Research On Crashworthiness Mechanism And Design Of Variable Thickness Thin-walled Structures

Vehicle lightweight is an effective solution to promote energy conservation and emission reduction of traditional energy vehicles and solve the new energy vehicles’ range anxiety,which has attracted more and more attention from automotive industry and academia.The application of new lightweight materials and the development of innovative structure can effectively achieve lightweight.However,the automobile design request often need the automobile structure to have the excellent crashworthiness,thin-walled structure has an important significance for energy absorption in the automobile crashworthiness design.The traditional thin-walled energy-absorbing structure are designed based on uniform thickness,which can’t realize the maximization of the energy absorption of structural materials due to uneven bearing in the deformation.In contrast,a new type of functionally graded or variable-thickness thin-walled structure could be exploited to further realize the material utilization ratio,so the design of new lightweight and crashworthy structure has become a new research hotspot.In order to achieve lightweight of vehicle body and improve the structural collision safety,this paper took the variable thickness thin-walled aluminum energy absorbing structure as research object,and carried out systematic research through experimental,numerical and theoretical analysis methods.First,the deformation mode and crushing force response of single-cell thin-walled structures with variable thickness were studied under bending and axial loading respectively,and the energy absorption mechanism of variable-thickness thin-walled structure were revealed.Secondly,the crashworthiness mechanism and the influ ence of graded thickness on the crushing behaviour of multi-cell thin-walled tubes were studied.Then,the research content of multi-cell and variable thickness was introduced into the design of the front anti-collision beam assembly,effectively reducing the weight of the body parts and improving the energy absorption.The research work of this paper reveal ed the energy absorption mechanism and energy absorption characteristics of thin-walled structures with variable thickness,forming the theoretical syst em of crashworthiness design of thin-walled energy-absorbing structures with variable thickness.The main research contents were as follows:(1)The bending behaviour of variable-thickness(VT)circular tubes under transverse loading was explored.In order to understand transverse load bearing behaviours,three circular variable thickness tubes with different linear thickness gradients from one end to its nearest indenter were investigated through three-point and four-point bending tests in this study.A series of loading conditions,namely different loading offset for three-point bending and different loading span for four-point bending tests,were considered to explore the energy absorption characteristics of the variable thickness tubes.The experimental deformation modes and force vs.displacement curves of variable thickness tubes were first compared with corresponding uniform thickness(UT)tubes.Second,the finite element(FE)models of circular VT tubes were established for comparisons with the experimental tests.Third,a dimensionless analysis was conducted to compare the deflections in the three-point and four-point bending tests of the circular VT tubes with different thickness gradient.It is shown that the VT tubes have more regions to participa te in deformation than the UT counterparts to bear the transverse loads.Based on the validated FE models,a parametric study was finally carried out to explore the influence of thickness gradient or thickness difference on the three-point and four-point bending behaviours of the VT tubes in comparison with the same mass UT counterparts.The results show that a proper selection of thickness gradient c ould effectively enhance the energy absorption of tubal structures under transverse loading,exhibiting that the VT tubes are considerably more advantageous over the uniform tubes.(2)The energy absorption mechanism of square tube with axial functionally graded thickness and lateral functionally variable thickness were revealed respectively.This paper introduced axial functionally graded thickness and lateral functionally variable thickness to thin-walled square structures separately,and then investigated their crashworthiness theoretically,numerically,experimentally under axial crushing load.The quasi-static axial crush experiments and the corresponding finite element models were first conducted to analysis the deformation mode and crushing force for uniform thickness and variable thickness tubes under the same mass.Then,theoretical models predicting the mean crushing forces of axial functionally graded thickness and lateral functionally graded thickness square tubes were established.The results showed that both theoretical solutions and numerical results for VT tubes agree well with the experimental resu lts.Energy absorption characteristics between VT and UT square tubes with same mass were compared based on the validated numerical models,in which the square tube with axial functionally graded thickness could effectively reduce the initial peak force compared to UT square tube;the lateral functionally variable square tube remarkably surpasse d the UT square tube in specific energy absorption(SEA)under axial crushing.Furthermore,parametric studies were performed to investigate the effects of gradient thickness variation on the energy absorption characteristics of VT square tubes.The results again demonstrated that VT square tubes can improve the crashworthiness of thin-walled square tubes.(3)The deformation and energy absorption characteristics of v ariable thickness multi-cell thin-walled structures were studied.As a relatively new sectional configuration with a higher efficiency of material utilization,laterally variable thickness(LVT)and axially variable thickness(AVT)structure has demonstrat ed its compelling features in energy absorption.To explore the crushing behavior of LVT and AVT multi-cell tubes as well as validate the corresponding finite element(FE)models and analytical solution for the mean crushing force,the quasi-static axial crushing experiments were first performed for the five-cell and nine-cell LVT and AVT tubes in this study.The FE models of LVT and AVT tubes were then created for investigating the crashworthiness of these structures;and the simulation results were found to agree well with the experimental data.Following the validated FE models,a parametric study was carried out to quantify the influence of the thickness gradient on the crashworthiness of the LVT and AVT tubes with the same mass as the uniform thickness(UT)counterparts,respectively.The results showed that the LVT and AVT multi-cell tubes were of certain advantages over the uniform counterparts to be an energy absorber.Based on Super Folding Element(SFE)theory,the analytical models for the mean crushing force(MCF)and energy dissipation of the LVT and AVT multi-cell square tubes had been established with regard to the thickness gradient,respectively.The analysis was also performed to explore the mechanism of energy absorption enhancement by the evolution of theoretical models.It was confirmed that the derived analytical solutions provided fairly good prediction of the mean crushing force and energy absorption of LVT and AVT multi-cell tubes.Moreover,the analytical prediction could be a powerful tool for designing crashworthy structures.(4)The newly variable thickness multi-cell front anti-collision beam assembly was proposed and optimized.In this study,crashing characteristics of conventional square tube and a new type of criss-cross tube were firstly studied using both experimental and numerical methods,in which the finite element(FE)models were well validated.The results showed that the energy absorption of the criss-cross sectional tube was about 150% higher than that of square column with the same weight.Further,a range of criss-cross sections were parametrically modelled with spline curves and a parametric study was subsequently performed to explore the effects of different parameterized shapes on crashing characteristics.It was f ound that the geometric parameters significantly affected crashworthiness of the criss-cross tubes,and the criss-cross tubes with a spline curve(CCT_SPL)surpassed the criss-cross tubes with a straight line(CCT_STR)in crashworthiness with the same weight.Finally,to optimize the crashworthiness of parameterized criss-cross tubes,the non-dominated sorting genetic algorithm II was adopted to seek optimal criss-cross shapes for improving specific energy absorption and reducing the peak crashing force,simultaneously.The optimization results indicated that the CCT_SPL profiles with reasonable geometric parameters were superior to the CCT_STR counterparts with an increase of 11.1% in specific energy absorption.For the conventional trip-cell bumper system with uniform thickness,the schedule of variable thickness five-cell energy absorber and axially variable thickness bumper beam was proposed following the research result of this paper.The optimization of variable thickness bumper system was conducted to further achieve the improvement of lightweight and crashworthiness.The result showed that the proposed VT multi-cell bumper system could further improve the crashworthiness and lightweight of vehicle body.