| New energy vehicles are often limited by unsatisfactory cruising range because of battery capacity and body weight.Lightweight vehicles are the key means to solve the current contradiction in terms of cruising range of new energy vehicles.Therefore,the demand for lightweight vehicles of automotive industry has become particularly urgent.Aluminum alloy has the characteristics of high strength,low density and good corrosion resistance,and is an important lightweight material in the automotive industry.However,aluminum alloys have low room temperature forming plasticity.When forming with traditional forging processes,aluminum alloys are prone to defects such as fractures and local necking.Studies have shown that aluminum alloys have excellent plasticity under high-rate loading conditions,so high-rate forming technology has become a method to solve aluminum alloy plasticity defects.The development and design of high-rate forming process puts forward higher requirements on simulation models,especially constitutive and failure models under high strain rates.So far,there were few studies on the failure criteria of aluminum alloys at high speeds,and it was difficult to obtain the fracture parameters of materials at high strain rates through experiments.Therefore,it is important to obtatin the failure law of aluminum alloy sheets under high-rate loading conditions through experiments,to obtain accurate high-strain-rate constitutive and fracture model of materials,and to establish a failure prediction and judgment system for high-strain-rate forming processes.The main research contents and research results of this article are as follows:(1)Established a constitutive model and simulation model suitable for high-speed forming of AA5052-O aluminum alloy.First,under quasi-static and medium strain rate,using Digital Image Correlation(DIC),the true stress-strain relationship curves of the tensile test specimen under a large strain range were obtained,and modified constitutive model with the Cowper-Symonds strain rate effect of was established.Then,based on electromagnetic forming,an electromagnetic bulging-tensile deformation conversion method of the test specimen was proposed.The strain rate stability of the method and the force measurement principle were analyzed.The high strain rate constitutive model established under the condition of magnetic pulse loading was examined.The results showed that the proposed new electromagnetic stretching method could obtain an ideal stretching process,and the established constitutive model was suitable for electromagnetic forming.Finally,using three-dimensional high-speed DIC,the dynamic deformation behaviour of the material during the magnetic pulse free bulging by flat spiral coil was studied,and the finite element-boundary element coupling model of the free bulging of the magnetic pulse flat coil was established.The results showed that the simulation results of the finite-boundary element method were in good agreement with the DIC test results,and the accuracy of the established coupling model of electromagnetic fields and material constitutive-related structural fields was sufficient to meet the subsequent research requirements.(2)Studied the fracture limits and damage process under different stress states and strain rates.Experimental methods were also studied.First,using the DIC method,through the quasi-static standard and notched specimen tensile experiments,as well as the traditional Nakazima experiments,the deformation history and fracture strain of the material under the quasi-static state were obtained.Subsequently,through high-speed tensile tests of notched specimens with a strain rate ranging from 1 to 100,the fracture failure strain,strain concentration and damage initiation law of the material under different stress states and strain rates were studied.The results showed that the fracture failure strain of aluminum alloy increaseed significantly with the increase of strain rate.The inertia effect at high strain rate could significantly retard the strain concentration caused by material softening.The effect of strain rate on the damage initiation point was not significant.Finally,based on the new electromagnetic stretching method,an electromagnetic cross-tensile method was proposed,a cross-shaped specimen suitable for high strain rate cross-tensile tests was designed,and the high strain rate fracture failure strain and deformation history of the material under the biaxial stress state were obtained.The results showed that the electromagnetic cross-tensile tensile method had a breakthrough in solving the problem of material fracture parameter testing under high strain rate biaxial state,and the designed high strain rate cross-shaped specimen solved the the problem of fracture caused by stress amplification of stress wave reflection,the deformation history could judge the damage and failure process of the material.(3)Revealed the damage mechanism under different stress states and strain rates,and established a high strain rate ductile damage model.The EBSD(electron backscatter diffraction)and SEM(scanning electron microscope)experiments were carried out to study the micro-structure changes and fracture modes of the material fracture under different strain rates and stress states.The results showed that near the fracture surface of the high strain rate fracture sample,a large number of dislocations were absorbed by the sub-grain boundary to make grains more refined.The impact effect inhibited the generation of preferred orientation of the structure during the deformation process.The cube textures of high-rate samples were partially retained,while samples that fractured under quasi-static conditions were completely transformed into the copper texture,brass texture and S texture with poor plasticity.Compared with the fracture of the quasi-static fracture specimens,the fracture of the high strain rate specimens had more dimples,and inertia effect made the holes less concentrated into large cavities.Based on the obtained fracture experiment data,the fracture model of AA5052 plate under quasi-static conditions was first established.Subsequently,the strain rate sensitivity characteristics of the fracture strain under different stress states were explored.The stress state related and stress state unrelated strain rate modified fracture models were established respectively.Combined with the Gissmo damage evolution model and the fracture initiation model,a complete ductile damage model established.Results showed that the strain rate sensitive effect of the fracture strain has obvious stress state correlation,and has the highest strain rate effect coefficient in the plane strain state,while the failure initial strain of the material did not have obvious stress state correlation within the studied.(4)The effectiveness and necessity of the established damage prediction model were verified,and the application research of damage prediction was carried out.The electromagnetic circle free bulging tests under different energies were designed.The final failure modes of the sheet under different discharge energy were studied.The calculation results of the simulation model loaded with the established damage prediction model were compared.The results showed that compared to the high strain rate failure model that was independent of the stress state,the high strain rate fracture model that was tress state related could more accurately predict the failure mode of the sheet.A visualized three-dimensional fracture limit diagram for high-speed forming were presented,and the concept of fracture margin for judging material deformation potential were proposed.Using the established fracture model,the important role of inertia effect on the strain concentration effect of high strain rate deformation was analyzed.The results showed that the high-speed forming three-dimensional fracture limit diagram and fracture margin diagram could effectively help process developers intuitively judge the fracture failure and deformation potential.The magnitude of the inertia effect at high strain rates was affected by the shape of the sample,and the inertia effect had a significant impact on the strain concentration of the material,which proved that the traditional FLC model that used necking to determine failure was difficult to directly apply to high-speed forming failure prediction. |
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