can describe the initial deformation,low-speed deformation and accelerated deformation stages of sandstone.Levenberg-Marquard?LM?algorithm in 1st Opt calculation software was used to solve the constitutive equation parameters according to the fatigue test data of sandstone,and the curve was fitted to verify the applicability of the sandstone non-linear rheological constitutive model.?6?By fitting the attenuation process of dynamic modulus of elasticity with least square method,a fatigue damage evolution model of sandstone based on residual stiffness is established.The attenuation model is redeveloped by finite difference software.On the basis of successful validation of applicability,the slope stability of a jointed rock slope under V,VI and VII seismic intensity is calculated.Comparing the results of M-C model with those of this model,the results show that under this model,the results of slope calculation tend to be safer and have certain advantages.Considering the maximum intensity of reservoir-induced earthquakes,the stability of slope model under multiple earthquakes is calculated by using small seismic waves with intensity III and IV.The results show that with the increase of earthquake loading times,the dynamic stability coefficient of slope decreases and the cumulative damage speed of slope increases.Under different intensity,the higher the earthquake intensity,the faster the cumulative rate of slope damage is lower.By recording the changes of velocity and displacement vectors of slopes under the cyclic action of seismic waves,the process of slope instability under the action of earthquakes is analyzed.