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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requirements like lowest component mass at highest structural integrity. A typical example is the design of liquid hydrogen propellant tanks for aerospace applications and future transportation systems with fuel cells. Mechanical properties at cryogenic temperatures, especially in liquid hydrogen, are necessary for design and construction of these propellant tanks. A candidate material for hydrogen propellant tanks is the aluminum alloy AA2198. It is well known that the mechanical properties of aluminum alloys are increasing with decreasing temperature (Reed and Clark 1983, Glazer et al 1987, Venkateswara et al. 1989 and 1990). Unfortunately, most of the available data are limited to monotonic loading and in the case of new materials often little or no mechanical properties at cryogenic temperatures are available. The present investigation shows the results of tensile and fatigue tests of the aluminum alloy AA2198 T851 in liquid hydrogen (LH2) and ambient conditions. The mechanical experiments are accompanied by fractographic investigations. The investigations were undertaken on Al-Mg-Cu-Li-alloy AA 2198 T851 sheet material with a thickness of about 3.2 mm provided by ArianeGroup. The specimens were taken from the sheet material with the loading axis parallel to the rolling direction. For the tensile tests specimens with a gauge length of 40 mm and a width of 8 mm were used. The tests were performed under stroke control with a loading rate of 1 mm/min. The fatigue experiments were conducted on specimens with a continuous radius between ends according to ASTM E 466 (2015) with a minimum width of 8 mm. All tests were performed under load controlled conditions with a frequency of 20 Hz at a stress ratio of R = 0.1. The tests were performed until failure of the specimen or up to a limit of 5 million cycles. In case of RT-experiments unbroken specimens were tested again on a higher loading level. The tests on room-temperature were performed on a servo-hydraulic testing machine. For the tests in liquid hydrogen a special constructed servo-hydraulic testing machine was used. Figure 1 shows a sketch and a picture of the testing machine. The specimens were cooled with liquid hydrogen to 20 K prior to the experiment and the LH2 level was monitored by level sensors. In the case of long fatigue tests, the tests were interrupted for refilling the cryostat with LH2. 2. Experimental Details 2.1. Material and Test Procedure

Fig. 1. Sketch and photograph of the testing machine for testing in LH2.