Project LeitHyb (completed 09.2014)

"Lightweight hybrid structure for the chassis of motor homes"

Project content and results

Cellular composites are syntactic foams in which cellular granules are embedded in a plastic matrix. With a suitable combination of granules and matrix material, the mechanical properties of the composite can be significantly improved compared to the same-weight cellular matrix material. Material models exist for predicting the mechanical properties of cellular composites from the properties of the base materials. The application of analytical and numerical homogenization theories in the elastic domain has already been investigated. In addition, a phenomenological material law exists for large distortions and high strain rates. Cellular composites have great lightweight potential as a core material in sandwich structures or as a support material in structures at risk of buckling, and as a core in deformation elements. An application of cellular composites as hollow support cores for reinforcements in frame structures as well as the influence of the connection between support core and sheet shell on stiffness, strength and energy absorption have not been investigated yet.
The aim of the project was to develop and design a hybrid lightweight structure as a chassis for motor homes with significantly improved occupant protection in the rear area. A significant increase in the degree of lightweight construction was achieved by using thin-walled closed higher-strength steel profiles (hat profiles), which are locally stiffened in the stiffness-determining and buckling-endangered areas by hollow support cores made of cellular composite material (glass foam granules in an epoxy resin matrix with a firm bond between the granules and the matrix). The thin-walled profiles, in combination with the cellular composite, enable improved stiffness behavior and an increase in critical buckling loads while reducing weight. In addition, improved occupant protection could also be developed in the rear passenger area through a hybrid deformation structure using a cellular composite with polyurethane matrix.

The design process was carried out according to the methodology of lightweight design, following VDI guidelines 2221 and 2222, with consistent inclusion of system analysis and synthesis as well as material, form and manufacturing lightweight design within the framework of lightweight system design. In order to achieve the greatest possible functional integration (including stiffness, strength and energy absorption), the superstructure floor was used to contribute to the stiffness and strength of the overall structure by means of a fixed connection with the steel sections. Using high-strength steels (DOCOL 800 DP), the wall thicknesses could be lowered to the point where buckling occurs as the first mode of failure, while taking advantage of the available design space. This loss of stability can be shifted to higher stress values by local integration of specific lightweight cellular composites with EP matrix. Linear and nonlinear FEM analyses (whole vehicle and cutaway models) and a servo-hydraulic test facility (quasi-static loading of the frame structures) were used to verify the stiffness, strength and increase in light weight. A weight reduction of 29% (50 kg) was achieved in the frame-floor area while maintaining the same static stiffness and strength.

The cyclic mechanical stresses occurring during driving require an operational strength verification and thus the investigation of the fatigue behavior of the cellular composites as well as the hybrid structures. For this purpose, the cellular composite material was characterized in terms of static and cyclic material behavior at room temperature. For the modeling of the fatigue behavior, uniaxial continuum mechanical damage models for static and cyclic damage were developed and verified on the basis of experimental results. This allows material Woehler curves and service life curves for variable stress-time functions to be generated with a significantly reduced experimental effort.

For reasons of economy, cellular composites with foam granules of recycled glass (approx. 0.6 €/kg, density = 0.2-0.3 kg/l) and EP matrix have been used for the frame structure. The support cores were manufactured using a low-pressure casting process suitable for series production with epoxy resin, which enable a firm bond with the foam granules. The prefabricated support cores are integrated into the steel structures by means of adhesive bonding.

The high specific energy absorption capacity and high energy absorption efficiency of hybrid structures with higher-strength hollow steel sections made of DP 600 and support cores made of cellular composites with PU matrix contribute significantly to improving the crash performance of the overall structure. For occupant protection, the applicable construction and testing regulations (EC Directive 70/156 EEC and 2007/46 EC) only provide test criteria for a quasi-static tensile test on the seat frame. In order to significantly increase occupant protection, a suitable crash equivalent load case for the lateral impact of a passenger car in the area of the rear seats was defined and numerically investigated in accordance with the ECE-R 95 directive. With the aid of an FEM overall vehicle model, the hybrid deformation structure developed was able to demonstrate a reduction in the penetration depth of the crash barrier to a quarter of the value of the reference vehicle without deformation structure.

The results obtained in the LeitHyb project for hybrid structures made of high-strength steel sheets and cellular composites show a high lightweight potential in terms of stiffness and strength of the frame-floor structure. In addition, the hybrid structures considered are very well suited as deformation structures for side impacts. In this project, it was possible to gain fundamental insights into the material and wall thickness combinations for which major lightweight construction advantages can be realized. In addition, experience was gained on primary forming, forming and joining techniques in the production of hybrid lightweight profiles and the frame-floor structure. Tools for the development and production of hybrid structures with cellular composites as well as a prototype of a lightweight chassis segment dimensioned with respect to stiffness, static and cyclic strength and a crash load case could be provided. The technologies developed can also be transferred to other vehicle concepts, such as vans and passenger cars.