# Design of Fiber-Composite/Metal-Hybrid Structures Made by Multi-Stage Coreless Filament Winding
## 1 Introduction  
Beginning in the 1960s, the adoption of carbon fiber reinforced plastics (CFRPs) in structural engineering was primarily motivated by their superior mass-specific mechanical parameters, which can be tailored to the individual application [1]. While CFRPs incorporate a low thermal expansion [2] together with non-magnetic [3] and non-corrosive properties [4], their drawbacks include limited recyclability [5], separability [6], and often non-destructive quality control [7], as well as a high environmental impact [8]. Coreless filament winding (CFW) is an emerging composite manufacturing process that has not yet reached its full potential for high-performing and efficient structures due to limiting software design tools. In CFW [9; 10; 11; 12], a roving is spanned freely between point-like anchors to additively manufacture a component, such as lattices [13; 14] or shells [15; 16]. The constantly tensioned [17] fiber strand is usually impregnated online [18] with a thermoset resin in a bath or by an end-effector [19]. Alternatively, pre-tows [20] can be deployed. CFW is distinguished from conventional winding techniques [21; 22] in that the component shape can be well described by straight lines connecting several anchor points in a specific sequence. Geometrical deviations in the fiber net can arise through fiber-fiber interaction only, whereas in hybrid forms [23; 24], the fibers are also partially supported by surfaces. The benefit of CFW is decoupling the tooling from the composite structure, which enables cost-effective adaptability of components and leads to minimizing tooling costs while increasing fiber-fiber interaction, which is hard to predict and control. Recent demonstrations of CFW were directed towards building applications [25] and validated design [26], simulation [27], manufacturing [28], testing [29], approval, and monitoring [30] procedures. Where these research-orientated custom solutions are fragmented between different software tools and platforms, widely used proprietary CAD/CAM tools are intended for prismatic geometries in isotropic materials. Software directed towards composite and textile-based characteristics mostly depends on a layer-based design [31], which CFW often does not exhibit. Recently, a first step towards an integrative software tool for CFW was undertaken by the creation of a common data framework, which was validated in a case study [20; 32]. One challenge of designing with CFW is to find the structurally most efficient framework structure in compliance with CFW fabrication characteristics. This is driven by two effects: minor reconfigurations of the fiber net can cause disproportional changes [33] in the mechanical behavior of the structure, and the addition of an additional node exponentially increases the number of possible configurations. The number of all configurations is given by \(n^{(n-2)}\) depending on the number of nodes \(n\)[34]. Although this includes self-similar configurations and ones that do not involve all given nodes, the number is still so large that evaluating all reasonable combinations is currently not economical for technical applications. Another challenge in CFW is the digital capturing of fabrication deviations to feed structural simulations. Due to the geometrical complexity, fiber-resin-, and fiber-fiber interactions, current simulation tools can resolve only simpler components (Figure 1) on the roving level [35]. Larger and more complex structures are calculated based on approximative models, which rely on a correct calibration of material properties, which are generated by full-scale destructive tests [36]. During the design process, less sophisticated structural simulations are sufficient to evaluate fiber net configurations, but the rather linear design process [33] (Figure 1) is still limited by human intuition and experience. In multi-material [37] components, the various materials can take on different tasks, and material parameters can be set depending on the location. Figure 1: Process flow of coreless filament winding.