PSI - Issue 19

Christian Schneider et al. / Procedia Structural Integrity 19 (2019) 370–379 Ch. Schneider, et al/ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 9 (left) shows the machine triggered edge pictures of the specimens exemplarily at specimen CEFT_01_i to expose the appearance of first cracks in the embedded 90°-plies. As expected, the sum of cracks in the 90°-plies was not strong enough to trigger fatale failure and so the two specimens did not brake until two million cycles, where the tests were stopped finally. The right picture shows the tested UD90 after fatale failure. Similar to the quasi-static tests, all specimens of this type failed close to the tabs.

90° Cracks 1800 cycl.

Final picture After 2M cycl.

First visible crack

Fig. 9: Left: Egde pictures, exemplary at CEFT_01_i, for recognition of first visible cracks in 90° plies; Right: Tested specimen with fatigue load after failure for UD90.

6. Summary

Two different layups for specimen were compared. On the one hand, a layup of UD 90° plies (UD90), on the other hand, UD 90° plies embedded in UD 0° plies (EMB090). A FEM simulation of different materials and different taper angles of the end tabs was done for embedded specimens. To this end, a quarter-model was analyzed and the ply stresses were assessed by the Puck failure criterion. Based on these results, the optimal tab configuration is GFRP with a taper angle of 14°. For this model, the failure is expected to start in the 90° plies and the risk of failure in the 90° plies is higher in the gauge section than at the end of the tabs. For the testing, a number of specimens were produced with the two different layups. The tab configuration from the simulation was used for the EMB090. To compare the results of UD90 specimen with existing results, Aluminum tabs were glued on these specimens. Quasi-static and fatigue tests were done with both types of specimen. With the fatigue tests an, additional DIC-system was used to take machine triggered edge pictures of the specimen. To compare the results for the embedded 90°-plies with the UD90 specimen, stresses in the 90° layers were computed by CLT. It could be shown, that the failure stresses for the 90°-plies within the EMB090 (calculated with the CLT) are higher than the failure stresses in the UD90 specimen. Besides that, the UD90 broke close to the tab region. Two reasons were mentioned for these results. On the one hand, the crack growth within the embedded 90°-plies is hindered by the additional UD 0° plies, the first crack does not lead to specimen failure and multiple cracks accumulate. On the other hand, the stress concentrations caused in the UD90 specimens by the standard tabs lead to failure at lower nominal stresses. Based on these results, further development of specimen geometry has to be done to shift the stresses from the tab region to the center of the specimen. Additional work will be done to monitor the initiation and propagation of (micro-)cracks in the UD 90 layers with different techniques (optical investigation, thermography, acoustic emission) and compare to measurements of the stiffness degradation.