Composite materials are a serious competitor for lightweight metals used in the aerospace and automotive industry. Uni-directional (UD) carbon fibre reinforced thermoplastics are favoured due to their high specific strength and stiffness, but also their good toughness, impact and chemical resistance properties. By heating UD reinforced thermoplastic laminates sufficiently above the melting point of the polymer, these can be stamp-formed to relatively complex geometries. The product is released after a relatively short cooling time. Hence, high production rates can be achieved, which makes this process very appropriate for the large volume production of high performance thin-walled products of complex shapes. Nevertheless, process-induced defects such as wrinkling are frequently encountered, which disqualify the final product. A thorough understanding of the deformation behaviour of UD laminates is required to anticipate those defects, which is therefore one of the objectives in this research. Forming simulation tools can be employed in the product design phases to anticipate the defects observed, ultimately leading to a reduction in product development costs. The predictive capability of forming simulations was therefore carefully analysed. Forming an initially flat laminate to a doubly curved surface invokes in-plane and out-of-plane deformations, such as intra-ply shear, inter-ply slippage, and bending. These are described with constitutive models, which require material data input. The sensitivity of composite forming predictions to this input was firstly investigated for a dome-shaped geometry. The resulting product shape was found to be determined by a delicate balance between the mechanisms considered, which highlights the importance of a thorough material characterisation. Wrinkle-free forming of UD laminates to doubly curved surfaces requires in-plane deformations of the plies, in particular by shear. The work therefore focuses on the intra-ply shearing mechanism, where fibres slide parallel to each other. A new shear characterisation test for UD fibre reinforced thermoplastics was proposed. Torsion bar specimens from polyetheretherketone (PEEK) with a UD carbon fibre reinforcement (UD-C/PEEK) were subjected to oscillating loads in order to determine the dynamic shear moduli from the linear visco-elasticity theory. The composite system shows a predominantly elastic behaviour for small strains, which is attributed to multiple fibre-fibre interactions. A low temperature and frequency dependency was found as well. The latter indicates the presence of yield behaviour at larger strains. Forming experiments were conducted with quasi-isotropic UD-C/PEEK laminates on a representative product geometry used in the aerospace industry: a wing stiffening rib. These laminates are sensitive to wrinkling near areas with double curvature. Limited intra-ply shear strains develop in the final stage of forming, where further bending and wrinkling are prohibited by the tooling. The formability issues of the UD-C/PEEK material are explained by the relatively high resistance to intra-ply shear. The wing stiffening rib was used to study the predictive capabilities of finite element based forming simulations. The laminate was modelled by incorporating the characterised behaviour of intra-ply shear and inter-ply friction. The predicted intra-ply shear strain fields and the large wrinkles match well with those observed in the experiments. However, the results were dependent on the unknown bending parameters, for which an extensive characterisation programme is necessary. The small wrinkles observed in practice cannot be predicted with the element size used, however, predicted waviness at the corresponding locations may indicate potential critical spots. The simulations conducted have proven to be instrumental in obtaining a better understanding of the laminate deformations during the stamp forming process. They can be employed for design optimisation, as well as to derive design guidelines in a more general sense.
|Qualification||Doctor of Philosophy|
|Award date||12 Apr 2013|
|Place of Publication||Enschede|
|Publication status||Published - 12 Apr 2013|