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The 3D skeleton winding (3DSW) technology consists of winding a fiber skeleton and thermoplastic injection overmolding of the fiber skeleton.  

The winding process uses thermoplastic hybrid yarns, which are first heated above the melting temperature of the thermoplastic and then compressed in a nozzle for impregnation. Subsequently, the molten and impregnated hybrid yarns are wound onto a winding tool using a robot to form the fiber skeleton.  

The fiber skeleton is usually wound around load introduction and support points, which are mostly implemented as metallic inserts. Based on this, the fiber orientation is ideally aligned with the load paths, yielding a high utilization of the paramount weight-specific mechanical properties of the continuous fibers. 

The winding process has proven to be suitable for both carbon and glass fibers as well as a wide range of thermoplastics, including high-temperature thermoplastics. 

Finally, the fiber skeleton is overmolded using thermoplastics injection molding. The overmolding structure supports and protects the fiber skeleton and enables load transmission. Using topology-optimized overmolding structures enables in combination with an overall low amount of fibers cost-efficient lightweight structures. 

Schematic illustration of the 3D skeleton winding (3DSW) technology [1]. 

PA6 being an engineering thermoplastic was chosen within the ACCORD project as material. Hybrid yarns with 63 wt.% glass fibers are used for the fiber skeleton. In contrast, a varying fiber concentration is considered for injection overmolding, ranging from the neat thermoplastic over 20 wt.% to 40 wt.% glass fibers.  

Materials characterization is conducted for the injection molding material under the variation of fiber concentration, moisture concentration, and fiber alignment. Tensile tests are conducted according to EN ISO 527 and the specimens are extracted in different orientations from injection molded plaques with sufficient flow length to ensure fiber alignment. 

The experimental results reveal a significant increase in mechanical properties for stiffness and strength with increasing fiber concentration and fiber alignment. This, however, goes along with a decrease in ductility. Besides this, the mechanical properties are significantly influenced by moisture, where a significant decrease of stiffness and strength but an increase of ductility is observed for the conditioned material state over the dried material state.

Materials characterization: Variation of fiber concentration (0°, dried). 

Materials characterization: Variation of moisture concentration (0°, PA6/GF20) 

Materials characterization: Variation of fiber orientation (PA6/GF20, dried) 

Schematic of the testing plaque and specimen orientation

The Abaqus built-in multi-scale material modeling approach has been adopted in addition to isotropic material modeling to capture anisotropy. It is observed that the approach is suitable for a material state with limited ductility. However, the modeling approach is not capable of capturing the anisotropy for more ductile behavior, as observed for the conditioned material state. Moreover, the multiscale approach requires a high computational effort due to the homogenization at runtime. Therefore, isotropic material modeling with low fiber alignment is preferred and used for conservative but efficient virtual design.

A virtual optimization and validation approach for 3DSW structures has been developed by Simutence. This approach uses Abaqus exclusively with built-in techniques and comprises two steps. 

The first step concerns the fiber skeleton. Initially, the design space, the metallic insert positions, boundary conditions, and load requirements are defined in a preprocessing step. Subsequently, a topology optimization is conducted to identify the load paths. Finally, the fiber skeleton is defined in an engineering step, considering the load paths as well as manufacturing constraints.

The second step concerns the overmolding structure and is an iterative procedure consisting of topology optimization and Finite Element Analysis (FEA). In each iteration, a topology optimization is used to reduce the component mass on an algorithmic basis. The result is then transferred to a CAD design and load requirements are validated using FEA. These two steps are repeated iteratively to successively reduce the component mass while ensuring that the load requirements are fulfilled.

The demonstrator part of the Korean German ZIM project ACCORD is a rear trailing arm for a Hyundai Santa Fe. The original part is a metallic component, which consists of 9 welded parts. The 3DSW rear trailing arm developed in the ACCORD is a single integral part and reduces the component mass by up to 37 %, depending on the choice of material grade.

The dimensions are 650 x 280 x 180 mm³.  

The fiber skeleton has been manufactured using the winding cell outlined established at Fraunhofer ICT. The overmolding structure is cast with epoxy for demonstration purposes, due to the project’s focus on the winding process development. Therefore, the load requirements are validated virtually only.

Photograph of the 3DSW rear trailing arm demonstrator

CAD of the 3DSW rear trailing arm assembly

The virtual optimization and validation approach developed by Simutence is used to design the 3DSW rear trailing arm. Once the fiber skeleton is defined, the starting point for optimization of the overmolding structure is a conservative design, which is high in mass but fulfills all load requirements. With each design iteration, the component mass is reduced, until a mass reduction of up to 37 % is achieved in the final iteration.

In each design iteration, an FEA is conducted to ensure the fulfillment of load requirements. On the one hand, it is ensured that at least a force of 32 kN is achieved in longitudinal compression, which is the load requirement defined by the OEM. It is observed that a fiber concentration of 20 wt.% outperforms the metallic reference. On the other hand, it is ensured that there is no failure in the fiber skeleton and the overmolding structure.

FEA results of the final design: Failure in the fiber skeleton

FEA results of the final design: Van Mises stress in the overmolding structure