Experimental Research On Nonlinear Behavior And Damage Of Cemented Sand And Gravel Material Under Cyclic Loading

The seismic resistance of dams has been a difficult problem in hydraulic engineering construction,it is related not only to the complexity of seismic load,but also to the dynamics performance of dam materials.Research the dynamics performance of dam materials is an important method to solve the seismic resistance problem,and also provide an effective way of thinking for the seismic research of water conservancy projects.Earthquake is a kind of complex vibratory process,taking the cyclic loading to simulate the seismic load is a good method to research the dynamics performance of materials,it lays a theoretical foundation for the development of cemented sand and gravel(CSG)dam to permanent high dam through researching the dynamics characteristics of this material.According to the gradation distribution of sand and gravel in natural rivers,taking the CSG material with a certain mix ratio as the object,on the basis of laboratory dynamic triaxial tests,analyzes the dynamic stress-strain relationship,failure law,nonlinear hysteretic characteristics and damage law of CSG material under cyclic loading,based on the dynamic stress-strain hysteretic relationship,a mesoscopic hysteretic model was founded,and discussed the dynamic damage model of CSG material combine with the theory of nonlinear damage mechanics.The main research work and results are showed as:(1)Based on the test results,analyzed the failure strain standard of CSG material under different cyclic loading mode,determined the three-stage distribution characteristics of cumulative strain curve and the corresponding critical strain values,the cycle number in different stages were affected by the confining pressure and the cement content.A calculation method of dynamic modulus was proposed according to the characteristics of hysteretic loops,studied the distribution law of dynamic modulus during the whole cyclic process,and the relationship between confining pressure,the cement content and the dynamic modulus,too.(2)Based on the envelope of stress and strain,the stress-strain curves of CSG material under dynamic loads can be roughly divided into three stages:elasticity,yield and softening.On this basis,the dynamic constitutive equation of CSG material was established.(3)The CSG material exhibit obvious non-linear hysteresis characteristics under cyclic loading,the hysteresis loops are crescent shape and unclosed in the bottom,and the degree of non-closure reflects the plastic deformation of a single hysteretic loop;the change trend of residual deformation is big-small-big with the cycle numbers;The hysteretic loop curve can be described as an exponential function based on E.(4)An energy calculation method based on hysteresis loop characteristics was proposed,analyzed the energy mechanism and damping ratio evolution law of CSG material.According to the macroscopic appearance in the process of specimen rupture,the crack propagation evolution were mainly the initial surface crack opening,micro-crack re-initiation,aggregation and propagation to form the main fracture zone,and accompanied secondary cracks.The failure mode of specimens was mainly shear dilatancy failure,and the connection between microcracks presents obvious zigzag lines around the aggregate due to the influence of aggregate shape.(5)Discussed the distribution of classical PM model and the influence of parameters on stress-strain curves.On the basis of the classical PM space model,a non-linear elastic-plastic micro-hysteretic model suitable for CSG material was established,and determined the parameters of the micro-hysteretic mode,the rationality of the model was verified by comparing with the experimental results.(6)Combining with the mechanical characteristics of CSG material,proposed a definition method of damage variable suitable for CSG material,the distribution law of damage corresponds to the distribution law of cumulative strain,it also reflected three stages of development:closure of original defects,initiation and propagation of new cracks and unstable propagation in CSG material,and shown further that the failure of material was caused by the accumulation of damage.A three-stage non-linear damage model suitable for CSG material was proposed,and the physical meaning of each parameters and theirs influence for the curve in the model were analyzed,and determined the range of three parameters for CSG material.