Mass minimisation is a key objective in aircraft design which can reduce the material cost,fuel consumption and environmental impact. To achieve this objective, composite materialsare often used to replace traditional metals in order to increase the strength to weight ratio.Additionally stiffened wing and fuselage panels often have a postbuckling reserve of strength,enabling them to carry loads far in excess of their critical buckling loads. Therefore allowingfor postbuckling in design can reduce their weight (Che et al., 2010).The software VICONOPT (Kennedy and Featherston, 2007) performs initial buckling,post-buckling and free vibration analysis of metal or composite prismatic stiffened panels,using an exact strip method and the Wittrick-Williams algorithm (Wittrick and Williams,1973). However the post-buckling analysis in VICONOPT assumes that the deflections of theplate assembly vary sinusoidally in the longitudinal direction (Che et al., 2010).The present paper extends the post-buckling analysis in VICONOPT to more accuratelyreflect the skewed mode shapes arising from shear load and anisotropy. Such mode shapes arerepresented by a series of sinusoidal responses with different wavelengths which are coupledtogether using Lagrangian multipliers to enforce the boundary conditions (Anderson et al.,1983),. The governing equilibrium equations therefore involve infinitely many unknowns andcannot be solved exactly. However suitable accuracy is achieved by truncating the series aftera few wavelengths. In postbuckling analysis it is found that the in-plane deflections involveresponses with additional wavelengths which are absent from the out-of-plane deflectionseries. Numerical results are presented and compared with finite element analysis forvalidation.
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