Design Optimisation of Practical Variable Stiffness and Thickness Laminates

Abstract

The use of composite materials in airplanes has been increasing over the last decades, mainly due to the high strength-to-weight and stiffness-to-weight ratio of composites. Traditionally, the possible fibre angles are often restricted to 0°, ±45° and 90°, referred to as conventional laminates. However, with the rise of fibre placement machines, not only can any ply angle be placed, the fibres can even be steered onto curved paths. By steering the fibres, the mechanical properties of the material are made spatially varying while maintaining material continuity; hence these laminates are called variable stiffness laminates.
This thesis proposes an optimisation approach that exploits the possibilities of variable stiffness laminates, while posing limitations to the steering radius to guarantee the optimised design is manufacturable. Furthermore, the design guidelines are interpreted for variable stiffness laminates and posed as constraints, increasing the (industrial) feasibility of the optimised design.
In addition to steering the fibres, layers can also be dropped to obtain laminates with varying mechanical properties. This is also incorporated in the optimisation algorithm, leading to variable thickness laminates. Finally, the combination of steering fibres and dropping layers is implemented as well, leading to variable stiffness, variable thickness laminates.