Optimum Design, Experimental Testing and Post-Buckling Analysis of Thick Composite Stiffened Panels

Abstract

The paper presents a strategy for the efficient opti mum design of composite stiffened panels using VICONOPT, a fast -running optimization package based on linear eigenvalue buckling theory, and embracing practical composite design rules. The VICONOPT and finite element models for the design and analysis of t he panel were validated with experimental compressive testing of two blade stiffened panels. The buckling and post buckling behavior of the two panels, with initial buckling in the stiffeners and skin, respectively, was investigated in a high load and high strain range. The optimization strategy, based on substitution of equivalent orthotropic plates for laminated plates, is evaluated by the design of a Z stiffened panel. Finite element analysis is performed to verify the design. It is demonstrated that the strategy is capable of efficient, accurate and practical optimization of composite stiffened panels. I. Introduction kin -stiffener structures are extensively used in the aerospace field due to their structural efficiency in terms of stiffness/weight and strength/weight ratios. The application of such panels is primarily within fuselages and wing boxes, where the weight saving potential of composite materials compared with aluminum alloys is well known. However, design of composite panels involves the opt imization of a large number of variables such as ply thickness and plate widths. Further complication arises when the expert knowledge required for laminate design is considered and when the panel is constrained by buckling under axial compression. Durin g the design of composite laminates against buckling, the full complexities of detailed modeling, analysis and optimization are compromised for the sake of efficiency 1 . The optimization of composite stiffened panels subjected to buckling constraints has be en considered in many previous studies. Some of these have focused on the simplest modeling method of closed form equations to investigate the structural efficiency of various stiffener shapes for minimum mass and costing. Kollar 2 reported a closed -form eq uation to determine the local buckling loads of composite structural members when the edges of the webs are rotationally restrained by the flanges. Using closed -form equations as buckling constraints for cost and weight minimization, Kassapoglou 3