Optimization Design Of Composite Hood Based On Stiffness

Hood is an important car body cover. Its external styling design mainly concerns vehiclestyling and aerodynamic requirements. As outer cover,the hood does not carry loads. Thehood structural design mainly requires stiffness and lightweight, and nowadays the pedestrianprotection is added.In other components, particularly in the application of bumper, the sandwich structurehas shown the ability of taking into account the stiffness and weight requirements, and crashenergy absorb ability.Sandwich structure is a typical of light weight and high stiffness structure. Thecomposite material, as the sandwich panel material, has lower density and higher stiffness,compared to other common materials. The stiffness can be guaranteed, while further weightreduction. The sandwich core can absorb impact energy, to compensate the lack of kineticenergy of the lightweight hood.Therefore, carbon fiber sandwich structure is a new hoodstructure, which has the potential to meet the stiffness, lightweight and pedestrian protectionrequirements at the same time. In this thesis, the nature of this structure is researched, andthe stiffness and mass reduction is optimized.For optimization the hood, the hood evaluation system should be determined at the verybeginning.after that,the optimization objectives and constraints could be selected.At present,there are several hood static stiffness evaluation methods, and they are very confusing.Therefore, first of all,these methods should be analysed. Use steel hoods and carbon fiberhoods whose panels are quasi-isotropic or anisotropic laminates, a total of14hood models assamples. Calculate them using the11kinds of evaluation methods. Do regression analysiswith the result, and identify the relations between these methods. By classifying theserelations and selecting them,4static stiffness evaluation methods are chosen. They are bending stiffness, torsion stiffness, slamming stiffness and latch stiffness.On the initial carbon fiber hood prototype, the frame mass is40%of the overall. Andthere is no consideration in frame in sandwich theory. Therefore a design of experiment isused, which aim is evaluating the affect of mass reduction methods on hood stiffness. Usetwo ways to reduce the frame mass. They are changing the structure and thinning the wallthickness. Choose frame structure, frame wall thickness and face sheet overlay groupnumber as factors, four static stiffness results as responses, use the full factorialsexperimental design, do ANOVA analyses, the relations between the responses and thefactors are got. The result shows that, using both two mass reduction methods will makeslight decrease of the stiffnesses, but the mass reduction is obvious. Therefore, the lightestframe is chosen.Analyse the foregoing results and modal analyse results of the14models, it could befound that the stiffnesses of the carbon fiber hoods exceed a lot comparing to steel hoods. Inthis way, the carbon fiber hood may be more lightweight. Appoint stiffness as constraint,use genetic algorithm to minimize stacking number. For this stacking number optimization,referring to the mRNA transcription in biological genetic process, an improved geneticalgorithm is proposed. Using matlab to program, abaqus to calculate the simplified model,and isight, the improved genetic algorithm runs. Compared to optimization prototype, theoptimization result achieves a25%reduction in stacking number, which shows that theimproved genetic algorithm is effective.The optimized carbon fiber hood stiffness increased by more than1.5times, comparedto stiffened steel hood. And it achieved a50.8%mass reduction, comparing to lightweightsteel hood. These results demonstrates the advantages of carbon fiber sandwich structure interms of hood stiffness and lightweight. Use the improved genetic algorithm, the lightestquality design could be gained, on the basis of the specified stiffness.This structure could beused by the hood design which will meet the pedestrian protection.