PSI - Issue 19

6

Fabian Becker et al. / Procedia Structural Integrity 19 (2019) 645–654 F. Becker et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 4. Experimental result of bolt loads and sti ff ness loss during the fatigue test

Fig. 5. Detail of experimental result of bolt loads and sti ff ness loss during the fatigue test between 0.7 and 1.0 of normalized cycles to failure

nation can be seen. The following loss in sti ff ness is caused by growth of the delamination (figure 8). The slower rate may be caused by the growth of the delamination into little stressed areas because of the three point bending loading. The second steep loss in sti ff ness can be attributed to a second delamination in another layer of the specimen or a split crack in the center of the specimen. The initiation of the first delamination and the loss in normalized dynamic sti ff ness correlates with local peaks in the mean values of the measured bolt loads. However, due to the long term oscillation in the data of the bolt loads, this behaviour cannot be confirmed in relation to the initiation of the second delamination. The damaged surface at the end of the experiment (when reaching the residual sti ff ness criterion) shows an asym metrical delamination in the case of specimen 10 on the tension side with local areas damaged by friction heating after the initiation of the delamination (Figure 9). On the compression side, inter fiber failure splits are visible. Since no split cracks could be observed in the reference fatigue testing, the split cracks are assumed to be introduced by the additional transverse tensile strain in ”2” direction on the compression side. No oblique inter fiber failure modes could be observed as predicted by static failure criteria for a loading in ”3” direction (Puck et al. (1998)). For specimens loaded with higher equivalent bending strains (Specimen number 8 and 9 in table 1), the damaged area shows asymmetrical delaminations at the surface of the tension side and inside the specimen. The delamination inside