PSI - Issue 23

Sascha Gerbe et al. / Procedia Structural Integrity 23 (2019) 511–516 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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crack initiation and propagation. Porosity led to strong scattering in number of cycles to failure, especially in VHCF regime. However, an analysis according to Basquin delivers a fatigue-cycle-dependent approach, which can be used to derive a multi-threshold Kitagawa-Takahashi diagram with modification according to El Haddad. For this purpose, it was shown that results from fatigue experiments connected to fractographic investigations are well describable. Data points, which do not fit with the threshold diagram, were correlated with different crack propagation behavior due to the absence of porosity on fracture surfaces. Crack initiation and propagation appeared shear stress controlled on facet-like crystallographic {111}-slip planes. Further, a high amount of striations influenced by eutectic silicon particles of interdendritic regions shows their barrier effect against dislocation movements for hypo-eutectic Al-Si cast alloys. In both cases the numbers of cycles to failure were significantly higher than for specimens where crack initiation was induced by big pores of sizes bigger than about two times the SDAS. For such specimens, crack propagation appeared independent from microstructure and normal stress-controlled due to higher SIF ranges caused by big pores. In intermitted HCF experiments with ex-situ CT analyses, multiple fatigue crack initiations at pores on different planes in the gage length could be observed. Here, the crack propagation is controlled by nominal stresses in the beginning, while the coalescence of fatigue cracks on different planes is caused in the plane of maximum shear stresses. The experimental setup of online damage monitoring by AC potential shows an excellent fit with ex-situ 3D fatigue crack propagation analyses and allows a defect- and cycle-dependent observation and understanding of fatigue crack initiation and propagation mechanisms in hypo-eutectic Al-Si cast alloys.

Acknowledgements

The authors gratefully acknowledge the German Research Foundation (DFG) and the German Federal Ministry of Education and Research (BMBF) for the funding of the investigations within the frameworks of the DFG research project 282318703 and BMBF research project 13FH027PX5. Additionally, the authors would like to express their special thanks to Nemak Dillingen GmbH and Ohm &Häner Metallwerk GmbH for the technical and material support.

References

Basquin, O. H., 1910. The exponential law of endurance testing, in Proceedings of American Society of Testing Materials 10, 625-630. Ben Ahmed, A., Nasr, A., Bahoul, A., Fathallah, R., 2017. The impact of defect morphology, defect size, and SDAS on the HCF response of A356 T6 alloy. International Journal of Advanced Manufacturing Technology 92, 1113-1125. Campbell, J., 2010. Discussion of “Fatigue crack initiation mechanism for cast 319-T7 Al alloy ”. Metallurgical and Materials Trans. A 41, 18. El Haddad, M. H., Topper, T. H., Smith, K. N., 1979. Prediction of non propagating cracks. Engineering Fracture Mechanics 11, 573-584. Gerbe, S., Knorre, S., Krupp, U., Michels, W., 2018. The significance of microstructure heterogeneities on the fatigue thresholds of aluminum castings. 12th Int. Fatigue Congress, Poitiers, France, MATEC Web of Conf. 165(14005), 1-7. Gerbe, S., Krupp, U., Michels, W., 2019. Influence of secondary dendrite arm spacing (SDAS) on the fatigue properties of different conventional automotive aluminum cast alloys. Frattura ed Integrità Strutturale 48, 105 -115. Houria, M. I., Nadot, Y., Fathallah, R., Roy, M., Maijer, D.M., 2015. Influence of casting defect and SDAS on the multiaxial fatigue behaviour of A356-T6 alloy including mean stress effect. International Journal of Fatigue 80, 90-102. Jang, Y. H., Jin, S. U., Jeong, Y. I., Kim, S. S., 2009. Fatigue crack initiation mechanism for cast 319-T7 aluminum alloy. Metallurgical and Materials Transaction A 40, 1579-1587. Kitagawa, H., Takahashi, S., 1976. Applicability of fracture mechanics to very small cracks, in ASM Proceedings of 2nd International Conference on Mechanical Behavior of Materials, Metalspark, Ohio, 627-631. Mrzljak, S., Hülsbusch, D., Walther, F., 2018. Damage initiation and propagation in glass-fiber-reinforced polyurethane during cyclic loading analyzed by in situ computed tomography, in Proceedings of 7th International Conference on Fatigue of Composites, Vicenza, Italy Siegfanz, S., Giertler, A., Michels, W., Krupp, U., 2013. Influence of the microstructure on the fatigue damage behavior of the aluminium cast alloy AlSi7Mg0.3. Materials Science and Engineering A 565, 21-26. Tenkamp, J., Koch, A., Knorre, S., Krupp, U., Michels, W., Walther, F., 2018. Defect-correlated fatigue assessment of A356-T6 aluminum cast alloy using computed tomography based Kitagawa-Takahashi diagrams. International Journal of Fatigue 108, 25-34. Tenkamp, J., Koch, A., Knorre, S., Krupp, U., Michels, W., Walther, F., 2018. Influence of the microstructure on the cyclic stress-strain behavior and fatigue life in hypo-eutectic Al-Si-Mg cast alloys. 12 th Int. Fatigue Congress, Poitier, France, MATEC Web of Conf. 165(15004), 1-8. Tenkamp, J., Bleicher, K., Klute, S., Chrzan, K., Koch, A., Walther, F., 2019. Advanced characterization of the cyclic deformation and damage behavior of Al-Si-Mg cast alloys using hys teresis analysis and alternating current potential drop method, in “Light Metals”. In: Chesonis, C. (Ed.). Springer, 167-175.