Research On Lightweight Materials And Batteries For Better EV Vehicle Crash Safety Performance

With global energy and environmental problems become increasingly prominent,the new energy vehicles have been incorporated in the emerging industry of national strategy.As an important part of new energy vehicles,pure electric vehicles have a great significance for China to achieve the strategic goal of automobile power.Breaking through the core technology and seizing the commanding point of automotive technology could promote the development of the new energy vehicle industry healthily,orderly,and rapidly.In 2017,the global sales of electric vehicles exceeded 1.22 million,where China ranks first with sales of 0.77 million,including 0.652 million pure electric vehicles.There are mainly two concerns from electric vehicle customers.One concern is the range and capacity fade of battery.The conventional ICE vehicles could easily reach 600 Km per charge,while the range per charge of the most production electric vehicles is around 300 Km.Another concern is about safety,consumers are taking a wait-and-see attitude towards the safety of lithium batteries due to the reports of vehicle self-ignition accident.Therefore,lightweight and safety are the key technology for the large-scale industrial application of pure electric vehicles.The development of automobile safety performance has roughly experienced three stages.The first stage is technology accumulation during product localization by “reverse technology”.It mainly involves the geometric design,instead of core technology.The second stage is to develop integration technology based on the "real vehicle test verification".At this stage,a certain amount of performance verification has been accumulated while the early design capability is insufficient,which relies on the vehicle test in the later period.The third generation of key technology is to enhance the theoretical basis from engineering experience.The character of this stage is product development guided by computer-aided engineering(CAE),which is a fully forward development process with independent intellectual property rights.Therefore,this project focuses on breaking through the mechanical calibration technology of aluminum alloy material and carbon fiber composite material,and obtains high-precision CAE analysis material cards.At the same time,it explores the failure principle of power lithium battery,and proposes a vehicle collision simulation analysis method to evaluate the failure of power lithium battery.Only when the key CAE technology in the early stage of have been broken through,can the forward development process of the whole vehicle being driven.The key innovations are as follows.For the mechanical calibration research of aluminum alloy materials,a lightweight material test platform for automobile collision was built,and the basic mechanical testing emergency of classic aluminum alloy materials was carried out.The collision failure and fracture prediction methods of aluminum alloy materials under complex stress and impact load were proposed.It broke through the bottleneck of data acquisition of accurate materials dynamic mechanics in CAE aided forward development in Chinese automobile companies.The dynamic mechanical properties characterization technology including the necking and fracture process was developed.The force signal measurement method based on optical image correlation technology is introduced to solve the problem of low accuracy of material tensile test data in the industry.The calibration factor is introduced into the material constitutive relationship innovatively,which solves the problem that the traditional material model cannot directly calibrate the material strength after necking,effectively improves the accuracy of the material strength curve after necking.The accuracy of the characterization of fracture failure behavior of metal materials is improved by 5.5%.As for the mechanical calibration of carbon fiber composites,the aim of this project is to realize the optimal design of carbon fiber composites from the aspects of structure and materials.In structural design,topology optimization technology is discussed to obtain load path information,which provides a reference for material layout.In the aspect of materials,the optimization methods of free size,size,and sequence of composite materials are studied to obtain better ply thickness,angle,and stacking sequence on the mesoscale.The basic card information for simulation analysis is obtained by multiple tensile and shear tests of material samples.Firstly,the design of the basic mechanical parameter test is carried out based on the parameter requirements of the basic material model.Secondly,a sample level test calibration method is proposed in order to obtain the material properties required by CAE simulation.By using the results of this stage,more relevant tests are simulated to verify the effectiveness of material properties and generate relevant sample-level precision material cards.The accuracy of the sample level card is usually calibrated by searching for the process and mechanical characteristics of a single crack,to make it closer to the part performance in the real collision.As for the research on the collision safety of power lithium battery,aiming at the problems of low efficiency and insufficient lightweight design of battery system collision protection under the guidance of experience,the failure analysis of battery collision extrusion deformation is carried out for three levels of battery components,cells,and modules.Based on the mechanical properties test of various stress states and multi-stage deformation rates,a characterization method of dynamic deformation response and fracture behavior of electrode and diaphragm was established.Based on the real-time monitoring of the electro thermal coupling response of the impact extrusion force of the cell,the failure mode and mechanical mechanism of the short circuit in the cell were revealed.Based on the battery module collision test,the internal short circuit and heat loss under different loads were analyzed risk control.According to the working condition of cell impact extrusion,the fine,semi homogenization,and homogenization modeling and analysis technology are developed.The influence of loading mode and speed on the short circuit in the cell is clarified.The comprehensive identification method of deformation tolerance of impact extrusion is given.The collision protection and lightweight design strategy that allows the battery deformation and avoids internal short circuit is proposed.In view of the new configuration design problems and anti-collision performance requirements of electric vehicles,based on the vehicle collision simulation analysis,the influence law of battery pack quality and layout on the collision waveform is clarified,and the collision energy conversion control strategy is proposed.Based on the analysis of impact damage and extrusion deformation of battery pack bottom guard,a structural optimization method for electric vehicle bottom collision protection was established.The basic physical parameters of all materials were measured by disassembling the battery cell,and the mechanical properties experiment was designed to study the constitutive relationship of materials,and the material card for the simulation model was made.After repeated testing,a set of feasible modeling scheme is developed,and the local intrusion test of two kinds of battery cells is used to verify and verify.The simulation results show a high consistency with the test data,which can effectively study and judge the risk of battery thermal runaway.Finally,in the application of vehicle development,we use the refined finite element model of the battery cell to analyze the safety development of pure electric vehicles with aluminum alloy lower body and carbon fiber upper body.On the premise that the safety of the lithium-ion battery pack meets the requirements of regulations,the collision simulation analysis of battery safety risk under basic typical road conditions is carried out.The results of the real vehicle crash test indicate that the research results ultimately achieve the targets of vehicle lightweight and safety.This project has formed a number of the key technological innovation in the field of electric vehicle safety,and the technological innovation achievements have won ten invention patents,nine papers and seven provincial science and technological progress awards.