Ratchetting Of Extruded AZ31 Magnesium Alloy And Its Constitutive Model

Magnesium(Mg)alloys have been widely used in aerospace,automobile,rail transportation and biomedicine industries as lightweight structural components due to their high specific strength and stiffness and good damping capacity as well as other excellent properties,but the components are inevitably subjected to a complicated cyclic loading.Systematic study on the cyclic plastic deformation and resulting fatigue failure of the material is the fundamental to assess the reliability of structural components and devices.Thus,it is extremely necessary to conduct systematic investigation on the cyclic plastic deformation of Mg alloys under different loading conditions.However,the exsiting researches mainly focus on the uniaxial cyclic plastic deformation of Mg alloys.The studies on the non-proportionally multiaxial cyclic plastic deformation of Mg alloys are rare.Meanwhile,the effect of temperature on the cyclic plastic deformation of Mg alloys has not reported in the existing literature.Therefore,it is necessary to perform the uniaxial and multiaxial cyclic tests on the widely usded extuded AZ31 Mg alloys at different temperatures to reveal the dependences of ratchetting on the non-proportionally multiaxial loading paths and temperatures;and then develop a crystal plasticity based constitutive model by considering the different plastic deformation mechanisms of Mg alloy.Thus,through the systematically experimental and theoretical studies on the uniaxial and multiaxial ratchetting of Mg alloys,the following new results are obtained in this thesis:(1)By performing series of uniaxial and non-proportional multiaxial cyclic tests with different loading paths,loading levels and ambient temperatures,the effects of abovementioned factors on the ratchetting of extruded AZ31 Mg alloys are discussed,and the evolution characteristics of uniaxial and multiaxial ratchetting dominated by different plastic deformation mechanisms are revealed.These results are helpful to construct a crystal plasticity based constitutive model of Mg alloys.(2)In the framework of crystal plasticty at small strain,a constitutive model reasonably considering the contributions of dislocation slipping,twinning and detwinning mechanisms is constructed,and then the uniaxial ratchetting of polycrystalline extruded AZ31 Mg alloy and its temperature-dependence can be reasonably reproduced.In the newly developed constitutive model,the classic Armstrong-Frederick kinematic hardening rule is adopted to describe the strain hardening caused by the dislocation slipping,and two new non-linear kinematic hardening rules are proposed to reflect the strain hardening caused by the twinning and detwinning;meanwhile,a linear isotropic hardening rule is adopted to describe the interactions among different plastic deformation mechanisms;furthermore,the temperaturedependent material parameters are introduced and the effects of temperature on the kinematic and isotropic hardening rules are also considered to reasonably reveal the temperaturedependence of the uniaxial cyclic plastic deformation of Mg alloy.The model is verified by comparing the predictions with the corresponding experimental results of extruded AZ31 Mg alloy.It is demonstrated that the newly developed model can reasonably predict the uniaxial ratchetting of Mg alloys with different loading levels and at room and high temperatures.(3)In the framework of crystal plasticty at finite strain,an additional dynamic recovery iterm is introduced to modify the Armstrong-Frederick kinematic hardening rule,so that the influence of the multiaxial loading paths on the ratchetting of extruded AZ31 Mg alloy can be addressed reasonably;meanwhile,an elastic–viscoplastic self-consistent(EVPSC)approach is developed at finite deformation;finally a crystal plasticity based constitutive model at finite strain is constructed to reproduce the multiaxial ratchetting of extuded AZ31 Mg alloy.Comparing the predictions by the proposed model with the corresponding experimental results,it is demonstrated that the proposed crystal plasticity based constitutive model at finite strain describes the multiaxial ratchetting of extruded AZ31 Mg alloy at room and high temperatures reasonably.