Study On Design Methodology Of Graded Thin-walled Structures And Its Application In Automotive Body

In recent years, lightweight design of vehicle body had been a key problem of automotive development. Among the existing lightweight structures, there is a class of the thin-walled structures whose performances are relatively advantageous. Their material grades and thicknesses are distributed according to a certain graded form, such as tailor welded blanks-TWB and variable thickness blanks-VTB. They can be designed to be some special automotive components according to their different characteristics and requirements. Thus, the structures could be flexibly fabricated. However, the graded structures could not affect other important performances when main requirements are guaranteed. In order to further understand the basic performance of graded structures and provide guide of lightweight design and make the graded structures be further applied into vehicle body, it is very meaningful and highly-value to investigate on the lightweight design of the graded structures.The dissertation studied on the key engineering problems for those graded thin-walled structures. The main content includes the refined modeling technology on the weld interface; the determination reasonably on the location of weld line for TWB structures and multi-objective optimization design method for front and side dynamic impacting events. The initial study and comparison analysis on crashworthiness^of VTBs is finally carried out. The detailed content is as follows:(1) A method is provided to effectively indentify the material parameters of weld interface based on experiments, simulations and computational inverse techniques. The method lays on a solid fundamental for modeling on TWB structures and has a certain engineering value to some extent. The numerical model of indentation test is conducted based on power exponent constitutive model. The parameters are changed until convergence by comparing simulation and experiments results. And strain exponent and strength coefficient are obtained by digital image correlation technique (DIC) and computational inverse. The error is controlled in 5% and the obtained parameters are much more accurate and reliable. The computational design method can improve simulation accuracy.(2) A design method is provied to determination of weld line for multi-thickness TWB door in vehicle body. The position of weld line is reasonably given under different stiffness by bi-directional evolutionary structural optimization (BESO) method. And the sensitive numbers are computed according to different stiffness requirements. The optimized objective is to make the TWB door maximum stiffness under the given loads, boundaries and volume fractions of different thicknesses. The optimized results show that the zones with different thicknesses are re-distributed and the positions of weld lines are located. The position is indentifed with the interface of zones with different thicknesses. The performance of TWB with the optimal weld line positions obtains best to some extent. In other words, the optimal TWB door has maximum stiffness when one or multiloads are applied. In addition, the weight is reduced and meanwhile the structrual stiffness is garanteed.(3) A design method of side muti-components TWB coupling system is proposed based on the previous research fundenmental. First, the weld line position of TWB door is quantitatively given according to the design method of multi-thickness TWB structures. Then, numerical model of coupling system including TWB door and B pillar is conducted and validated. The accuracy of approximate models is different for different mechanical problems. And those models are comparied for side impacting response of the coupling system. The results demonstrated that the RBF model is suitable for the safety design of side impacting of multi-components. And multiobjective optimization is performed by different optimization method. Thus, lightweight design is carried out for autobody side multi-components coupling system. By optimizing the thickness of different parts and height of weld lines, crashworthiness performance of the coupling system is improved. For example, the intrusion displacement of B pillar could be reduced by 26.57%.(4) An optimization algorithm using orthogonal array is proposed for design on multiobjective optmization of TWB front longitudinal beam. Methods for discrete design such as genetic algorithms are extremely expensive in computational cost. In this method, the thickness and material grades are set as discrete and special values. A characteristic function is defined to consider the constraint feasibility. A new design in a certain iteration is determined from analysis of means (ANOM) with the characteristic function. The constrained optimization problems with discrete variables are solved. The number of finite element analyses is considerably reduced by the proposed method. The method can be applied with low computing cost and high computational effective. Multiobjective optmization is perfomed to crashworthiness of TWB structures by combining with the orthogonal array. The opitimized results can improve the crashworthiness performance of TWB structures. And the peak crashing force is reduced by 49.2% and mass by 9.84%. It shows that the optimization method is more suitable for crashworthiness design of TWB. The proposed method could deal with two main problems:expensive computational cost of general discrete optimization methods and tranditional orthogonal array does not solve crashwithiness design of TWB with many variables and levels. And it can be extended into other enigeering problems, which showed a certain application value.(5) To avoid the defects of TWB structures such as stress concentrate and mateiral distributions, crashworthiness design method of VTBs whose thickness is assumed as pow exponent distrubitions are perfomed based on design criteris and optimization design. And the graded exponent is defined as design variable and two design ranges are divided into. The Pareto solution is obtained by optimization method and the crashworthiness of VTBs are improved by reasonable thickness distrubitions. The crashworthiness performance of variable thickness structures under axial and mutiple crashing angles is qualitily carried out. And the analytical parameters relationship of VTBs, uniform thickness straight and tapered tubes (SUT and TUT) is conducted and the energy absorption charateristics from those three kinds of thin-walled tubes are compared. A new indicator, i.e., energy absorption per unit area ETA is introduced. It provides a measured method for different tubes with same weight but different occupied space area. Refer to TUT, a design criteria about gradient exponent is given when VTBs are fabricated. Comparision results show that the VTBs are more advantgous than TUT and SUT. The graded variety property makes the structures deformed more gently. Thus, the ability to resist bending deformations can be impoved under much bigger crashing angles.