자연 섬유 복합재를 적용한 자동차 후드구조 최적화 설계 연구

Abstract

Recently due to increasing interest in eco-friendly materials, studies on fiber obtained from nature have been actively performed to the area of composite. Although the natural flax fiber has less strength than the high strength fiber such as the carbon fiber, it has similar strength to glass fiber. Accordingly, it can be applied as very advantageous composite when an appropriate resin has been selected. In this study, the design of eco-friendly structure using flax fiber was performed after investigation on mechanical properties of natural composite. The selected target structure is a hood of a small compact automobile. The optimization of structural design using the FEM can be divided largely into size optimization, shape optimization, topology optimization. The size optimization was performed to obtain design parameters including thickness through performing repeatedly the finite element analysis in a given shape. Therefore it has a disadvantage of the degree of freedom falling. The topology optimization was applied to find an optimal model within given constraints by changing the topology. In order to evaluate the structural design results of the hood, the structural analysis was performed by the finite element method. The element used for the analysis was the composite shell element PCOMP. Through this analysis, stress, deformation, safety factor, etc. at various load cases with constraints were investigated, and the structural safety was confirmed. The impact analysis was carried out by modeling the adult head to confirm the head impact safety. In the impact analysis of the hood, stress, displacement and safety were found when an adult male head hits on the hood central region at car velocity of 40km/h. For manufacturing of the hood using flax fabric/vinyl-ester composites, the RIM(Resin Injection Method) was adopted. In order to finding the proper RIM conditions of the food, the resin flow analysis was performed. The flow analysis is to predict of resin flow filling time and to confirm no dry-patch. The resin flow analysis was performed using the Polyworx FEM flow simulation solver. The permeability coefficient that is an important factor for resin flow analysis was obtained by experimental test. Through performing repeatedly the analysis by changing the location of resin inlet and outlet and vacuum pressure and by adding the runner, the proper resin filling time and pattern were found within the vinyl-ester resin gelling time of 3000 seconds. The prototype hood was manufactured by applying the RIM based on the resin flow analysis results, it was confirmed that the measured resin infusion time agreed well with the analyzed resin infusion time. The manufactured prototype was tested in accordance with the structural loading condition, it was confirmed that the structural test results meet similarly with the analysis results. Through this study, it shows that the automobile hood using the flax fabric/ vinyl ester composite can be used as an alternative structure instead of the currently used metal hood.