PSI - Issue 8

V. Dattoma et al. / Procedia Structural Integrity 8 (2018) 444–451 Author name / Structural Integrity Procedia 00 (2017) 000–000

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Fig. 6. Comparison of typical stress-strain curves: OHC tests.

3.2. Failure mode

Failure process has been studied recording with a high speed camera the test. Failure mechanism is not really influenced by different layup. On the other hand the study of failure mechanism is more easy in the case of severe wrinkles (ratio W/D equal to 10), which enhanced the delamination process before failure. However, tension and compression failure have different behaviour. In tension, delamination starts at 43 % of the ultimate strength in the case of 50-33-24-WD10 laminate. At increasing load, delamination interests all the curved plies in sequence and corresponds to the irregularities in the stress-strain curves (Fig. 7). For ratio W/D higher than 10 of the same laminate, these phenomena are limited, since the first delamination appears at 83% for the ratio W/D equal to 20 and it is absent in the case of W/D equal to 40. Nevertheless the wide extension of delamination area, the specimen is able to hold relevant loads until failure, which is dominated by fiber rupture. Finally, other layups show the first delamination between 55% and 59% of its ultimate strength. In compression, failure is generally catastrophic and not forecasted by evident delamination. The only exception is constituted by the specimen 17-67-24-WD10, which is characterized by lowest stiffness. In this case, a first delamination occur practically at the beginning of the test, as highlighted on the left side of the specimen in Figure 8, which lead to a localized damage and the final failure. Final failure results faster than the speed recording of the camera that in this case was settled to 1000 frame per second. In case of other layups, final failure is not foreshadow by any evident phenomenon. 4. Conclusions Failure mechanism induced by wrinkles in Open Hole configuration has been experimentally studied both in tension and compression. Knockdown of ultimate strength is coherent with results that can be founded in literature. Initial delamination in traction appears at load comprises between 43% and 59% of the ultimate strength in the case of the more severe wrinkles, while is generally absent in compression, with the exception of the layup characterized by the lower stiffness.