On the impact of pre-deformation on the fatigue performance of metallic laminates

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Due to their ability to combine opposing material properties, modern metallic multi-layer materials are able to outperform their monolithic counterparts. As a result, these metallic laminates are used in a variety of applications and have been in focus of various research studies for many years. Current progress in processing now allows to use hot-rolled metallic laminates in the automotive industry. The cost-effective sheet materials suitable for mass production with specifically adjustable, graded properties are very promising for numerous applications in this field. In order to ensure a reliable use of such materials, a profound understanding of their fatigue properties is needed. As components made from hot-rolled laminates are produced by forming and, thus, are subjected to pre-deformations during processing, the influence of pre-deformation is of particular importance in this context. However, analysis of the current state-of-the-art reveals that no systematic investigations on the fatigue properties of layered composites are available and that the influence of monotonic pre-deformations on the fatigue performance has not been addressed so far. Well-known factors significantly influencing the fatigue behavior are defect density and the residual stress state of a component. Regarding formation of residual stresses in roll-laminated composite materials and their evolution upon pre-deformation and under cyclic loading no comprehensive data can be found in open literature. In light of this lack of data the present project was designed. The fatigue behavior of a two-layer composite produced by hot rolling will be thoroughly investigated. The role of the individual layers of the composite as well as the boundary layer will be in focus. With respect to the envisaged fields of application of the two-layer composite material, the effects of pre-deformation on the microstructure development, defect density and distribution, the residual stress state and the fatigue behavior of the material will be investigated and an adequate model allowing to capture relevant relationships will be proposed. The analysis methods used in the project will allow for the evaluation of the influence of the surface and boundary layer on the fatigue behavior of the two-layer laminate material. In addition, the influence of targeted pre-deformations is investigated and modelled, thus, providing insights into whether the resulting residual stress states, defect densities and microstructural changes lead to superior or inferior fatigue performance.