Keywords: cost optimization, weight optimization, composite stiffened panels.

The design and analysis of stiffened composite panels, for the most part, do not account for the cost until the very late stages of the design process. As a result, the design of such panels is rarely optimized for cost and cost-weight trades that show how various design decisions affect the one or the other are not performed.

With the development of various cost estimating tools, it became possible to include cost as one of the variables in the design process [1]. Even when relatively simple mathematical formulations for part strength and stiffness are used, the multiplicity of failure modes that must be accounted for, combined with a variety of design configurations (for example different cross-sectional shapes for the stiffeners) and manufacturing processes, lead to a very complex, computationally intensive problem even with one objective function (either cost or weight). This complexity increases as the applied load increases because the laminates needed are thicker and the number of acceptable stacking sequences increases exponentially. Finding robust efficient designs is further complicated by that fact that there are no simple stiffness or strength parameters of the candidate layup that can be expressed in closed form as functions of simple design variables. Finally, the thickness is a discontinuous variable and using standard gradient-based optimization approaches requires extra caution.

The present study develops a method to determine efficient configurations that can be used to perform trades between minimum cost and weight especially for highly loaded panels under compression. These trades help establish basic trends that can later be used in setting up more efficient multi-objective optimization approaches. This investigation builds on previous work [2,3,4] and extends it by combining weight and cost as parameters in the design process. In addition, the present approach establishes an efficient method for generating candidate layups for the different components incorporating industry-accepted design guidelines for robust performance and deriving governing equations for failure of skin and stiffeners. A number of failure modes and design constraints are used to rank designs to determine the lowest weight and cost configurations. Thickness is treated as a discontinuous variable. A Pareto set of optimum or near-optimum designs is derived. Combinations of stiffener spacing and dimensions are found that minimize the weight or cost. The global optima for weight and cost are very different with minimum cost requiring a minimum number of large stiffeners.

1

G.E. Mabson, B.W. Flynn, L.B. Ilcewicz, and D.L. Graesser, "The use of COSTADE in developing composite commercial aircraft fuselage structures", in "Proceedings of 35th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference", HiltonHead, SC, AIAA Paper 94-1492, 1994.

2

P. Apostolopoulos, C. Kassapoglou, "Recurring Cost Minimization of Composite Laminated Structures - Optimum Part Size as a Function of Learning Curve Effects and Assembly", Journal of Composite Materials, 36, 501-518, 2002. doi:10.1177/0021998302036004556

3

C. Kassapoglou, A. Dobyns, "Simultaneous Cost and Weight Minimization of Postbuckled Composite Panels Under Combined Compression and Shear", Structural Optimization Journal, 21, 372-382, 2001. doi:10.1007/s001580100116

4

C. Kassapoglou, "Minimum Cost and Weight Design of Fuselage Frames: Part B - Cost Considerations, Optimization and Results", Composites A, Applied Science and Manufacturing, 30, 895-904, 1999. doi:10.1016/S1359-835X(98)00191-2

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