PSI - Issue 37

P. Santos et al. / Procedia Structural Integrity 37 (2022) 833–840 P. Santos et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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load of 2.5 kN was applied for 24 hours to maintain a uniform laminate thickness. During the first 4 hours the bag remained attached to a vacuum pump to eliminate any air bubbles existing in the composite (Fig. 1c). The post-cure was followed according to manufacturer datasheet in an oven at 40 ºC for 24 hours. The overall dimensions of the plates prepared were 330  330×4 mm 3 . This laminate was used for comparability of results, as this study aims to analyze the impact benefits of a sandwich with a cork core and similar lay-up on the skin. Therefore, using similar methodology and materials a sandwich was produced according to Fig. 1b). For this purpose, a 2 mm thick NL 20 CORECORK core layer (supplied by Amorim Cork Composites) was inserted in the middle of the laminate. The overall dimensions of the sandwich plates were 330  330×5.6 mm 3 .

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Fig. 1. a) Leaflets after preparation; b) Leaflets lay-up sequence; c) Final plate into a vacuum bag.

The specimens used in the experiments were cut from those plates with dimensions of 100×100 (mm 2 ), using a diamond saw and a moving speed chosen to reduce the heat in the specimen. Finally, low-velocity impact tests were performed using a drop weight-testing machine IMATEK-IM10 (IMATEK, Old Knebworth, Herts, UK). A hemispherical impactor of 20 mm diameter with masses of 3.005 kg was used. The tests were performed on square section samples of 75×75 mm and the impactor stroke at the centre of the samples obtained by centrally clamping the 100×100 mm specimens. This machine is composed of two guiding columns, the impactor (guided by bearings) and a device preventing second impact. The impact energy is supplied by gravity only and controlled adjusting the falling height (up to 3.5 m). Data acquisition is controlled by a computer, and the results are treated and presented by using ImpAqt software (version 1.3; IMATEK). The impact load is measured by a piezoelectric load cell, able to collect 32000 points and placed in the impactor. The system has a precision of 1% of peak force. The specimen deflection is obtained from double integration of acceleration versus time curve. The impact energies used in the tests were 1, 2 and 3 J, and the impact tests carried out at room temperature according to the procedure described in standard EN ISO 6603-2. For each condition, three specimens were tested. 3. Results and Discussion Impact tests were carried out at different incident impact energy levels, namely, 1, 2 and 3 J. Fig. 3 shows the load and energy versus time response of the laminate (Fig. 3a) and sandwich laminate (Fig. 3b). These curves represent the typical behavior for each laminate and are in good agreement with the literature (Amaro et al., 2013; Mortas et al., 2014; Reis et al., 2013). In detail, from the load versus time curves it is possible to observe an increase up to a maximum value, P max , followed by a drop after the peak load. The value of P max is very dependent on the impact energy and represents the peak load value that the composite laminate can tolerate, under a particular impact level, before undergoing major damage. Regarding the energy-time curves, the beginning of the plateau corresponds to the loss of contact between impactor/specimen, which evidences that the impact energy is not enough to obtain the complete perforation of the specimens. Therefore, the elastic recovery, also defined in many literature texts as restored energy, is obtained by the