Bilevel optimization and postbuckling of highly strained composite stiffened panels

The paper presents a bilevel strategy for the efficient optimum design of composite stiffened panels using VICONOPT, a fast-running optimization package based on linear eigenvalue buckling theory, and embracing practical composite design rules. Panel level optimization finds a minimum weight cross-sectional geometry based on a substitution of equivalent ortbotropic plates for laminated plates. Optimization at the laminate level finds stacking sequences satisfying laminate design rules. VICONOPT models are validated with ABAQUS finite element models, and with experimental compressive testing of two blade-stiffened panels. The buckling and postbuckling behavior of the two panels, with initial buckling in the stiffeners and skin, respectively, is investigated in a high load and high strain range. The bilevel strategy is evaluated by the design of a relatively short Z stiffened panel which has been manufactured and tested, and also by design of a long wing cover panel with combined loads. The weight saving from the wing cover panel is 13% compared with an existing datum design. This demonstrated that the strategy is efficient, reliable, and extendable into the long panel range.