| According to the goals and requirements of the development plan of the national "13th Five Year",energy-saving,emission-reduction and lightweight will still be the important development trend of the automobile industry in the future.In the ways of realizing automobile lightweight,people pay more and more attentions to the new technology of producing periodically variable thickness strips by using the Flexible Rolling Technology,and then the automobile parts with continuously variable thickness can be achieved by the subsequent stamping processes.Because the desired thickness distributions of the variable thickness parts obtained by this technology can be achieved by adopting the Flexible Rolling Technology to realize load margin optimization of parts in different regions.The TRBs were firstly obtained by using the Flexible Rolling Technology.After then,related technologies were carried out on the TRBs to obtain the required Tailor Rolled tubes(TRTs)with axial thickness variation.Combined with the test and finite element simulation(FEM),the crushing characteristics and energy absorption characteristics of the TRTs under quasi-static and dynamic axial loading were revealed,and the theoretical model of the TRTs under axial compression was also established.In addition,aiming at the front structure of a domestic vehicle,the automobile energy absorbing box with optimization parameters has been developed by introducing the surrogate model and optimization algorithm,which improves the structural crashworthiness.The main works and experimental results can be concluded as follows:(1)Based on the classical theoretical model of Uniform thickness Tube(UT),the folding element characteristics and deformation mechanism of the TRTs were analyzed,and the theoretical model of the TRTs under axial crushing was also established.By using it,the influences of structural parameters on the axial bearing capacity and folding element heights of TRTs were analyzed.The results show that,the prediction results of the theoretical model considering the variation of axial thickness,properties distribution and folding element heights are basically consistent with the test results.The average load errors between the theoretical model and the experimental results are 0.8%and 5.5%when the section is square and circular.Under the condition of equivalent strength,the components with continuously varied wall thickness and material properties have the highest bearing capacity,which is 13.7%and 12.5%higher than those UTs with square and circular section.By comparing the mean loads of TRTs with different ratios of thickness difference,it's can be found that the component with 1:100 ratio of thickness difference has higher load-bearing level than the component with 1:50 ratio of thickness difference.In contrast,the bearing capacity of the structure can be significantly improved by improving the equivalent strengths.(2)In order to explore the typical deformation mode,the load-displacement curves and energy absorbing characteristics of TRTs were studied,quasi-static axial loading tests of were carried out on the TRTs,and the influence of structural parameters on the compressed behaviors was also investigated.The results show that,the plastic instability is firstly occurred in the local region near the thin zone,and gradually expanded to the thick zone.Meanwhile,the folding element heights would be gradually increased with crushing.The bearing capacity of components would be gradually increased with the deepening of deformation,and there will be a low initial peak load appeared.Comparing the axial compression performances of different tube structures,it can be found that the overall energy absorption of TRTs is about 8.4%and 13.1%higher than that of UTs and Tailor Welded tubes(TWTs),while the initial peak load is 52?63%lower than that of UTs.Through the study of the influence of structural parameters,the section geometry,transition zone distribution and layout mode have the most prominent influence on the load carrying capacity.As the way no need to increase the material consumption,the energy absorption efficiency of the TRTs with circular section is 43.4%higher than that of the triangular section,the energy absorption efficiency of TRTs with 30 mm section length is 119.6%higher than that of the 20 mm section length in the same volume.In addition,the overall energy absorption can be improved by increasing the tube heights,but it reduces the energy absorption efficiency.(3)The FEM of TRTs under quasi-static axial loading was established,and the crushing behaviors of TRTs with variety of structural size and material properties distribution were investigated by using it.Results show that,the load history,energy absorption characteristics and deformation modes obtained by the FEM are basically in agreement with the test results.Through studying the influence of structural parameters,the thickness and height of thick zone are the two most significant factors affecting the energy absorption performance.When the thickness of the thick zone increased from 1.2 mm to 1.8 mm,the energy absorption of the component increased by 86.5%,while the energy absorption can be increased by 114.7%when the axial length of thick zone increased from 0 mm to 100 mm.Under the condition of ensuring the geometric parameters of structure remain unchanged,only changing the material strength distribution of the transition zone also can greatly affect the energy absorption capacity of the TRTs,and even change the deformation order and crushing mode of the workpieces.When the strength increases linearly with the thickness thickening,the energy absorption performance and equivalent load of the component with strength distribution of 300+5x is 71.9%and 62.5%higher than that of the component with strength distribution of 300+x.In addition,the axial compression behaviors of TRTs and UTs with induction grooves were compared.The load level of the latter is higher in the early stage of axial crushing,but the load level of TRTs is significantly higher in the later stage.After complete crushing,the energy absorption of TRTs is higher about 26.6%than the UTs with induction grooves.(4)The energy absorbing characteristics of TRTs under axial impact were studied by using the high-speed impact equipment.Based on the data of material properties at different strain rates,the dynamic axial compression FEM of TRTs has been established,and the influence of various geometric parameters on the crashworthiness of TRTs were analyzed by the model.The results show that the energy absorption efficiency of the hexagonal TRTs is 19.4%higher than that of the quadrangular TRTs under high-speed impact,and the energy absorption efficiency has been increased by 76.8%when the circumference of the section increases from 189 mm to 210 mm.Comparing the performances of TRTs under quasi-static and dynamic axial compression,the initial peak load of components under high-speed impact is 50?120%higher than that of components under quasi-static axial loading.Meanwhile,the overall load level has been increased by 54.1%?76.7%.According to the energy absorption performances of different geometric parameters,the thickness and the axial height of thick zone are still the most significant factors affecting the energy absorption.Besides,the energy absorption increases by about 58%when the cross section edges are odd,and 70?80%when the cross section edges are even compared with the quasi static results.(5)Aimed at a domestic vehicle,the crashworthiness of the front-end structure equipped with the energy absorbing box components with axially varied thickness under the two high-speed impact conditions were studied by using FEM.The optimal structural parameters of which were obtained by combining the mathematical surrogate model with the optimization method,which improves the crashworthiness of automobiles.The results show that the front-end structure fitted with the energy absorbing box components with axially varied thickness has good deformation stability under 100%overlap and 40%offset positive impact conditions.The optimal values of the optimal variables obtained by the response surface methodology and NSGA—? optimization algorithm are:thickness of thin zone is 1.46 mm,thickness of thick zone is 1.79 mm,axial length of thin zone is 50.59 mm and the axial length of thick zone is 26.72 mm.The energy absorption capacity of the novel car crash box with thickness variation is 14.33%higher than that of the original car crash box when it collides for 13 ms under 100%positive impact condition,and 24.95%higher than that of the original car crash box under 40%positive impact condition.Meanwhile,the optimized car crash box loses weight by 12.9%.In addition,the peak load of the optimal component is not more than 120 kN,which is 17.83%lower than that of the original component. |
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