Residual stresses in non-symmetrical carbon/epoxy laminates

The curvature of unsymmetrical [0/90] laminates moulded from AS4/8552 uni-directional tape has been measured. A linear thermoelastic approach has been applied to predict the related residual stress state before demoulding, giving an estimate of the stress induced by polymerisation strain. The results from the linear approach are confirmed by a viscoelastic finite element model including the cure conversion and related change in viscosity. It is concluded that the curvature measurement of unsymmetrical laminates is an accurate method for the prediction of the transverse residual stress, making it suitable as a benchmark for complex stress modelling

Spring forward of woven fabric reinforced composites

Continuous-fibre-reinforced plastic products are usually formed at elevated temperatures. They exhibit distortions when they are cooled to room temperature and released from the mould. For example, the enclosed angle of an L-shaped product decreases, see Fig. 1. This effect is known as spring-forward. It is mainly due to the anisotropic thermal shrinkage of the composite, which is small in the fibre direction and relatively large in the direction normal to the fibres. The costs of forming a product with the demanded dimensions by trial and error are high. To reduce these costs, the objective of the research described in this paper is to develop a model, which predicts the occurring distortions

Modelling the thermo-elastic properties of skewed woven fabric reinforced composites

Woven fabrics prove to be a very convenient fibre reinforcement when prepreg layers have to be draped on to double curvature moulds. The process of draping causes the angle between the warp and weft yarns to vary over the product with this double curvature. As a result, the thermomechanical properties of the fibre reinforced composite material show a corresponding distribution. These thermo-elastic properties must be known in order to predict the shrinkage\ud and warpage of the product. Normally, composites consist of multiple fabric layers. These layers are oriented and skewed differently, and each contributes to the overall composite properties. Therefore, in order to predict the overall thermo-elastic properties of the composite as a whole, the properties of each individual layer must be known. In this paper, the inplane thermo-elastic\ud properties of a woven fabric reinforced composite with an arbitrary weave type are analysed as a function of the skew angle, using micromechanics. Three different levels of material structure are modelled, the micro-, the meso- and the macro level. The inplane thermo-elastic properties of four different basic elements are determined at the micro level, using geometrical shape functions and a two-dimensional thermo-elastic model. The inplane properties of one fabric layer are determined at the meso level, using the fabric pattern and the properties of the basic elements. At the macro level the homogeneous properties and warpage of woven fabric composites are considered. Here the composite structure and the properties of the individual layer are used. The method proves to be a convenient way to model the skew deformation of the woven fabric composite and the resulting variation in the thermo-elastic properties. The theoretical predictions are verified by experiments on multiple-layered satin 5H woven fabric composites

Warpage of rubber pressed composites

The rubber pressing process is applied for the rapid production of thermoplastic composite products. However, rubber pressed products show geometrical distortions, such as warpage, due to process-induced residual stresses. It is believed that these stresses build up as a result of the large thermal gradients that are present during consolidation. An experimental study is performed to measure the curvature after rubber pressing of initially flat woven fabric glass/PPS composite panels. A material model is proposed that incorporates the solidification of the composite in order to predict the residual stresses and the warpage due to inhomogenous cooling. The model is employed in Finite Element simulations of the rubber pressing process. The simulations are compared to the experimentally obtained curvatures. It shows that inhomogeneous cooling has a minor effect on the warpage in this case, and that another mechanism is present

Spring-forward in composite plate elements

Spring-forward is a distortion of corner sections in continuous fibre reinforced composite products. The linear thermoelastic prediction for the spring-forward of single curved geometries is incorporated in a FE formulation for plate elements in order to simulate the spring-forward of doubly curved products. The proposed methodology is demonstrated with the simulation of the process-induced distortion of a wing leading edge stiffener

Modelling of fabric draping: Finite elements versus a geometrical method

Thermoplastic composite materials can be processed by Rubber Press Forming at elevated temperatures. Process specific boundary conditions are difficult to incorporate in the classical geometric drape simulation methods. Therefore, a fabric reinforced fluid model was implemented in the Finite Element package DIEKA, which is capable of modelling the boundary conditions as well. The model predictions of both types of simulations are compared for a double dome geometry, having separated contact areas, leading to different results