| Magnesium alloy is one of the lightest structural metallic materials,and has been widely used in the fields of aerospace,automotive components and electronics.Such structure components made by magnesium alloys are often subjected to an impact load in service.Numerical simulation is often used to study the plastic deformation of engineering structures subjected to a dynamic loading(such as explosion,high speed impact,etc.),but the accuracy of numerical simulation is determined by the adopted constitutive models.It means that the key issue is how to establish a constitutive model to truly reflect the physical essence of the impact dynamic stress-strain response of the materials.However,the specific crystal structure of magnesium alloy results in a significant macroscopic anisotropy of its impact dynamic stress-strain response,which makes the establishment of a reasonable impact dynamic constitutive model very complicate.In recent years,many scholars have established some impact dynamic constitutive models based on the impact dynamic experimental results of magnesium alloys.Those models are mainly macroscopic phenomenological ones,which do not consider the microscopic mechanism of the impact dynamic deformation of the materials,and then the prediction ability is limited.A few of impact dynamic constitutive models of magnesium alloys are proposed by considering certain microscopic deformation mechanisms,but the models still have not considered the deformation mechanisms completely,and do not describe the strain rate sensitivity reasonably.It can be seen that the existing researches on the impact dynamic constitutive models of magnesium alloys are not perfect.In view of the shortcomings of the existing researches,the impact dynamic mechanical tests of a widely used AZ31 B magnesium alloy are systematically performed at first;then,based on the experimental results,the impact dynamic constitutive models of magnesium alloys are established in this work.The obtained innovative results are listed as follows:1)The macroscopic impact dynamic deformation of the magnesium alloy at high strain rate was studied by performing a systematic test of AZ31 B magnesium alloy.First,a series of quasi-static uniaxial tensile and compressive experiments were carried out on the AZ31 B magnesium alloy by MTS testing machine,so that the difference between the dynamic and static plastic deformation of magnesium alloys can be compared.Then the impact dynamic compressive stress-strain responses of AZ31 B magnesium alloy were tested at room and high temperatures by using the Separation Hopkinson Pressure Bar(SHPB)device;the dynamic tensile ones of AZ31 B magnesium alloy were investigated at room temperature by using the Separation Hopkinson Tensile Bar(SHTB).The effects of strain rate and temperature on the macroscopic impact dynamic stress-strain responses of AZ31 B magnesium alloy were obtained.2)The microscopic observation of AZ31 B magnesium alloy was carried out after the dynamic deformation in order to study the microscopic deformation mechanism of magnesium alloy: First,to analyze the failure mechanism of magnesium alloy under the impact loading conditions,the fracture face of AZ31 B magnesium alloy after impact loading was observed by scanning electron microscope(SEM);then the changes in the metallographic structures of AZ31 B magnesium alloy before and after dynamic deformations were observed by optical microscope(OM),and the effects of temperature and strain rate on the deformation mechanism of magnesium alloy were analyzed;finally,the dislocation patterns of AZ31 B magnesium alloy were observed by transmission electron microscope(TEM),and the influences of temperature and strain on the dislocation movement of AZ31 B magnesium alloy were analyzed.3)Based on the experimental observation,the microscopic deformation mechanisms of magnesium alloys under the impact loading conditions are summarized,and then three impact dynamic constitutive models of magnesium alloys are established,respectively,according to different applications:(1)It is assumed that under certain loading conditions,the deformation mechanism with a low resistance will dominate the plastic deformation of the magnesium alloy.From such an assumption,in this work,a new impact dynamic constitutive model(i.e.,the dislocation dynamics based model)was obtained to describe the impact dynamic stressstrain responses of the magnesium alloy by defining the evolution equations of dislocation slipping and twinning individually.This model can accurately describe the dynamic stress-strain behavior of the magnesium alloy at room temperature,and its numerical implementation is convenient,which is suitable for conducting the engineering structure analysis.(2)Based on the crystal plasticity theory,an impact dynamic constitutive model(i.e.,a crystal plasticity based model)is constructed for the magnesium alloys by introducing a significant strain rate sensitivity control and considering the influence of temperature on the deformation mechanism of the material.Also,the numerical implementation of the developed model is demonstrated.It is seen that the impact dynamic stress-strain behaviors of the magnesium alloy at room and high temperatures are successfully predicted by the model.The evolution of twin volume fraction during the dynamic compressive loading of the magnesium alloy at room temperature is also investigated.The obtained results verify the accuracy of the model.(3)Considering the twin mechanism under the framework of thermal activation theory,a dynamic constitutive model of magnesium alloy is also established.The model retains the separate judgment of the dislocation slipping and twinning mechanisms,which means the model can describe the impact dynamic mechanical behavior of the magnesium alloy at high temperature,and the adopted explicit operation can greatly reduce the computational cost.The model is suitable for engineering application to predict the stress-strain of magnesium alloy under dynamic loading at room and high temperatures. |
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