Constitutive Models Of Magnesium Alloys And The Application Of Lightweight Vehicle Frontal Sturcture

Stricter regulations on fuel economy,growing concerns over automobile emissions and increasing consumer comfort requirements have led to be matured and improved automotive lightweight technology urgently.With the development of automobile electrification and intelligence,battery packs,controllers,various vision and sensing devices will increase the weight of vehicle,and pose new challenges to the structural optimization.Structural layer optimization such as structural topology and parameters'optimization,continuous development of joining processes and application of lightweight materials such as magnesium alloys,aluminum alloys,plastics and composite materials are effective ways of lightweight.The application of lightweight materials is the most effective lightweight technology in the lightweight approach.As the lightest metal,magnesium alloy has a density of 1/4 of high-strength steel and 2/3 of aluminum alloy.It has a specific stiffness comparable to that of high-strength steel and aluminum alloy,and its specific strength is superior to them.It has excellent shock and sound absorption,good anti-fatigue effect.It is easy to mold large parts and can be completely recycled,which saves costs and has great application potential.At present,there are many studies on magnesium alloy materials,but the dynamic characteristics of magnesium alloy are complicated due to the microscopic grain structure,and the lack of research on strain rate related constitutive equations for engineering applications,the engineering application?especially in the field of automotive engineering?of magnesium alloy materials is still rare.It is very necessary to solve the problem of engineering constitutive models in the wide strain rate range and the engineering application of lightweight magnesium alloy materials,which is of great significance for the car to be lightweight,electric and intelligent.Based on the mechanical properties of magnesium alloy materials,this paper studies the whole process of steel magnesium integrated structural engineering application from the cognition of the microscopic nature of material properties to the detailed design stage.Based on the nonlinear characteristics such as stiffness mode and the basic characteristics,crashworthiness and fatigue life,lightweight design development and application research covering the whole performance is carried out.The"performance-material-structure"integrated lightweight optimization design process was proposed,and the idea of"appropriate materials for the right place"was incorporated into the optimization design development process.The quasi-static characteristics of magnesium alloy materials such as AZ31B-H24 and the dynamic characteristics of quasi-static to 7000 s^-1 high strain rate range were studied.Using the finite element method to estimate the range of strain rate of materials in structural collision,the Hopkinson pressure bar test is carried out in accordance with the requirements of high strain rate of vehicle collision.This experiment solves the disadvantages in high-strain rate test data reference of the literature,which possess data below 3000 s^-1 only.At the same time,the strain rate of 4000 s^-1 is found as the critical value of the change law of dynamic mechanical properties of magnesium alloy materials,and the mechanical properties beside the critical values are different.A hybrid macroscopic phenomenological constitutive model conforming to the application of automotive structural engineering is proposed.The microscopic deformation mechanism of metallographic and SEM was studied.The mechanical characteristics were explained from the microscopic point of view,and the microscopic dynamic constitutive equations were sorted out.In the numerical analysis of structural model,in view of the increasing use of numerical analysis techniques as a design reference,an inverse problem identification method for unknown parameters of structural local characteristics is proposed.The consummate modeling method of the system considers the nested optimization problems of uncertain parameters such as the structural complexity of the structural model and multiple material complexity,and provides the method basis and basis for the reliability of numerical analysis driving design and optimization.Used a free mode for numerical analysis model calibration as example structure.In the process of establishing the analytical model,the mass,stiffness and damping composite parameters of local characteristics are identified according to the inverse problem identification method of local characteristic unknown parameters proposed in this paper.By comparing the experimental mode with the traditional standing wave theory free mode and the traveling wave theory free mode with local damping characteristic in this paper,the correctness of using proposed method to deal with the complex structure of the example assembly is obtained.The use of it make it more consistent with the modal characteristics such as frequency and mode shape of the test.A coupled vehicle model of steel-magnesium integrated front end structure was established by using the proposed constitutive equation of magnesium alloy material and the inverse problem identification method of local characteristic parameters.Refer to the performance index of the stiffness,modality,crashworthiness safety and fatigue life of a certain standard vehicle as the performance target and constraint for the development of the front-end structure model with light magnesium alloy material.Using the optimized Latin hypercube test design,the experimental design performance array of the front end structure of the magnesium-containing alloy material with comprehensive coverage is obtained.The experimental array data is approximated by the Kriging proxy model,and an approximate model with consistent consistency of each discipline is obtained.The NSGA-II multi-objective genetic algorithm was used to optimize and obtain the optimization results based on comprehensive performance.The optimization results are adjusted to engineering data that can be applied by engineering,and the data model is used for verification,and a performance optimization scheme is obtained.Utilized the Kriging approximation model of the Matern Cubic correlation function,which has the test design covering the overall performance and best proxy accuracy,and the idea of"experimental design-approximate model-multi-objective multidisciplinary optimization"of NSGA-II multi-objective genetic algorithm.Combined the features lightweight ideas?such as based on stiffness and modality,based on crashworthiness,based on fatigue life,etc.?with static properties,dynamic properties and constitutive equations of materials.The"performance-material-structure"integrated lightweight design process is proposed,and has been verified in the front end structure of the car.