| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2001. === Includes bibliographical references (leaves 197-204). === For the weight efficient and crashworthy design of the structural body of a transportation system, a thorough understanding of crushing behavior of thin-walled structural members such as spot-welded sheet metal beams or extruded aluminum beams must be gained. In the present thesis, the complex crushing process of three-dimensional thin-walled structures subject to combined loading is solved analytically and numerically. Also, several new design concepts of strengthening "S" shaped frame with regard to weight efficiency and energy absorption are proposed. The mechanics of biaxial bending collapse and the collapse under combined bending and compression of thin-walled prismatic member are formulated and initial and subsequent shrinking interaction curves between the loading components are constructed. All the analytical derivations show close correlations with the results of the accompanying finite element analysis. Based on these two complex crushing mechanisms, the analytical derivation of the crushing resistance of three-dimensional "S" shaped frame is presented. Extensive study on the strengthening of the three-dimensional "S" shaped frame is performed with two types of internal reinforcing member, diagonally positioned sheet metal stiffener and ultralight metallic foam-filler. The optimization process involving varying the cross-sectional shape and the type of reinforcing member for both aluminum-extruded member and spot-welded hat-type cross-section member is developed. === (cont.) Using the analytical closed form expression of the crushing force of "S" shaped frame, the optimization process was performed based on Sequential Quadratic Programming. As a more realistic application, a front side rail and subframe structure of a mid size passenger car is analyzed. The combinational optimization process of "Design of Experiment" and "Response Surface Method" is carried out with the objective of weight minimization while maintaining the same or higher level of crash energy absorption. Both methods of internal reinforcement show high increase in the energy absorption and weight efficiency. The gain in terms of the specific energy absorption varies from 37% to 267% depending on the method. The proposed theoretical understanding and the design methodologies could be used as crash oriented early-stage component design tools. === by Heung-Soo Kim. === Ph.D.
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