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Publication Access status: Open Access , Auslegung eines Hybridverbunds aus UD-faserverstärktem, duroplastischem und kurzfaserverstärktem, thermoplastischem Kunststoff am Beispiel eines hoch beanspruchten Fahrwerkbauteils(2026-06-24)The development of lightweight structures is a complex and interdisciplinary process that inte-grates the fields of design, materials science, and manufacturing engineering. Increasing re-quirements due to e-mobility influence the subcomponents of the chassis directly - higher loads and demands for such lightweight structures with unchanged cost pressure. Hybrid structures were proven to be effective within this context. In the state of the art, different metal-plastic hybrids are presented. The objective of this work is to further advance such hybrid systems by substituting steel with a unidirectional glass-fiber-reinforced thermoset (UD-GFRP). Owing to its high specific strain energy and simultaneously high strength, UD-GFRP is well suited for load-path-optimized structural components. How-ever, the introduction of load transfer elements into such structures is both complex and often accompanied by a reduction in structural performance. By combining the continuously fiber-reinforced thermoset carrier structure with thermoplastic injection-molded material to form a hybrid component, these disadvantages can be largely mitigated. Nevertheless, research gaps remain with respect to the interfacial adhesion of thermoplastic–thermoset hybrids and the influence of surface pretreatment methods. The combination of dissimilar materials in a hybrid structure inherently leads to residual stresses due to differences between the curing or bonding temperature 𝑇𝐻 and the operating temperature 𝑇𝐸. These residual stresses may result in a reduction of the overall bond strength within the hybrid system. For thermoset–thermoplastic hybrids, thermally induced residual stresses have so far only been insufficiently investigated. The intrinsic hybrid structure developed in this work – consisting of a thermoset serving as a stiffening element and a thermoplastic serving as a load introduction element – is designed for application in highly loaded components. The development focuses on three main aspects: First, the bond between thermoset and thermoplastic is examined with particular attention to the adhesion mechanisms and potential methods for their improvement. The second part of the study investigates the residual stresses arising during the thermal manufacturing process are investigated, which result from the mismatch in the coefficients of thermal expansion of the constituent materials. In the final part, the interaction within the hybrid composite is examined using an application-oriented demonstrator. Specifically, it is assessed whether residual stresses can be relieved and how this affects the demonstrator’s service life. Based on the findings, a recommendation for an optimal interface between the thermoplastic and thermoset materials will be provided. Finally, a recommendation is derived for an optimized interface de-sign between thermoplastic and thermoset materials.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Werkstoffsysteme für den Fahrzeugleichtbau im Institut für WerkstofftechnikVolume number:65 7 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Zur Bestimmung des Kriechverhaltens martensitischer Federstähle im Niedrigtemperaturbereich(2024)The global transformation process, which consists of the transition from conventional drives, i.e. internal combustion engine, to battery electric vehicles, is forcing the automotive and automotive supply industries to undergo a significant change. However, the CO2 targets for 2030 cannot be achieved solely by expanding electromobility. This also requires an optimization of existing components. The reduction of CO2 emissions can be attained by reducing friction losses in the valve train. The thermal loads up to 423 K within these valve trains demand a high level of creep or relaxation resistance from the high-strength spring steels. Within the scope of this dissertation, the two existing theories for describing the creep behavior of spring steels in the low-temperature range will be verified and the creep rate-controlling mechanism of dislocation glide will be examined. For this purpose, creep tests with valve spring wires under tensile and torsional loading as well as incremental creep tests, i.e. stress-step or temperature-step tests, will be conducted. Finally, it will be reviewed whether the creep behavior of valve spring wires under biaxial loading can be predicted by the knowledge about the creep behavior of martensitic spring steels under uniaxial loading. This will be done by means of numerical methods, which enables the prediction of the creep behavior of helical compression or valve springs in future.