Study Of Cyclic Plasticity And Time-dependence Of Metals In Macro And Microscopic Scale

The cubic polycrystal metals,which possess symmetry mechanical and physical properties,are widely used in engineering.The deformation mechanisms of materials have profound guiding significance for research and engineering design,especially for the materials that serve under elevated temperature and severe condition.This thesis mainly concentrates on the constitutive model for cubic polycrystal metals,considering the underlying mechanisms and comparison with experimental results.For real cases,external loadings vary,and one of the most common loading is cyclic loading.Thus,to propose a general multiaxial constitutive model is necessary.Otherwise,creep obviously affects the stability and lifespan of components and structures on the behalf of time-dependence of materials.Therefore,how to combine the deformation and time-dependence of materials and propose a precise simulation and prediction is the ultimate purpose of this thesis.In the macroscopic scale,this study analyzed the unified viscoplasticity model that base on thermodynamics and continuum mechanics,and discussed the constitutive equation and determination of corresponding parameters.The previous experiment data were employed to verify the utility of the model.With the finding that classical unified viscoplasticity model cannot describe the effect of history loading on the evolution of decay stress during stress relaxation,a modified unified viscoplasticity model was proposed.Through the modification for the static recovery term in back stress,the new model successfully simulated the stress relaxation under the influence of loading history.Moreover,to have a further investigation on the cyclic plasticity and time-dependence of materials,a series of tests of 316 L stainless steel for creep and fatigue were designed.Some novel mechanical phenomena were observed and discussed.However,from the simulation with Chaboche unified viscoplasticity model,it is found that although static recovery term can improve the accuracy of description for time-dependence of materials,it also causes an inappropriate evolution of plastic strain rate during creep and stress relaxation interaction tests.On the other hand,according to the “non-unified” principle,this thesis also discussed a non-unified viscoplasticity model.From the comparison between the test results of 316 L stainless steel with the simulation by the non-unified viscoplasticity model,it's seen that the steady part introduced into viscopalsticity model can enhance the simulation for time-dependent behavior,with no inappropriate description for plastic strain rate for creep and stress relaxation interaction tests.As the traditional phenomenal constitutive models lack the intrinsic length and physical mechanism,they cannot be used to interpret some peculiar size effect in micro scale.In need of the models that work in micro scale,the cyclic torsion tests of micro polycrystalline copper wires were studied.Starting from Taylor law and the concept of Geometrically Necessary Dislocation(GND),a cyclic plasticity model considering GND and dislocation density was proposed,and successfully simulate the strength increasing,Bauschinger effect,and plasticity recovery.Furthermore,the study of torsion stress relaxation for micro polycrystalline copper wires was conducted.The classical Orowan equation,which elucidates the relation between plastic strain rate and mobile dislocation density and dislocation velocity,should contain the information of GND when the materials are subjected to non-uniform deformation in micro scale.Finally,the in situ test of bending stress relaxation for micro copper single crystal beams was conducted to manifest the time-dependence of materials under non-uniform deformation.The size of micro beams is around several micron meters.Discrete dislocation cannot be averaged in such a small scale,and continuum mechanics is inapplicable.From the evolution of plastic strain rate during stress relaxation,it's shown that plastic strain rate increased as the beam size decreasing.Based on the thermal activation theory,some GNDs induced by strain gradient pile up in the neutral plane and cannot escape from the out surface,and make contribution to the total mobile dislocation density.That is why the thinner the beam,the higher the plastic strain rate in this test.