PSI - Issue 17

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

303

4

Table 1. Results of the fit of the Gecks-Och function on the fatigue results. Parameter RT

LH2

 UTS [MPa]  FL [MPa]

399.87 ± 9.027 222.19 ± 4.516 4.994 ± 0.0422 11.342 ± 1.4934

585.08 ± 7.728 355.65 ± 9.646 5.978 ± 0.0473 14.909 ± 2.1234

 

Regression coefficient R 2 Mean value ̅ Standard deviation s Number of experiments Statistical factor k’ T k 10% (confidence level 95 %) k 90% (confidence level 95 %)

0.97974 399.89 12.929

0.97807 585.08 13.553

15

19

2.068 0.933 1.045

1.949 0.955 1.067

The measured cyclic lifetimes and the curves resulting from the Gecks-Och evaluation for room temperature (red) and liquid hydrogen (blue) are shown in figure 2. All valid experiments are given as filled symbols, the open symbols represent invalid experiments. The Gecks-Och function allows a good mathematical description of the measured S N-curve. The scatter of the measured data is low and therefore the lines for a failure probability of 10 % and 90 % are close to each other. The ultimate tensile strength in LH2 was found to be 185 MPa (46 %) higher compared to the value determined at ambient conditions and the fatigue limit raises about 133 MPa corresponding to an increase of 60 %. Both S-N curves show a distinct S-shape but also significant differences. Under LH2 environment only a slight decrease of the stress up to 10 5 cycles is found, whereas the curve determined at room temperature shows in the same interval a decrease of more than 100 MPa. At room temperature the transition to the fatigue limit is reached after about 5·10 5 cycles, in LH2 this transition is shifted to higher cyclic lifetimes of about 5·10 6 cycles.

600

500

400

300

RT LH2 maximum load [MPa]

P F =10% P F =50% P F =90%

200

1 10 4

10 5

10 6

10 7

cyclic lifetime

Fig. 2. Results of the fatigue Tests at room temperature (RT) and in liquid hydrogen (LH2) with the fitted Gecks-Och function (P F =50 %) and calculated lines for a failure probability P F of 10 % and 90 %.