Structural optimization of a hat-stiffened panel using response surfaces

A design study for structural optimization of a hat-stiffened laminated composite panel concept for an upper cover panel of a typical passenger bay of a blended wing-body transport airplane configuration is described. The feasibility of a hat-stiffened composite panel concept for an upper cover panel is investigated and is compared on the basis of weight to a thick sandwich panel concept that has been selected by designers as the baseline concept for the upper cover panel. The upper cover panel is designed for two load cases: internal pressure only and combined internal pressure and spanwise compression due to wing bending. The structural optimization problem is formulated using the panel weight as the objective function, with constraints on stress and buckling. The spacing of the hat stiffeners, the thickness of the skin, and the thickness of the components of the hat stiffener are used as design variables. The initial geometry of the hat-stiffened panel design is determined using the PANDA2 program by restricting the design to have uniform cross section in the spanwise direction. Because of the pressure loading, a more efficient design has variable cross section. Such designs are obtained by combining the STAGS finite element program with the optimization program in the Microsoft EXCEL spreadsheet program using response surfaces. Buckling and stress response surfaces are constructed from multiple STAGS analyses and are used as constraints in the optimization. The optimization conducted with the response surfaces results in considerable weight savings compared to the uniform cross section design, albeit a more complex design. Initial optimization cycles identify a design space where simple approximate analyses, such as Euler-Bernoulli beam theory and Kirchhoff plate theory applied to laminated composites, can be used to predict the behavior of the structure.