Study On The Uniaxial Ratchetting Behavior Of 6061 Aluminum Alloy And Corresponding Crystal Plastic Cyclic Constitutive Model

The ratchetting effect is the cyclic accumulation of plastic deformation of a material under an asymmetric stress-controlled cyclic loading.Under the influence of ratchetting effect,the fatigue life of components will be reduced or the deformation of components will exceed the limit and fail to work properly,which must be considered in structural reliability evaluation and life evaluation.After decades of development,a large number of experimental studies of ratchetting behavior of metallic materials had been conducted by scholars at home and abroad,and the constitutive model describing the ratchetting behavior had also been greatly developed.However,most of the macroscopic experiments and theoretical studies rarely involved the study of the micromechanism of material ratchetting deformation and the constitutive model based on micromechanism.The developed constitutive models had their own limitations,and each had its advantages and disadvantages in the prediction of ratchetting behavior.It still needs to continue to develop.Therefore,this paper carried out the following research work:1.At room temperature,the uniaxial strain-controlled cycle and uniaxial asymmetric stress-controlled cycle of 6061 aluminum alloy were experimentally studied,and the stress-strain evolution of the material under uniaxial strain-controlled cyclic loading condition and uniaxial asymmetric stress-controlled cyclic loading condition were studied.The results showed that:when the material was subjected to uniaxial strain-controlled cyclic loading,the response stress amplitude would increase rapidly at the beginning of the cycle.With the accumulation of the cycle,the response stress amplitude basically remain unchanged and the response stress amplitude tend to be stable;under uniaxial asymmetric stress-controlled cyclic loading,ratchetting strain increased with the increase of mean stress and stress amplitude,and increased with the decrease of stress ratio.Ratchetting strain had the historical dependence of mean stress and stress amplitude loading,and was affected by stress loading rate,loading waveform and creep.The experimental study provided reference data for the simulation and prediction of ratchetting behavior by constitutive model.2.In the framework of small deformation crystal plasticity,a new cyclic crystal plasticity constitutive model was established based on dislocation micro mechanism.By improving the Chaboche nonlinear kinematic hardening model,the power function and critical surface were introduced to reduce the influence of dynamic recovery term,and the cyclic crystal plastic constitutive model on single crystal scale was constructed by combining the Kocks-Mecking-Estrin isotropic hardening rate model related to dislocation mechanism.Using the explicit transition rule of single crystal to polycrystal,the model was extended to metal polycrystals to describe the ratchetting behavior of metal polycrystals.The explicit numerical integration algorithm was used to derive the formula of numerical integration,and the model was embedded in the numerical software MATLAB.Then the relevant material parameters were determined,and the corresponding working conditions were changed for simulation and prediction.By comparing with relevant experimental data,the advantages and disadvantages of the constitutive model in describing the ratchettingbehavior of metal polycrystalwere discussed.