서강대학교

초록 (요약문)

The complete design of a recycled-carbon-fiber reinforced-plastic / metal hybrid (PMH) automotive front subframe is realized is realized via finite element (FE) analyses and multiple structural optimization techniques. The structure considers a low-cost recycled carbon fiber (rCF) and PMH technologies. The design is divided into three main steps: the conceptual, preliminary, and detailed design. Conceptual design aims at constructing and analyzing different ideas and approaches before the construction of the actual geometry. This stage is fairly important, since the design of PMH parts is normally complicated; testing the concepts helps predicting future issues in the design, building knowledge regarding the manufacturing process and unique material characteristics, and drawing a proper design approach for the actual product. In Part I, the design and evaluation of PMH front subframes are realized via the design of concept models and FE analyses. Firstly, the design of two PMH front subframe concepts is carried out. The initial design approach consists of creating a C-type and a hollow-type insert and modifying the plastic component according with insert type. Additionally, the design variables considered in the design process are the dimensions of insert, insert thickness, and inclusion of plastic ribs. The PMH front subframes are later evaluated by performance criteria, including weight reduction, stiffness, natural frequency, stress levels under endurance loads, and strength. The results showed that the integration of rCF with injection overmolding of metal inserts was able to promote weight reduction, where the hollow-type and C-type PMH front subframes achieved a weight reduction of 19 % and 16 % in comparison to the reference steel front subframe, respectively. Moreover, the PMH front subframes achieved the desired performance targets set by the evaluation criteria. The preliminary design stage deals with the construction of the initial product geometry and specifications of additional performance requirements. In Part II, the design is realized via combined optimization techniques and their injection molding (IM) suitability verified. Topology optimization (TO) and combined TO and free-size optimization (FSO) techniques are exploited in the design process to find the optimal rib configuration and location of metal inserts. A material exchange technique to account for fiber orientation elementwise is employed and the models analyzed in terms of functionality and producibility. The robust design procedure was shown to be effective in developing sCFRP (short-recycled-carbon fiber reinforced-plastic) and PMH parts while considering IM restrictions and plastic part design guidelines from the early design stages. The PMH model obtained the most outstanding results with the greatest balance between functionality and producibility, with a 36 % weight reduction from the steel baseline model. Although the sCFRP models achieved weight reduction over 50 %, they presented difficulties in simultaneously obtaining the required levels of stiffness, strength and producibility. Detail design refines the preliminary product geometry into a shape that is functionally acceptable and compatible with the IM process. In Part III, the design is carried out via composite size optimization, which considers specific IM design guidelines in order to improve part quality. The complete FE modeling is discussed, where thickness-dependent material properties of the reinforced plastic utilized in this study is automatically updated in the optimization process. In the optimization modeling, user-defined constraints are utilized to improve the efficiency of the algorithm. Moreover, a proper design guideline for the front subframe case is selected based on simulation results. With the procedure, the quality and IM suitability of the PMH subframe was significantly improved, while weight reduction was increased to 38 % with a further simplification of the model. The results of this study contribute to the efficient multimaterial design of PMH components in the auto industry and the advance of a new class of lightweight subframes with improved structural performance, fabricated by fast-capability manufacturing processes and low-cost carbon fibers.