Methodology Study On Lightweight And Crashworthiness Design Of Front Longitudinal Beam With Tailor Rolled Blank

With the increasing sales of automobiles year by year,energy crisis and environment deterioration have been the main challenges in vehicle industry.Ligihtweight design of vechile is one of the most important way to handle aforementioned problems properly.However,it should not compromise vehicle crashworthiness performance when performing lightweight design.It is well known that the front longitudinal beam(FLB)is the most significant deformable part and energy absorption structure under vehicle frontal impact loading.The design quality of FLB will directly determine the vehicle safety to some extent.Due to stricter safety regulations and lightweight pressures,more attentions have been paid on crashworthiness and lightweight design of FLB.To balance the lightweight and crashworthiness performance of FLB,integrating tailor rolled blank(TRB)concept into structural design optimization of FLB is one of the solid approaches.To the authors9 best knowledge,the researches on crashworthiness and lightweight design of front longitudinal beam with TRB(FLB-TRB)are limited,and many difficult problems are needed to be solved.To handle the structural design problems of FLB-TRB,the following research works are performed in this paper:the fundamental mechanical properties of FLB-TRB,the optimal design of thichness distribution of FLB-TRB,the cross-sectional shape,size and topology optimization of FLB-TRB based on implicit parameterization technique,as well as the multiobjective system reliability-based optimization for design of FLB-TRB under uncertainty.The above works have formulated a systematic design methodology and theory for lightweight and crashworthiness design of FLB-TRB,and the main research works and innovative achievements are summarized as follows:(1)A novel TRB top-hat thin-walled structure(TRB-TH)is proposed.A general thickness distribution function and the corresponding mathematical formulation of the strict geometrical constraints are defined according to practical TRB manufacturing capacity.Meanwhile,the mass model of the TRB-TH structure is also derived.The proposed approach can provide some useful guidance for designing the FLB-TRB.Eight different kinds of variable wall thickness structures can be derived from the TRB-TH structure when the geometric parameters of specific conditions are satisfied.It should be noted that the feasible distribution of parameters are the main advantages of TRB-TH.Parameter study and multiobjective design optimization are performed to study the lightweight and crashworthiness of TRB-TH structures with variations in the thickness of constant thickness zone(CTZ),length and position of thickness transition zone(TTZ).Finally,the optimal thickness distributions and deformation patterns under different optimization problems are compared.(2)Dividing principle of different functional zones of FLB-TRB is established,and the relationship between the functional zones and equivalent dual-trapezoids wave(EDTW)is established.By using the EDTW,crash energy management of FLB-TRB can be achieved in displacement field and time field.Finally,design optimization method for the optimal thickness distribution of FLB-TRB is formulated.The inner part of front longitudinal beam with TRB(TRB FLB-inner)is taken as an example to illustrate the design methodology.The design optimization formulation for the TRB FLB-inner is established under the constraints of TRB manufacturing capacity,EDTW and the crashworthiness indicators,ε-support vector regression(ε-SVR)technique is used to construct the surrogate models for the dynamic responses of full-scale vehicle crash problems,and artificial bee colony(ABC)algorithm is adopted as an optimizer for crashworthiness design of the TRB FLB-inner.This work first conduct the design optimization of the thickness of CTZ,length and position of TTZ for TRB FLB-inner.(3)A high fidelity and locally parameterized subsystem frontal impact finite element(FE)model is established.Parameterization of cross-sectional shape,size and topology design variables can be achieved by using implicit parameterization technique.Then,the cross-sectional shape,size and topology optimization methodology for design of FLB-TRB is formulated based on the established locally parameterized subsystem model.To maximize the lightweight and crashworthiness design of FLB-TRB,structural design optimization is performed from the aspects of reducing the reinforcement part of FLB,optimizing the thickness distribution of TRB FLB-inner,optimizing the cross-sectional shape and position of beads or triggers.In the optimization process,parameterization of cross-sectional shape,size and topology design variables are achieved by using implicit parameterization technique,a full automated design of experiment(DoE)loop is established.ε-SVR technique and non-dominated sorting genetic algorithm-Ⅱ(NSGA-Ⅱ)are integrated to optimize the cross-sectional shape,size and topology parameters of FLB-TRB simultaneously.(4)To deal with the reliability-based design problem for structural system under multiple probabilistic constraints,an improved multi-modal radial-based importance sampling(MRBIS)method is developed by combining the advantages of importance sampling(IS)and radial-based importance sampling(RBIS).Numerical results have shown that the improved MRBIS method provides a good balance between accuracy and efficiency,and it can be effectively applied to the design problem of FLB-TRB.When performing the reliability-based design optimization of FLB-TRB,system reliability requirement is considered as the constraint function,and the MRBIS method is integrated into multi-objective system reliability-based design optimization(MOSRBDO)process of FLB-TRB.The numerical implementation of MOSRBDO employs the double loop method with design optimization in the outer loop and system reliability analysis in the inner loop.The MRBIS is utilized as the system reliability solver for multi-constraint analysis in the inner loop.NSGA-II is used as the outer loop optimization solver.In this thesis,MOSRBDO for design of FLB-TRB has been successfully applied.The results have shown that the MOSRBDO method integrated with MRBIS method is more efficient than the traditional MOSRBDO in achieving the same accuracy.