Issue 30

N. Mortas et alii, Frattura ed Integrità Strutturale, 30 (2014) 403-408; DOI: 10.3221/IGF-ESIS.30.48 404 In this context, Hosur et al . [9, 10] showed that the sandwich composites with nano phased foam sustain higher loads and present lower damage areas as compared with neat sandwiches. Avila et al . [11] reported benefits in terms of energy absorption and failure modes with the nanoclays fillers on fiber glass/nano-modified epoxy face sheets and polystyrene foams. According to Reis et al . [2], the lowest displacements and the highest elastic recuperation occur for sandwiches with resin nano-enhanced. The best performance of the sandwiches with epoxy resin enhanced by nanoclays was also confirmed by the residual flexural strength, which increases with the introduction of the nanoclays [2]. On the other hand, Avila et al . [12] observed that the natural frequencies of the structural sandwich composites are altered by the addition of nano-reinforcements. However, the balsa planks (boards) present finite dimensions. Consequently, there are discontinuities of the core that affect the mechanical properties. Therefore, the aim of this work is study the effect of these discontinuities (gap between cores) on the impact performance. For this purpose, a gap with 20 mm of length will be analyzed and the results will be compared with other ones obtained for sandwiches manufactured with continuous cores (control samples). The benefits of resins nano-enhanced on the impact response of sandwich composites made by fiber glass/epoxy skins and balsa wood core will be studied. The epoxy resin was enhanced by nanoclays with special treatment to improve their dispersion and interface adhesion. The results are discussed in terms of load-time, load-displacement and energy-time diagrams. E XPERIMENTAL P ROCEDURE he sandwich composite specimens were fabricated using skins of glass fibre/epoxy resin and core of balsa wood with 6 mm of thickness. The skins are composited by six ply laminates, all in the same direction, of woven bi- directional glass-fibre 1195-1000 (195 g/m 2 ). SR 1500 epoxy resin and a SD 2503 hardener, supplied by Sicomin, were used. The system was placed inside a vacuum bag and a load of 2.5 kN was applied during 24 hours in order to maintain a constant fibre volume fraction and uniform laminate thickness. During the first 10 hours the bag remained attached to a vacuum pump to eliminate any air bubbles existing in the composite. The post-cure was followed according to the manufacturer`s datasheet (epoxy resin) in an oven at 40 ºC for 24 hours. In order to improve the dispersion and interface adhesion matrix/clay, nanoclays were previously subjected to a special treatment appropriate to the epoxy resin. More details about the treatment and the dispersion/exfoliation on the epoxy matrix can be found in [3, 4]. The nanoclays content used in present study is 3 wt.% because, according with studies developed by the authors [4], is the best amount for this epoxy system. Fig. 1 shows the square specimens, with 100 mm side and 8 mm thickness (100x100x8 mm 3 ), used in the present study. Low-velocity impact tests were performed using a drop weight-testing machine IMATEK-IM10. More details of the impact machine can be found in [13]. Impactor diameter of 10 mm with masses of 2.903 kg was used. The tests were performed on square section samples of 75x75 mm and the impactor stroke at the centre of the samples obtained by centrally clamping the 100x100 mm specimens. The impact energies used in the tests were 5 J. This energy was previously selected in order to enable the measuring of the damage area, but without promote full perforation of the specimens. For each condition, five specimens were tested at room temperature. a) b) Figure 1 : a) Control samples; b) Samples with different gap lengths (  0 = 20 mm). T 6 mm 100 mm 100 mm 8 mm 100 mm 100 mm 6 mm 8 mm  0