Design considerations for variable stiffness, doubly curved composite plates

It is widely acknowledged that laying up composite tapes and fabrics on doubly curved surfaces can introduce wrinkling and overlap defects. It is also not a straightforward task to define fibre angle directions. The latter problem amplifies when considering curvilinear fibre paths on doubly curved surfaces. Herein, we define a new fibre orientation datum curve and use this as a reference for optimisation purposes. A novel implementation of lamination parameters is introduced whereby we constrain the values of lamination parameters based on minimum turning radii of automated fibre placement machines and maximum ply angle change for stiffness optimisation. As an example, this approach is applied to a pre-twisted plate with 68 layers subjected to combined centrifugal and pressure loading. Lamination parameters are used to reduce the complexity and multiplicity of composite laminate design with many layers to a convex and continuous design space using a reduced number of variables. Lamination parameters are varied over the geometry using spline surface interpolation over a grid of control points and the optimal lamination parameter values are found using a gradient based optimiser. The fibre paths for each ply are then designed from the optimal lamination parameter variation using a two-stage optimisation process.