Crystal Plasticity Cyclic Constitutive Modeling And Low Cycle Fatigue Crack Initiation Life Prediction Of Ultrafine Grained 6061 Aluminium

The ultrafine grained 6061 aluminum alloy(UFG AA6061)with excellent comprehensive properties shows a wide range of applications in civil engineering,marine engineering,aerospace,etc.However,the UFG AA6061 with average grain size of 1μm has fine grains and numerous grain boundaries,exhibiting significant improvement in high cycle fatigue performance and deterioration in low cycle fatigue performance.When the UFG AA6061 endures low cycle fatigue loading,it is prone to triger early fatigue failure and avalanche collapse,and its fatigue life is dominated by fatigue crack initiation life.Therefore,it is of great significance to carry out experimental research on cyclic plasticity response behavior of UFG AA6061 to master its plastic deformation mechanism,and establish a reasonable cyclic plastic meso-constitutive model to accurately predict the low cycle fatigue crack initiation life,which are also huge challenges.For this purpose,taking UFG AA6061 processed by equal channel angular pressing(ECAP)as the research material,symmetrical cyclic strain controlled loading and asymmetrical cyclic stress controlled loading were conducted for UFG AA6061 at room temperature,respectively.Then,the microstructure of UFG AA6061 before and after cyclic deformation was characterized and analyzed by electron back diffraction(EBSD);Subsequently,a crystal plasticity cyclic plastic constitutive model based on dislocation mechanism was developed by considering the micro-deformation mechanism of UFG AA6061 under cyclic loading;Finally,with the help of crystal plasticity finite element method and statistical extreme value theory,the cyclic constitutive model of UFG AA6061 was verified and fatigue life prediction was made.The main innovations and research results are as follows:(1)UFG AA6061 exhibits cyclic softening under symmetrical cyclic strain controlled loading,and it has obvious ratcheting effect in asymmetric stress control tests.The corresponding EBSD test shows that: The specimens after cyclic loading show different degrees of grain coarsening,and the one endure mean compressive stress cyclic loading is the most obvious.The main reason for this difference can be attributed to the activation of NOC<100>{110} slip system and the asymmetric evolution of lattice rotation during material deformation,which involves a dynamic continuous recrystallization process driven by both plastic deformation and the release of energy produced in ECAP process.In addition,the appearance of the brass orientation {111} <112> during deformation promotes the growth of Goss oriented grains,which is accompanied by obvious low angle boundary dislocation annihilation.The EBSD analysis shows that dislocation slip is the dominated microscopic mechanism of cyclic plastic deformation of UFG AA6061,while grain boundary sliding and continuous dynamic recrystallization may serve as auxiliary mechanism for the slip-induced grain rotation.(2)Considering the "pinning-depinning" effect of dislocations from grain boundary obstacles and the contribution of the annihilation of low angle boundary dislocations to grain coarsening,a cyclic crystal plasticity constitutive equation based on dislocation mechanism was improved and developed,and an improved time integration algorithm was proposed by considering the evolution characteristics of the internal variables in the equation.The constitutive equation takes into account different evolution laws of multiple dislocation populations and all stages of dislocation activities were considered especially their interactions with grain boundaries.In order to describe the softening effect caused by cyclic loading,intergranular back stress and intragranular back stress related to dislocation evolution were introduced into the classical Armstrong-Frederick nonlinear kinematic hardening model.The isotropic hardening was described by the critical shear stress considering the stacking effect of dislocations at the grain boundaries.The crystal plasticity finite element(CPFE)simulation shows that the constitutive model can not only predict the cyclic stress strain curves,but also describe the evolution of microstructure parameters.(3)A statistical method was proposed to describe the inhomogeneous deformation of polycrystalline aggregates,i.e.,the standard deviation of the dot product of the longitudinal strain and the maximum normal stress on the slip plane.This statistical method was used as a new fatigue indicator parameter(FIP)to describe the low cycle fatigue damage.Taking the UFG AA6061 with large numbers of precipitates as an example,the effects of the size and distribution of precipitate inclusions,as well as the loading conditions on the inhomegeneous deformation of polycrystalline aggregates were analyzed by establishing SVEs with different microstructures and conducting the CPFE simulation combined with the statistical extreme value theory.Comparative analysis between SD model and classic Fatemi-Socie model was conducted to prove whether the SD model can be regarded as the fatigue indicator parameter.The results show that the proposed SD model can describe the low cycle fatigue characteristics of UFG AA6061,and the fatigue nucleation life can be predicted by numerical simulation by obtaining the critical value of the SD model through only one strain amplitude test.(4)A low cycle fatigue crack initiation life prediction method based on physical mechanism of UFG AA6061 was developed by explicitly distinguishing the deformation mechanism of nucleation and microcrack propagation stages.The nucleation life was calculated by the deformation inhomogeneity measured by SD model;The microcrack growth life was calculated by the microcrack growth rate equation considering NPACH effect and mean strain effect;Finally,the low cycle fatigue crack initiation life of UFG AA6061 was predicted by discussing the proportion of two different physical stages in the crack initiation life.The prediction results show that the prediction accuracy of the traditional prediction method lied within ±3 error band,while the prediction accuracy of the proposed model lied within ±1.5 error band;Meanwhile,it can be concluded that when the nucleation life accounts for 50%-70% of the total fatigue crack initiation life,the prediction accuracy of the model is the best.