Systematic identification of damage mechanisms of short fibre reinforced thermoplastics under fatigue loading and development of a method for time efficient determination of the high cycle fatigue strength
Final Report Abstract
The present work focuses on the fatigue characterisation of short fibre reinforced thermoplastics (SFRT) and aims to explore time-saving experimental methods to evaluate fatigue behaviour. The anisotropic behaviour of SFRT and various factors influencing the mechanical properties require extensive investigation. To reduce the effort and improve the design process, modern computer techniques are used for data processing and virtual testing. Three different approaches are investigated: analysis of fatigue test data, acoustic emission testing and virtual testing by simulating a digital twin. A uniform specimen geometry is optimised to meet the requirements of the different test methods. Specimen optimisation is achieved by using LS-OPT® software to enable additional high resolution X-ray microscopy and AE testing. Initially, cyclic tensile tests are performed to generate S-N diagrams. Specimens that do not fail before 107 cycles are not included in the S-N curve estimation and are examined for incipient damage using X-ray microscopy. Further tensile-tensile fatigue tests in the HCF and VHCF range provide machine data to determine stiffness degradation and dissipated energy. Stiffness degradation is described by two logarithmic curves, with faster stiffness degradation in the first few cycles. Test times can be significantly reduced by testing up to 105 cycles. The rate of stiffness degradation is analysed as a function of the applied maximum stress and characteristic stress levels are identified with which an estimation of the ranges "low cycle fatigue" (LCF), "high cycle fatigue" (HCF) and "very high cycle fatigue" (VHCF) can be carried out. The hysteresis studies show a large scatter of the dissipative energy. The mean dissipative energy is analysed as a function of the applied maximum stress and shows the same trend as the reduction in stiffness. The dissipative energy also allows the determination of the characteristic stress levels. In the second part of the experiments, quasi-static tensile tests with acoustic emission analysis are performed. The acoustic signals are studied in residual strength tests to analyse the damage behaviour in the VHCF region. It is found that SFRTs do not exhibit a fatigue limit in the conventional sense. However, analysis of the acoustic emission results provides evidence of a fatigue limit at which damage increases rapidly. In summary, the investigations carried out illustrate the high effort required for a comprehensive fatigue characterisation of SFRTs. The presented time-saving methods, based on cyclic tensile-tension tests, acoustic emission tests and virtual testing by simulation of a digital twin, show promising approaches to efficiently evaluate the fatigue behaviour of SFRT. The results provide valuable information for the design process and performance improvement of SFRT structures.
Publications
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Determination of a Threshold for Initiation of Damage in Short Glass Fiber Reinforced Polyamide 66, 8th International Conference on Fatigue of Composites, Virtual Conference, 23. – 25.06.2021
Krummenacker, J. & Hausmann, J.
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Determination of Fatigue Damage Initiation in Short Fiber-Reinforced Thermoplastic through Acoustic Emission Analysis. Journal of Composites Science, 5(8), 221.
Krummenacker, Janna & Hausmann, Joachim
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Transfer of mechanical properties from specimens to injection molded structures under consideration of local fiber orientation. EUROMAT 2021 - Virtual Conference, 12. – 17.09.2021
Hausmann, J.; Esha, E.; Schmidt, S. & Krummenacker, J.
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Investigations on the high cycle fatigue strength of short glass fiber reinforced polyamide 66, Dissertation, Technische Universität Kaiserslautern, Juni 2022
Krummenacker, J.