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Werkstoffsysteme für den Fahrzeugleichtbau im Institut für WerkstofftechnikVolume number:53 7 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , An investigation of low temperature creep controlling mechanisms in a martensitic spring steel(2023)The rising demand for electric vehicles urges automotive suppliers to push their limits on the sag resistance of suspension coil springs. However, the mechanisms contributing to sag loss in a coil spring or low temperature creep (LTC) in martensitic spring steel (MSS) are still unclear. Furthermore, the LTC rate-controlling mechanisms in MSS remain elusive. In the current research, an attempt has been made to unfold the LTC rate-controlling mechanisms in SAE 9254 steel grade. First and foremost, the LTC mechanism-based models, i.e. strain hardening theory (SHT), and exhaustion creep model (ECM) are developed to make them capable of describing the stress σ and temperature T dependent LTC strain of a material. Subsequently, the modified SHT and reworked ECM are verified and validated at the example of SAE 9254. The temperature-dependent LTC behavior of inductively quenched and tempered (IQT) SAE 9254 and martempered SAE 9254 are studied at the temperatures 298 K ≤ T ≤ 353 K for a duration of 1 hr at each condition. The stress σ dependent LTC behavior of IQT SAE 9254 is studied at the stresses 1071 MPa ≤ σ ≤ 1634 MPa, and the stress σ dependent LTC behavior of martempered SAE 9254 is studied at the stresses 421 MPa ≤ σ ≤ 632 MPa. A combined analysis by means of (i) the reworked ECM, and (ii) advanced microstructural characterization prior to and post LTC suggests that dislocation glide, mainly localized in the metastable retained austenite phase, is one of the basic LTC contributing mechanisms in SAE 9254. Stress-assisted martensitic transformation (SAMT) and, at elevated temperatures, strain-induced martensitic transformation (SIMT) are the additional LTC contributing mechanisms. Furthermore, the considered approach suggests that the LTC rate-controlling mechanisms are stress-assisted recovery (SAR), SAMT, and, at elevated temperatures, SIMT.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Werkstoffsysteme für den Fahrzeugleichtbau im Institut für WerkstofftechnikVolume number:45 8 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Einfluss der thermischen Eigenspannungen auf die statische und zyklische Festigkeit eines Hybridverbunds aus GFK und Stahl(2022)Material compounds are becoming more and more important in the field of lightweight construction. In material compounds, different materials are combined in order to exploit positive synergy effects regarding mechanical properties. To increase the multi-axial strength, fiber-reinforced plastics (FRP) are reinforced with metal in a hybrid composite. These two materials are bonded in a thermally-activated manufacturing process. The different thermal expansion coefficients of the materials and a difference between application and curing temperature result in residual stresses within the hybrid composite. It is assumed that the residual stresses have a negative influence on the component strength. The hypothetical assumption that there is a negative influence should be verified and quantified. For this purpose, hybrid composites, which have a layered structure of FRP and metal, are examined within this work. Residual stresses act as shear stress in the boundary layer area and as well as normal stresses in the single layers. They are initially determined based on models for different laminate structures. X-ray stress analyses of the metallic components of the hybrid laminate validates the theoretical determination. The residual stress analysis is followed by the characterization of the shear strength of the boundary layer areas. By using two different test methods, the short bending test and the edge shear test, the influence of the residual shear stresses on the shear strength is analyzed. Furthermore, the influence of the residual stress on the strength of the single layers, especially orthogonal to the fiber direction in the FRP, is shown. The determined strength values for the boundary layer area and the single layers are finally used to design a component of a hybrid FRP leaf spring. In comparison to a pure FRP leaf spring, the influence of the hybridization and the resulting residual stresses on the strength is analyzed using the example of the hybrid leaf spring. In this context, the guidelines for the design of hybrid composites with respect to the residual stresses are presented.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Werkstoffsysteme für den Fahrzeugleichtbau im Institut für WerkstofftechnikVolume number:32 12
