Influence of stiffeners on a variable stiffness laminate optimisation

Engineers are still looking for ways to improve the performance of materials used in aircraft. A major step was going from metal parts to laminates in order to save weight significantly. Those laminates existed of fibres with a constant orientation, and became laminates with changing fibre directions. Variable stiffness laminates (VSL) are used for its possibility of exploiting the anisotropic properties of composites. In order to improve the buckling load of a panel, VSL are used. Those laminates steer the loads through the laminate. This is beneficial for maximising the buckling load of a panel. An optimiser is built by the Delft University of Technology to optimise VSL by changing its fibre orientations throughout the entire panel. A manufacturing constraint is added to the optimiser to prevent the fibre paths to have a turn radius of smaller than 0.333[m]. Plate designs with fibres paths having a smaller turn radius than this constraint, are assumed to be unmanufacturable. The software optimises fibre paths of VSL against buckling and keeps the stiffness of the entire plate equal or higher than the quasi isotropic design. Since the aircraft of nowadays are stiffener dominated, an extra functionality is added to the optimiser. The optimiser is further developed to be capable of optimising stiffened plates. Stiffeners are taken into account during the optimisation. Extra functionalities such as integrated stiffeners are introduced. These are stiffeners sharing layers with the plate. The extended optimiser is verified by comparing the internal loads from five cases with the values calculated with ABAQUS. After having the optimiser verified, it is used to calculate results for twelve different cases. Those cases range from square unstiffened plates to rectangular plates with two stiffeners. The results show the influence of stiffeners on the optimised panel with changed fibre paths. Those cases are compared to the plate designs with the stiffeners excluded from optimisation. Performance increases of up to 174% are obtained for balanced symmetric laminates. The results show clearly that when stiffeners are added to a panel, the area in-between the stiffeners can be treated as a simply supported plate. Integrated stiffeners also show a buckling load increase of 113% with respect to the quasi isotropic case. The fibres of the buckling optimised plates show a relating behaviour. Fibre orientations going towards [45,-45] are present in buckling critical areas. Near stiffeners and near edges, [0,0] areas are present in order to introduce the load through the structure. A relation is found between having more [0,0] orientations in the inner plies of the laminate and significantly more [45,-45] orientations at the outer plies of the laminate. The outer plies are more critical for buckling, which requires more fibres being close to a [45,-45] in order to higher the buckling load. The inner areas are less vulnerable for buckling, which creates the opportunity of creating stiffness in the inside of the laminate.Aerospace Structures and Computational MechanicsAerospace Engineerin