# Design of Fiber-Composite/Metal-Hybrid Structures Made by Multi-Stage Coreless Filament Winding
## 3 Results
### Multi-Stage CFW Process  
For the production, Teijin Tenax-J UMS45 F22 12K [56] carbon fiber with 385 tex at 1.83 g/cm\({}^{3}\) and MGS LR635, MGS LH635, and MGS LH637 [57] epoxy resin by Hexion was used in a 100:10:20 ratio. The fiber was selected because of its comparatively high tensile modulus of 425 GPa. To achieve the highest possible dimensional accuracy of the CFRP component, neither curing the component on the metallic winding fixture nor tempering of the demolded component in an oven was permissible. The aluminum frame could warp due to the high coefficient of thermal expansion, or the CFRP component could distort because of its inhomogeneous material distribution. Consequently, the resin was selected to achieve the highest possible stiffness without temperature treatment. As this resin requires several hours to cure, the time between winding sessions was correspondingly long. An application that allows a fast-curing resin could achieve better cycle times. The utilized winding equipment was state-of-the-art, except for a modular winding fixture (Figure 12a), which was used to mount the winding pins. It helps to shorten preparation times, maintains relatively high geometrical accuracy, and can be reconfigured easily but restricts the position of the winding pin to a predefined pattern. In the first stage, the lower region was completed, and the two outer tension cords in the upper region were fabricated (Figure 12b). After the first curing, the winding fixture was removed, and in  
Figure 11: 2D visualization of the fiber-composite structure of the case study with relevant nodes (1, 10, 20, 27, 42, 44, 59, and 66) marked. (**a**) Distance matrix; (**b**) Resolution matrix; (**c**) Edge bundling diagram of the graph color-coded by regions of the fiber net: (**A**) longer lower outer edge; (**B**) shorter lower outer edge; (**C**) lower center rib (section 10–20); (**D**) lower transverse struts (e.g., section 2–18); (**E**) lower diagonal ribs; (**F**) raised outer edges of corners; (**G**) longer upper outer edge; (**H**) shorter upper outer edge; (**I**) upper center rib (section 10–20); (**J**) lateral cross-bracing; (**K**) diagonal inner struts; (**L**) vertical inner struts. the second stage, the central tension cords in the top and the lateral diagonal struts were wound (Figure 12c). In the third stage, the inner struts were wound (Figure 12d). Finally, the component was mounted onto the aluminum base plate (Figure 12e). After winding a stage, occasionally, rovings within a segment did not form a coherent bundle. This results mainly from deformations of the fiber net during the stage causing tension loss. To establish a force flow between the rovings, these rovings must be locally bound together with a polytetrafluoroethylene yarn. The yarn can be removed after curing the resin. The digital design tool allows predicting the material consumption for each production step (Figure 13). From this, the cross-sectional area of each segment of the fiber net can be estimated and used to feed the framework analysis. The FVR is a determining parameter for this calculation and must be estimated a priori. For this, several interrelationships can be used. One of them is deriving it from the fiber and resin parameters and the cross-sectional area at the winding tool nozzle [33]. After each stage, the actual cross-sectional area of each segment was measured by a caliper (Figure 13). The rectangular area obtained by the width and height of the bundles does not represent the cross-sectional area well. A better representation is given by an ellipse or alternatively by a correction factor of \(\pi\)/4. The difference between the caliper measurements and the prediction results from a local variation in FVR, internal and external voids, and overestimation due to the rugged fiber bundle surface. The standard deviation of the caliper measurements is also predominantly dominated by an irregular division of the fiber bundle into individual bundles. Figure 12: Multi-stage CFW process.