Crashworthiness Analysis And Design Optimization Of Lightweight Materials And Structures For Energy Absorption

Lightweight (means low energy consumption) structures with high reliability are the pursuing goals in the design of modern cars and vehicle transport. With the request of lightweight design, it has become unfeasible to improve the load-carrying capacity and crashworthiness of the vehicle body only through increasing the sheet thickness. So, how to meet the requirements of lightweight design, but also to ensure the collision safety is a challenging task before the automotive design engineers. With this motivation, the crashworthiness analysis and design optimization of lightweight materials and structures for energy absorption are carried out in the present work, which aims at the collapse mode of the main energy-absorbing components and the corresponding energy absorption characteristics during the collision accidents. Several typical thin-walled structures with high energy-absorbing efficiency are obtained, which meet the requirements of both safety performance and lightweight design. The detailed research contents and results includes the following aspects.(1) A hollow thin-walled beam with internal stiffener is presented for potential application in vehicle energy-absorbing components. Furthermore, the shape optimization of the internal stiffener is performed for improving the crashworthiness when subjected to the transverse impact. The shape of the internal stiffener is depicted by a spline curve defined through a few control points and the shape optimization is implemented by designing the longitudinal coordinates of the controlled points. A multi-objective optimization model, including maximum energy absorption, maximum specific energy absorption and minimum initial peak force, is constructed through the ideal point method. Compared with the empty and foam-filled square beams with the same weight, the energy absorption of the optimum rib-reinforced beam is greater than the other two sections and the initial peak force is distinctly lower. Thereinto, in order to clarify the reasons of shift in deformation mode of empty square beam under bending loads, some possible factors that may influence the shift of bending mode are investigated. It is found that the main reason of changing the bending mode in the present work is that the cross section sizes of the beam are too large. In addition, the explicit finite element analysis based on LS-DYNA is validated by experimental test through the lateral bending of door crash beam.(2) An innovative impact energy absorber is presented, called honeycomb sandwich cylindrical columns. This new type of energy absorber is made of two circular aluminum tubes filled with core shaped as a large-cell honeycomb lattice. Taking the kagome sandwich column for example, the deformation mode and energy absorption characteristics of this type of honeycomb sandwich columns are studied. Owing to the interaction effects between the honeycomb and tubes, the collapse mode of the outer and inner tubes are changed, which increases the plastic deformation areas and improves the plastic deformation level. The energy absorption of the composite tubes is greatly increased. The geometrical parameters of kagome and honeycomb type are discussed on the effect of axial crushing. For comparison, the foam-filled tubes with three different adhesive conditions are also studied. It is found that the energy absorption is improved and the deformation mode is transformed as the adhesive strength increases. Furthermore, both types of sandwich columns are compared. It is found that the honeycomb sandwich columns, especially the kagome sandwich column, have better crashworthiness.(3) To improve the crashworthiness of S-shape rails, two aspects of works are carried out, including changing the cross-section shape and optimizing the axis configuration. Firstly, three groups of cross-section shapes are studied on the effect of crashworthiness. It is found that the energy absorption gradually declines from the square to circular sections, and the large aspect ratio of rectangular section has higher energy absorption capacity. For the reinforced sections, the vertical stiffener section absorbs more energy than the horizontal stiffener; the cross stiffener section absorbs more energy than the double vertical stiffener rail. Next, aiming at the localized behavior of plastic deformation for S-rails when subjected to axial loading, the axis configuration optimization problem is presented for improving the crashworthiness. Because of the highly nonlinear of collision analysis and time-consuming of explicit finite element analysis, the response surface methodology is adopted to carry out the optimization design. After optimization, both the energy absorption and specific energy absorption are greatly improved. Examining the deformation process, it is found that four localized fold occur and two plastic shear zones are formed. This greatly increases the energy absorption performance. Applying the optimum axis configuration to the spot welded hat-type thin-walled rail, four localized folds and two plastic shear zones are also formed. So the optimum configuration has stable collapse mode when subjected to axial loading. This offers a valuable reference and guidance for the design of vehicle body.(4) Taking the spot welded hat-type thin-walled tubes for example, the effect of gradient distribution of material properties is investigated on the crashworthiness. Firstly, the theoretical prediction formula of top-hat tubes is amended using the flow stress of energy equivalent. The applications are expanded from the mild steel to high strength steel, and validity is performed with three high strength steel tubes. Next, the spot welded hat-type tubes are designed with the gradient distribution of material properties. When subjected to axial crushing, this rigid-flex composite tube progressively folds and shows gradient. As the proportion of high strength steel increases, the energy absorption efficiency is greatly improved. However, the initial peak force keeps lower level. This corresponds to introducing the material imperfection. In addition, the above gradient distribution of material is applied to the outer plate of B pillar. The structural design is simplified, and the crashworthiness of side crash is improved.(5) Owing to the importance of energy-absorbing bumper systems in low speed collision and pedestrian protection, the crashworthiness optimization is presented for the thermoplastic energy-absorbing bumper systems. A multi-objective optimization model is constructed including minimum intrusion and minimum peak force by taking the energy absorber cross-sectional shape for design variables. A new type of energy-absorbing bumper systems is obtained by successive response surface method. The optimum bumper system has excellent energy absorption capacity in low speed collision with lower level of intrusion and peak force. Compared with the foam energy absorber widely used in bumper systems, it is found that the thermoplastic energy absorber has better protection performance for both the vehicle body and pedestrian.This work is supported by National Natural Science Foundation of China (No.90605002, 10721062,90816025), National Basic Research Program (973 Program) of China (No.2006CB601205), and the program for new century excellent talents in university of China (NCET-04-0272). The financial supports are gratefully acknowledged.