TopComposite – Almost tension-free placement of in-line impregnated fiber bundles in radii

Fiber-reinforced plastics (FRP) are ideal for lightweight applications. However, geometric limitations in established manufacturing processes (e.g. winding or pultrusion) or the need for expensive semi-finished products (e.g. prepregs) often stand in the way of a wider use of FRP.

In order to overcome these limitations, a novel manufacturing process for FRP components called wet fiber placement (Fig. 1) is being developed within the junior research group “TopComposite” at Leibniz-Institut für Verbundwerkstoffe (IVW). In this process, dry fiber bundles (so-called “rovings”) are in-line impregnated with a thermoset resin system and then directly deposited on corresponding tools. Finally, the component is cured in a press process by applying temperature and pressure. The in-line impregnation eliminates the need for expensive semi-finished products. Due to the almost tension free direct deposition, the fiber bundles can be placed in curved paths or radii, which makes it possible to realize even complex-shaped parts with a load-adapted fiber orientation and minimzed material usage.

However, various defects can occur when fiber are deposited in radii. Due to the difference in length between the outer and inner fibers during radius placement, the fibers on the inside are compressed while the fibers on the outside are subjected to tensile loads. This typically causes the formation of fiber waviness on the inside (Fig. 2, left), to the up folding of the outer fibers (Fig. 2, center) or, in the case of small radii, even to a complete twist of the fiber bundles (Fig. 2, right). In order to assess the extent to which these defects are permissible and to take them into account in component design (e.g. in topology optimization), their characteristics and their influence on the mechanical properties have to be known. In order to investigate the influence of the placement radius and the roving titer on the occurring defects, radii between 5 mm and 1000 mm are placed with a 24K and a 50K carbon fiber roving (24,000 and 50,000 individual filaments, respectively) (Fig. 3). The placed radii were cured in an oven and visually inspected with respect to their defects. Furthermore, photographic images of the rovings were taken and evaluated by Python routines to make quantifiable statements about the extent of the defects (Fig. 4). It was found that – as in related processes (e.g. Automated Fiber Placement) – the defects increase with a decreasing radius and they defects are higher when using a larger roving titer.

Based on those investigations, test specimen with different radii will be produced and mechanically tested as a next step in order to determine the influence of the defects on the mechanical material properties. Subsequently, methods are developed to consider the influence of the defects depending on the placement radius directly in component design, e.g. in topology optimization.

The project "TopComposite – Topology-optimized and resource-efficient composites for mobility and transport" is funded by the Federal Ministry of Education and Research (funding reference 03XP0259).

Kontakt:

Dipl.-Ing. Maximilian Eckrich
Nachwuchsforschungsgruppe TopComposite
Leibniz-Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Straße 58
67663 Kaiserslautern
Phone: +49 631 2017-228
E-Mail: maximilian.eckrich@ivw.uni-kl.de