Friction in Textile Thermoplastic Composites Forming

A previously developed mesoscopic friction model for glass/PP textile composite laminates during forming is evaluated for glass and carbon/PPS laminates, at higher temperatures and lower viscosities than before. Experiments were performed for tool/ply and ply/ply configurations in a new friction test set-up. The experimental results indicate that this contact is indeed in the hydrodynamic regime. The model results are sensitive to inaccuracies in the geometric representation of the textile structure, whereas the experimental results are sensitive to even slight misalignments. INTRODUCTION Textile reinforced thermoplastics offer a cost reduction compared to their thermoset counterparts due to promising fast production methods such as diaphragm forming and press forming, usually starting from pre-consolidated laminates. Friction plays an important role in these forming processes [1]. The constraints imposed by friction between subsequent plies and between the laminate and the tools are a major factor in the laminate deformations (such as wrinkling and folding) generated during composite forming. This friction depends on the forming process parameters such as the pressure, the temperature and the sliding velocity. In addition, it depends on the material properties of the fibres and the resin, the fibre distribution and the reinforcement architecture, as for any composite property. A model was developed to describe the frictional phenomena in the general case, taking into account the effects of the various governing parameters [2]. While previous studies have resulted in empirical models [3,4,5,6,7] or report experimentally obtained values [8], this model predicts the friction between thermoplastic laminates and a steel tool by assuming hydrodynamic lubrication on a meso-mechanical level. Thus, the frictional properties can be calculated, solely based on the rheological properties of the matrix constituent and the fabric weave geometry. The model was validated experimentally with a novel pull-through friction tester, in which a laminate is pulled at constant velocity, while clamped by two stiff blocks at C. Binetruy, F. Boussu (Eds.) Recent Advances in Textile Composites, 2010 p.271-279 processing temperature. Force displacement diagrams are generated from these pullthrough experiments (Fig. 1). These diagrams typically show an overshoot after which (in general) the friction force attains a steady state value. The steady state forces can be used to determine a friction coefficient, which usually increases with velocity and decreases with temperature and pressure. The model predictions in general agree well with our experimental results. Our results on glass-PP Twintex fabrics (around 200oC) compare well with Harrison et al [9], but opposite trends were found by Lebrun et al [8]. This has been the main reason for starting a benchmark exercise on this particular topic, initiated at the Esaform 2010 conference. Further measurements were performed on glass/PPS and on carbon/PPS Cetex laminates (around 300oC). The lower viscosity leads to lower predicted film thicknesses, approaching the limits of hydrodynamic lubrication. Here, we will discuss the results and highlight the limitations of our approach. FRICTION MODEL A meso-scale model was developed [9, 10] based on a geometrical description of the tows within the fabric. One of the advantages of the model is that the film thickness can be predicted from the normal pressure and velocity. This avoids the use of some arbitrary thickness of this lubrication film. Figure 2 presents a schematic cross section of the composite material. Hydrodynamic lubrication is assumed between the bundles and the tool surface. The total friction force per unit width follows by integrating the surface shear stresses over the length of the cross section, disregarding the bundle curvatures out of the plane of this cross section for the time being. The contributions of the longitudinal warp and transverse weft yarns can be analysed separately and added up to the total friction force. Figure 2. Schematic cross section of a 2x2 Twill ply on a tool surface. Fn Ff , U pull out length [mm] Fo rc e [N ]