| With the continuous popularization of new energy vehicles and the increasing market demand,the demand for lightweight vehicles has become particularly urgent.Light-weight materials such as aluminum-magnesium alloys are widely used due to their low density and high strength.However,its room temperature formability is poor,and defects such as local necking and fracture are prone to occur during the forming process.Studies have shown that high-strain-rate forming can effectively improve the problem of plastic defects in aluminum alloys.Therefore,the mechanical response of aluminum alloys at high strain rates has attracted attention,which puts forward higher requirements for material testing equipment,especially the development of high-speed tensile testing equipment.Therefore,it’s necessary to design a set of high strain rate testing equipment.Therefore,a set of high strain rate test equipment was designed and developed to obtain the necking and fracture limits of materials at different strain rates and different stress states.It is of great significance for the testing of material properties and the establishment of databases.The main research contents of this paper are as follows:Firstly,a constitutive model suitable for high-speed forming of AA5052-O aluminum alloy and an electromagnetic high-strain-rate material testing device were established.The modified J-C constitutive equation was established under quasi-static and medium strain rates,and the basic parameters of the material were obtained.Aiming at the problems of limited testing strain rate,high testing cost and high technical barriers of high-speed tensile machine,a high-strain-rate material tensile device based on the principle of electromagnetic forming is proposed,including universal bench module,uniaxial stretching module,and biaxial stretching module.It consisted of a tooling stand,a measuring system,a coil and a clamping device.The results showed that the combination of iron mesh and permalloy could effectively reduce the electromagnetic interference of the electromagnetic field to the measurement system,and the shielding effect was good.Secondly,the influencing factors of the strain rate of the material under electromagnetic high-speed stretching were studied and a mathematical model of the true strain rate was constructed.According to the basic parameters of the material and the propagation characteristics of the stress wave,the uniaxial stretching specimen was designed,and the experiment was carried out on the electromagnetic uniaxial stretching platform to analyze the factors of the electromagnetic stretching process.A mathematical model of strain rate was established for the two factors of specimen width and discharge energy,and applied to the study of electromagnetic tensile curve and fracture limit.The results showed that the tensile curve of electromagnetic forming with high strain rate was in good agreement with the modified J-C model,and the single tensile fracture limit increased with the increase of strain rate.Thirdly,combined with LS-DYNA finite element software to design the electromagnetic biaxial tensile specimen.The influence of the distance from the slots to the center area,the number of slots and the shape of the thinned area on the fracture mode and strain distribution in the central area of specimen were analyzed.And the single-slotted and sector-shaped thinned electromagnetic biaxial tensile specimen was finally determined.Based on its structure,other electromagnetic biaxial-stretching strain path specimen was developed.Finally,the feasibility of the electromagnetic biaxial-stretching specimens were verified by experiments and the ultimate strain of the material was studied.The tests were carried out on the electromagnetic biaxial-stretching platform.Stable biaxial-stretching strain paths were obtained and the fracture started from the central area,which verified the feasibility of the electromagnetic biaxial-stretching specimens.The necking strain had little sensitivity to the strain rate and was mainly affected by the stress state,while the fracture limit had a strong positive sensitivity to the strain rate. |
