Non-crimp fabric permeability modelling

A qualitative study to the in-plane permeability modelling of Non-Crimp\ud Fabrics has been carried out. A network flow model was developed to describe flow through\ud inter bundle channels (meso level). To improve this model, it was extended with details that\ud consider stitch yarn influenced regions. The model predicts a highly anisotropic permeability.\ud The predicted permeability in the machine direction of the fabric corresponds with the\ud experimental results. However, prediction of permeability perpendicular to the fabric’s\ud machine direction does not correspond with the experimental results. Possibly, flow through\ud fibre filaments (micro level) is significant and the network flow model has to be extended to\ud include this type of flow

Complex stamp forming of advanced thermoplastic composites

The inherent advantages of thermoplastics over the traditional thermoset composite systems are well recognized in the aeronautics community. The main advantages are the much faster processing and the higher toughness. The current advanced thermoplastic material systems provide excellent mechanical performance but their main disadvantage is the difficult processing. Ideally, future aircraft subcomponents with complex shapes can be readily formed on the basis of pre-consolidated tailored laminates based on uni-directional plies. However, to exploit the full potential of thermoplastic composites models are to be developed in order to predict the process feasibility and product performance in an early stage of development. This paper addresses a number of steps to increase the accuracy of stamp forming simulations and highlights promising results for identifying intra-ply shear and tool-ply behavior of thermoplastic composites. A comparison of the forming behavior of a doubly curved reference part with simulations will be presented. The paper concludes with remarks on necessary future researc

Forming of UD fibre reinforced thermoplastics

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.\ud 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.\ud 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.\ud 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.\ud 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.\ud 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

Shear characterisation of uni-directional fibre reinforced thermoplastic melts by means of torsion

Intra-ply shear appears during the forming process of hot thermoplastic laminates with a uni-directional fibre reinforcement. This paper proposes a torsion bar test to characterise the longitudinal shear mechanism, which can be performed with a standard rheometer. Sensitivity analyses showed that most reliable shear property measurements can be obtained by using torsion bar specimens with a close to square cross section. The method is implemented in practise and critically evaluated. Storage and loss moduli were determined for carbon UD/PEEK specimens at high temperatures. Non-linear material behaviour was found for relatively small shear strains. The linear regime was focussed on subsequently, where the characteristics were found to be similar to that of a visco-elastic solid or weak gel, confirmed by a dominant storage modulus and a weak frequency dependency. Future work is recommended to be focussed on the large strain regime, for which this paper provides a found basi

Constitutive modelling of UD reinforced thermoplastic laminates

Intra-ply shear is an important mechanism in thermoforming processes of UD fibre reinforced thermoplastic laminates. Various methods have been developed to characterise this shear mechanism, but measured properties differ for several orders of magnitude. The potential of another technique is shown in this paper. This approach considers a strip with a rectangular cross section, subjected to torsion. Several analyses have been carried out to determine the significant material properties for this loading mechanism. For clarity, isotropic and orthotropic elastic material behaviour was considered first. The response torque mainly depends on the in-plane shear modulus G12. An extension was made to the analysis of anisotropic viscous material behaviour. Closed form solutions and FE simulations were compared, using a fibre reinforced viscous fluid model. The torsional response of a partially clamped strip was highly influenced by the fibre stiffnesses. Nevertheless, it is possible to relate the longitudinal viscosity to a closed form solution directly, if bulging is not suppressed and constant through the length of the strip. A smart clamping design is therefore required