Multi-scale strain energy-based fatigue damage modeling of fibre reinforced polymers

• Applicants: Dr.-Ing. Caroline Lüders; Professor Dr.-Ing. Raimund Rolfes
• Subject Area: Lightweight Construction, Textile Technology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 532766298

Within this project it is investigated which strain energy density parts describe the fatigue process in unidirectional fibre reinforced plastics at different scales. The fibre-matrix-level (micro scale), the homogenized unidirectional composite (meso scale) and the multidirectional laminate (macro scale) are considered. According to the research hypothesis, a combination of the plastic and the tensile elastic part of the strain energy density is a measure of the fatigue process and is directly related to the number of cycles to failure and to the property degradation. If these strain energy density parts are directly calculated based on the scale-specific constitutive material law – this is the assumption – the mean stress effect and multiaxial stress states are inherently considered within the fatigue calculation. Thus, correction functions, commonly used in stress- and strain based fatigue approaches, would no longer be necessary and the experimental effort for model calibration would be significantly reduced. Knowing the fatigue relevant strain energy density parts furthermore contributes to a deeper understanding of the fatigue process in fibre reinforced composites. The relation between the relevant strain energy density parts and fatigue (number of cycles to failure, degradation) is experimentally identified. The findings are transferred into micro and meso mechanical fatigue damage models. To determine the strain energy density parts within the simulation, constitutive laws are developed that quantitatively describe the elastic, plastic and rate dependent parts of the material behaviour. Material modelling focusses on the epoxy resin matrix at micro scale and on the homogenized, unidirectional glass fibre composite at meso scale. The transfer of the meso mechanical model to the macro scale is demonstrated by its application to multidirectional laminates. Finally, a sequential multi-scale approach links the micro and meso mechanical model by determining calibration data for the meso model by micromechanical analyses. This illustrates the potential of further reduction of the experimental calibration effort.

• DFG Programme: Research Grants