PSI - Issue 38

Kimiya Hemmesi et al. / Procedia Structural Integrity 38 (2022) 401–410 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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fatigue strength can be expected. Note, however, that this hypothesis seems to be reasoned only for the alloy EN AW-6082 demonstrating cyclic hardening behaviour, whereas the steel 42CrMoS4 exhibits cyclic softening. Moreover, it does not explain a decrease in the fatigue strength after OL 2. Another possible explanation of the experimental findings is based on the assumption that overloads may cause local plastic deformations around micro cracks and material defects like non-metallic inclusions. Consequently, crack tip blunting and compressive residual stresses can arise at the microscopic level, thus leading to a retardation of micro crack nucleation and growth. On the other hand, severe overloads like those at OL 2 may evoke considerable material damage, for instance, through breaking non-metallic inclusions, and thus instantaneously cause fatigue crack propagation. To clarify the relevance of the latter damage mechanisms for the materials and loading conditions investigated in this study, detailed fractographic examination of the test samples is in progress. 8. Conclusions A series of fatigue tests on notched specimens made of the 42CrMoS4 steel and the aluminium alloy EN AW-6082 demonstrate a beneficial effect of moderate overloads on the fatigue life. The respective overload level OL 1 is defined as 75 % of the static strength estimated according to the FKM Guideline (2020). For a more severe overload level OL 2 corresponding to 75 % of the experimentally determined static strength, a reduction of the fatigue strength as compared to OL 1 is observed. Thereby, the fatigue strength of 42CrMoS4 at OL 2 remains higher than the respective value with no overloads. In contrast, the fatigue strength of EN AW-6082 considerably decreases due to OL 2. The analysis approaches in FKM (2020) and FKM (2019) provide no consistent explanation of the experimental findings. Neither consideration of residual stresses nor estimates of fatigue damage caused by overloads are able of predicting an increase in the fatigue strength due to OL 1. Moreover, residual stresses alone predict a monotonic decrease of the fatigue strength with increasing overload level, which contradicts to the experimental results. Nonetheless, this study confirms the validity of the limiting criterion in FKM (2020) specifying an admissible overload level of 75 % of the static strength estimated according to that Guideline. As a proper mechanism explaining the experimental observations on specimens subjected to overloads, plastic deformations at the microscopic level are regarded. On the one hand, these may induce both blunting of micro cracks and compressive residual stresses at volumetric material defects, and thus retard the nucleation and growth of micro cracks. On the other hand, severe overloads may cause particle breakage and an instantaneous crack initiation and propagation in the material. Further investigations, including fractographic examinations of the test specimens, are in progress to explore the relevant failure mechanisms. Acknowledgements This study is a part of the research project “Überlasten” performed within the Research Association of Mechanical Engineering (FKM – Forschungskuratorium Maschinenbau e.V.) under the BMWi/AiF Grant no. 20086 BG. The financial support by the Federal Ministry for Economic Affairs and Energy (BMWi) and Industrial Collective Research (IGF) is gratefully acknowledged. References ABAQUS, 2020. Dassault Systèmes SIMULIA Corp. Dixon, W., Mood, A., 1948. A Method for Obtaining and Analyzing Sensitivity Data, Journal of the American Statistical Association 43, No. 241, 109-126. FKM, 2019. Analytical Strength Assessment of Components under Explicit Consideration of Non-Linear Material Behaviour (in German), FKM Guideline, 1st Edition, VDMA Verlag, ISBN 978-3-8163-0729-7. FKM, 2020. Analytical Strength Assessment of Components, FKM Guideline, 7th Edition, VDMAVerlag, ISBN 978-3-8161-0745-7. Hück, M., 1983. Ein verbessertes Verfahren für die Auswertung von Treppenstufenversuchen. Materialwissenschaft und Werkstofftechnik 14, No. 12, 406-417. Lemaitre, J., Chaboche, J.L., 1990. Mechanics of Solid Materials, Cambridge University Press. Miner, M.A., 1945. Cumulative damage in fatigue, ASME Journal of Applied Mechanics 3, 159-164. Palmgren, A, 1924. Die Lebensdauer von Kugellagern. VDI-Zeitschrift 68, 339-341. Ramberg, W., Osgood, W.R. 1943. Description of stress-strain curves by tree parameters. NACA Techn. Rep. 902. Smith, R.N., Watson, P., Topper, T.H., 1970. A stress-strain parameter for the fatigue of metals, Journal of Materials 5, 767-778.