Optimization of Composite Structures to Delay Mode Jump Instabilities

Numerous studies have shown that postbuckling stiffened panels may undergo abrupt changes in buckled mode<br/>shape when loaded in uniaxial compression. This phenomenon is often referred to as a mode jump or secondary<br/>instability. The resulting sudden release of stored energy may initiate damage in vulnerable regions within a<br/>structure, for example, at the skin-stiffener interface of a stiffened composite panel. Current design practice is to<br/>remove a mode jump by increasing the skin thickness of the postbuckling region. A layup optimization methodology,<br/>based on a genetic algorithm, is presented, which delays the onset of secondary instabilities in a composite structure<br/>while maintaining a constant weight and subject to a number of design constraints. A finite element model was<br/>developed of a stiffened panel’s skin bay, which exhibited secondary instabilities. An automated numerical routine<br/>extracted information directly from the finite element displacement results to detect the onset of initial buckling and<br/>secondary instabilities. This routine was linked to the genetic algorithm to find a revised layup for the skin bay, within<br/>appropriate design constraints, to delay the onset of secondary instabilities. The layup optimization methodology,<br/>resulted in a panel that had a higher buckling load, prebuckling stiffness, and secondary instability load than the<br/>baseline design.