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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Figure 4 shows typical images of crack surfaces of specimens fatigued at room temperature and in liquid hydrogen. Specimens fatigued at room temperature show a clear fatigue crack area which appears in form of half-elliptic or semi elliptic regions from the specimen surface or the specimen corner, respectively. In all cases a distinct border between fatigue crack and final fracture is visible. In the case of the specimens fatigued in liquid hydrogen, this clear difference is not visible. In the upper right image, the fatigue crack in the form of a half ellipse is slightly visible, whereas in the lower image no fatigue crack area can be observed. All specimens tested in liquid hydrogen show clear delamination leading to the formation of a “ sheet structure” with sometimes large secondary cracks between the individual sheets. This plate structure is clearly visible in the SEM-images of the fatigue crack area shown in figure 5. The specimen tested at room temperature shows a smooth crack surface with a faintly visible line structure. The specimen tested in liquid hydrogen shows the sheet structure with pronounced steps and several cracks between the sheets.

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Fig. 5. SEM images of a fatigue crack initiation sites of a specimen fatigued at room temperature (RT) and in liquid hydrogen (LH2).

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Fig. 6. SEM images of the final fracture of a specimen fatigued at room temperature (RT) and in liquid hydrogen (LH2).

The sheet formation takes also place in the final fracture area of specimens fatigued in LH2. In specimens tested at room temperature the typical dimple formation of a ductile fracture can be observed (figure 6a). The specimen tested in LH2 shows also the formation of dimples, but they are smaller compared to room temperature. Additionally the