Background

Increasing requirements due to e-mobility influence the subcomponents of the chassis directly - higher loads [1] and demands for lightweight construction with unchanged cost pressure. Hybrid structures proved to be effective within this context [2]. In the state of the art, different metal-plastic hybrids are already presented, such as combining metal with short fiber reinforced plastic (FRP) or with FRP with continuous fiber reinforcement as Fiber-Metal-Laminate (FML) [3]. A next step in the further development of hybrids is the substitution of steel with continuous fiber-reinforced thermosets. The intrinsic hybrid composite of thermoset as a stiffening structure and thermoplastic as a load introduction element is developed for application in components subject to high cyclic loads. Especially for chassis parts the substitution of steel leads to weight savings and creates potential for CO2 savings. Furthermore, no coatings are required for corrosion protection and value chains can be designed more deeply. Among lightweight construction, FRP-thermoplastic hybrid composites allow more freedom in design compared to metal-plastic hybrids. Load paths can be optimized by introducing the fibers of the FRP in a targeted manner. The thermoplastic in the hybrid composite allows targeted functional integration [4, 5].

Especially for the thermoset-thermoplastic hybrid composite without an additional interlayer, failure is predicted to occur in the boundary layer region [7, 8]. To our knowledge only a few studies investigate the possible adhesion improvements in thermoset-thermoplastic hybrids produced by means of injection moulding [7-12]. Current research considers different adhesion mechanisms, such as peel ply and sandblasting [9], chemical coupling layers [10], plasma treatments [7, 11], and laser treatments [8, 12]. The adhesion is evaluated by the achievable tensile shear strengths. In contrast to our work, carbon fiber reinforced plastic (CFRP) is used instead of glass fiber reinforced plastic (GFRP). Plasma and laser treatments result in a shear strength up to 15.1 MPa [7, 8, 11, 12], peel ply and sandblasting 6 MPa [9]. Studies describe nominal stress values to characterize the robustness of a boundary layer. However, two issues emerge: (1) Local stress peaks are disregarded, and (2) residual stresses are omitted from analyses. Consequently, documented stress values are inadequate for component dimensioning and only suitable for qualitative comparisons of material combinations or technologies.

This work focusses on the development and the characterization of the bonding between thermoset and thermoplastic with the focus on the residual stresses which are implemented during the thermal manufacturing process due to different coefficients of thermal expansion of the materials and the significant influence of the lifetime of components [6]. Furthermore, the interaction between adhesion and residual stresses in the hybrid composite is explored. The objective is to investigate and assess the occurrence of residual stresses in thermoset-thermoplastic hybrids, with the aim of providing a foundation for application-specific dimensioning that incorporates the effects of residual stresses.

The hybrid composite analyzed in this study consists of thermoset GFRP and thermoplastic material. The thermoset GFRP is characterized by continuous unidirectional glass fiber reinforcement, comprising approximately 79% of the total fiber weight content (equivalent to around 66% volume content). As thermoplastic, polyamide 6.6 with 30 % short glass fiber reinforcement is used (PA 6.6 GF30).

Initially, suitable test specimens are designed for the buildup of residual stress. Subsequently, the test specimens are tested for adhesion strength. Next, measurements of residual stresses in the specimens are made with the help of Fiber-Bragg-Grating sensors. The results are further validated by calculational and computational simulated determination of residual stresses.

Results

Results show a well-fitting behaviour between calculational, computational simulated and measured normal residual stresses σ_RS in the centre of the specimen. Furthermore, the plausibility of the computational simulated results is confirmed, allowing the derivation of normal residual stresses perpendicular to the fiber direction. However, the integration of an FBG perpendicular to the fiber direction is not feasible in the conventional UD pultrusion process to experimentally verify these values.

Concerning the computational simulated determination of the shear stresses τ_RS, it is not possible to measure the resulting curves. The well-matching curves of calculational and computational simulated normal residual stresses allow the conclusion to be drawn that computational simulated τ_RSis at the correct level. In particular, the shear stresses influence the failure of the hybrid composite by preloading the interface and predicting the composite failure due to adhesion.

Conclusion

We conclude that thermal joining processes can cause significant residual stresses in thermoset-thermoplastic-hybrids, which can preload the bond and lead to a reduction in strength. The prerequisite is adhesion between the composite partners. Furthermore, the level of normal residual stresses in intrinsically bonded thermoset-thermoplastic-hybrids can be estimated calculational - normal stresses using Hook's law for the plane stress state. If a semi-crystalline thermoplastic is used, the corresponding crystallization temperature must be assumed as the stress-free temperature.

References

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