PSI - Issue 17

Jürgen Bär et al. / Procedia Structural Integrity 17 (2019) 300–307 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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loading, leading to a sliding between the individual sheets. This assumption is supported by the formation of a smooth layer which is found on the surface of the secondary cracks (figure 6). Obviously this layer is formed by the sliding between the individual sheets leading to a leveling of the surfaces of the sheets and with it to a dissipation of energy. This dissipation of energy ultimately leads to the improved fatigue behavior found in the experiments. In order to substantiate this thesis, further investigations will be carried out to understand the failure mechanism of this alloy under cryogenic conditions. 1. Compared to room temperature the aluminum alloy AA2198 T851 shows a significant improved fatigue behavior in liquid hydrogen. 2. The improved fatigue behavior can be attributed to a change in the failure mode, which manifests itself in LH2 through the formation of a sheet structure with several secondary cracks. 3. Likely, the sliding between the individual sheets leads to a dissipation of energy, leading to an improved fatigue behavior of AA 2198. 4. A prediction of the fatigue behavior for cryogenic conditions based only on the S-N-curve determined at room temperature and the changes in the UTS with existing models is not possible. 5. For reliable fatigue data an experimental determination of the S-N-curve under LH2-conditions is necessary. 5. Conclusions ASTM E466, 2015. Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials, ASTM International, DOI: 10.1520/E0466-15. Gecks, M., Och, F., 1977. Ermittlung dynamischer Festigkeitskennlinien durch nichtlineare Regressionsanalyse, MBB-Bericht DU-208-77. Glazer, J.; Verzasconi, S.L.; Sawtell, R. R.; Morris, J. W., 1987, Mechanical Behavior of Aluminum-Lithium Alloys at Cryogenic Temperatures, Met. Trans. 18A, 1695-1701. Graf, U., Henning, H.-J., Stange, K., 1966. Formeln und Tabellen der mathematischen Statistik, Springer-Verlag, 2. Auflage. Kohout, J., 2000. Temperature dependence of stress–lifetime fatigue curves, Fatigue Fract. Engng. Mater. Struct. 23 , 969-977. Kohout , J.; Vĕchet, S., 2001. A new function for fatigue curves characterization and its multiple merits, Int. J. Fatigue 23 , 175-183. Reed, R. P.; Clark, A. F.; 1983. Materials at Low Temperatures. ASM International, Metals Park, Ohio. Venkateswara Rao, K. T.; Yu, W.; Ritchie, R. O.; 1989. Cryogenic Toughness of Commercial Aluminum-Lithium Alloys: Role of Delamination Toughening, Met. Trans. 20A , 485-497. Venkateswara Rao, K. T.; Ritchie, R. O.; 1990. Mechanisms influencing the cryogenic fracture-toughness behavior of Aluminum-Lithium Alloys, Acta Met. Mater. 38 , 2309-2326. Xu, Y. B.; Wang, L.; Zhang, Y.; Wang, Z. G.; Hu, Q. Z.; 1991. Fatigue and Fracture Behavior of an Aluminum-Lithium Alloy 8090-T6 at Ambient and Cryogenic Temperature, Met. Trans. 22A , 723-729. References