Unidirectional glass fiber reinforced plastic (UD-GFRP) represents itself as an excellent lightweight spring material due to its high strength R_m, a moderate Youngs modulus E and its low density ρ. A major drawback, however, is the relative weakness of the matrix under more complex loading conditions leading to a rather multi-axial state of stress. Therefore, a local hybridization of the UD-GFRP by use of a more isotropic material, e.g. a high strength steel, seems to be an adequate measure to ensure the robustness of the lightweight spring material.
Multi-layer laminates of UD-GFRP and high strength steel serve as intrinsic hybrid specimens. The material testing program sets the focus on the investigation of the strength of the interface between UD-GFRP and steel. Material tests, like double cantilever beam tests (mode I) and shear tests (mode II) are conducted in order to measure the energy release rate G_I and the shear strength τ_S of the considered interface at room temperature RT and at an elevated temperature T=80°C. Several methods of surface treatments of the steel layers have been characterized by this approach. The results show up that primers, such as silane and titanium dioxide, are leading to a good adhesion between the constituents. However, the variation within the data is still significantly higher than for other surface treatment methods. Furthermore, a fiber bridging effect is apparent which leads to an increase of the energy release rate G_I with the growth of the length a of the crack. A power law is well suited to describe this strengthening and its exponent n serves as a further measure to describe the strength of the interface.
Ageing of the material and testing at an elevated temperature may compromise the performance of the intrinsic hybrid material as a new lightweight spring material. Hence, all material tests are conducted by use of intrinsic hybrid specimens in the as manufactured state as well as in an aged state where the ageing has been done by exposing the intrinsic hybrid laminates to water at a temperature T=80°C for one week.