Characterization and modeling of load- and cycle-dependent residual stress development and damage evolution of electrically conductive fiber metal laminates with process-based residual stresses

• Applicants: Professor Dr.-Ing. Joachim Hausmann; Professor Dr.-Ing. Frank Walther
• Subject Area: Polymeric and Biogenic Materials and Derived Composites
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541803764

The objective of this project is the quantitative determination of the residual stress influence on the fatigue behavior of fiber metal laminates consisting of steel and carbon fiber reinforced polymer. For this purpose, defined residual stress states are set in specimens by the manufacturing process or mechanical post-treatment in order to correlate the different residual stress states with the mechanical properties. The development of residual stresses is logged during the fatigue process by special metrological instrumentation for the in situ acquisition of characteristic material values and is interpreted for the accompanying development of failure models to predict fatigue life as a function of residual stresses. Influencing factors considered are the type of residual stress (tensile/compressive) and magnitude, as well as the cyclic tension-tension stress prevailing during loading, and ambient temperature. The model development is accompanied by continuous experimental validation, both in terms of calculation, influence, measurement and development of residual stresses, and in terms of prediction and experimental determination of damage mechanisms and fatigue life. The objective is to obtain an advanced process-structure-properties relationship understanding necessary for tailored design of fiber metal laminates. In this regard, the following research hypotheses arise from the current state of the art and preliminary work: • Thermal residual stresses develop depending on the curing temperature. • Thermal residual stresses influence fatigue life and damage behavior. • Thermal residual stresses can change during cyclic loading. • Creep and relaxation of the polymer matrix play a minor role with respect to residual stress development in the fiber direction due to the fibers and the metal. The following research objectives were derived: • Producing fiber metal laminates with different thermal residual stress states using modified manufacturing processes and mechanical post-treatments while ensuring comparable matrix curing levels and properties. • Thermal residual stresses in the initial state and their progression during cyclic loading can be measured and described by models. • Prediction of the damage behavior is made possible by models depending on thermal residual stresses and their development. • An analytical description provides the relationship between thermal residual stresses and fatigue life. Building on the results of the determined process-structure correlations from the first funding phase, a significantly expanded level of knowledge is expected to be achieved in the second funding phase. This includes findings on materially and structurally different fiber metal laminates as well as the characterization of the fatigue behavior up to the range of very high cycle fatigue so that a basis for practical, optimized laminate production can be derived.

• DFG Programme: Research Grants