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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For the correct evaluation of quasi-static properties, the failure of specimens should be far from the clamping and furthermore far from the tabs. Fig. 7 shows the EMB090 (left) and UD90 (right) specimens at the quasi-static tensile tests after failure. For EMB090 this specification, all specimens except one broke in the defined test field and away from the tabs. For UD90, the test results should be handled with caution (Adams and Adams 2002). Only one specimen broke away from the tabs. Nevertheless the calculated values for E and ν for the different specimens are almost the same, so the further evaluation was done with all specimens.

Fig. 7: Tested specimen with quasi-static loads after failure for EMB090 (left) and UD90 (right).

5.2. Fatigue test results

For the EMB090, the tests were stopped finally after 2 Mio. cycles because the specimen did not brake before. The edge pictures of the tested specimen where evaluated after the tests, to determine the appearance of first 90°- cracks and to get some information about possible fibre debonding later on in the fatigue tests. For UD90, the tests only ran a few cycles after the appearance of first cracks in 90°-plies before fatale failure occurred. In Fig. 8 (left), the stress-based "Wöhler" approach with experimental S/N curves (stress amplitude vs. number of cycles) is shown with the global stress amplitude for both types of specimen. In Fig. 8 (right), the values for the embedded UD 90° plies were calculated with the CLT. With these diagrams, the comparison of the S/N-curves for the two types of specimen is possible. As expected, the gradients of both types of specimen are nearly the same but the values of stress amplitude in the UD 90° plies are different. The level for the embedded UD 90° plies in EMB090 is much higher than the level for the UD90. To describe S/N-curves correctly, the 10 %- as well as the 90 %-failure probability were calculated and drawn in the diagram.

Fig. 8: Fatigue S/N-curves for both types of specimen for the whole layup (left) and with calculation to compare the 90°-plies (right).