Design of compressed variable stiffness panels with steering-thickness coupling

Automated manufacturing techniques of composite materials such as automated fiber placement (AFP) and continuous tow shearing (CTS) can be configured to produce fibers that follow curvilinear paths, resulting in variable laminate properties that can be tailored to a large range of engineering applications. The present study focuses on the design and optimization of a wing upper skin exploring the coupled thickness build-up that is inherent to the CTS process and appears in the AFP process when continuous tows are used with an overlapping design approach. The steering-thickness coupling comes from constant-volume requirements and is an extra nonlinear constraint that poses additional challenges to the design and optimization, rendering conventional two-step approaches based on lamination parameters and total thickness ineffective. The number of longitudinal stiffeners, cross-section shape and laminate configuration are treated as design variables in a single-step optimization driven by a classical genetic algorithm. Knowing the current state of angle distributions proved to be important while calculating the coupled thickness build-up. The optimization problem is constrained by the critical linear buckling load, herein calculated using finite elements with MSC Nastran (R), and by manufacturing and design constraints, such as the minimum steering radius of AFP and CTS and common design guidelines for laminated composites. The results are compared with an optimized baseline design using conventional straight-fibre laminates to quantify how the design is changed in terms of overall geometry, buckling loads and structural weight.