Evaluation And Application Of Typical Material Models Used In Fe Modelling Under Monotonic And Cyclic Loading Conditions

With the development of computer technology,the finite element（FE）method plays an important role in the field of reinforced concrete（RC）structure analysis.Due to the behaviours of RC structures under different loading conditions are highly complex,establishing advanced numerical models based on continuum mechanics by using the robust concrete material model is necessary,of which the mechanical response and failure mechanism of RC structure under different loading can be better understood.The selection of concrete models,preliminary benchmark tests of the chosen concrete model as well as the numerical application of the concrete model under monotonic and cyclic loading were studied respectively.The failure characteristics of the RC structure under different loading conditions as well as the analysis characteristics and requirements for the meso and coarse-scale FE modelling was firstly discussed.The elastic-plastic model,elastic damage model and elastic-plastic damage coupling model based on continuum mechanics were compared and evaluated.The concrete damaged plasticity（CDP）model with the thermodynamic consistency was finally selected as the analysis model based on the analysis requirements of damage accumulation simulation.In addition,based on the existing research on the concrete material properties and the FE benchmark test（uniaxial compression/tension,uniaxial compression/tension cycle test）,the application of the concrete material properties including the fracture energyG_F,crushing energyG_c,uniaxial stress-strain curve and damage calculation model in CDP model was discussed.On this basis,FE models of a RC simple supported beam under monotonic loading and a RC column under cyclic loading were established based on CDP model.The dilation angle?and viscosity parameters?related to the plastic flow potential G and the viscoplastic regularization in CDP model were discussed.The compression stiffness recovery factorsw_c related to the stiffness recovery of concrete opening-closing behavior was also studied.FE models of the normal concrete（NC）,RC,ECC,reinforeced ECC（R/ECC）linings were established based on the CDP model.The FE models were validated by comparing the numerical and empirical results of the four kinds of linings.The concepts of residual load-carrying capacity ratioR_DL comparing with the crack developments were used to evaluate the damage levels of the NC lining.TheR_DLequal to 80%,60%,40%,20%)and 0%were defined as five different damage levels.Subsequently,the numerical simulations on damaged NC linings strengthened by the RC arch,ECC arch,R/ECC arch,steel plate and CFRP sheet were discussed in detail.The mechanical behavior and strengthening effects of retrofitted NC linings with different strengthening methods under different damage levels were carefully analysed.In order to solve the algorithms and application problems for the compressive stiffness recovery factorw_c,which was used to describe the compression stiffness recovery of concrete at the stage of closing after crack opening under cyclic loading conditions.An exponential softening stress?and opening w relationship was established based on concrete fracture energyG_F.After comparing the existing concrete damage calculating models,a typical damage evolution law suitable for modelling RC structures was adopted.Based on this,a new algorithms onw_c was proposed,which was equal to the?-th power（??1）of the ratio of the concrete residual fracture energy g_FR and the fracture energyg_f both in per unit volume.The algorithm of the proposed model was also discussed in detail.Due to the limitation of thew_c evaluation law in CDP model,of whichw_c can be only defined as a certain constant.A new simplified method for RC structure modelling based on tensile damage level（d_t）was further developed.The different RC joint members under cyclic loading conditions were simulated and compared with the experimental data,and the feasibility of thew_c algorithms and the simplified method was validated.The above methods were extended to RC frame structure modelling,the FE results performed good agreement with the test results.