Issue 30

P.N.B. Reis et alii, Frattura ed Integrità Strutturale, 30 (2014) 431-437; DOI: 10.3221/IGF-ESIS.30.52 433 01 [16], using an electromechanical machine, Shimadzu model AG-X, equipped with a 1kN load cell. The ENF tests were carried out in the same machine in accordance with the literature [17, 18-19]. Fig. 2b is a schematic view of the three point bending apparatus and the specimens’ dimensions with an initial crack (a o ) of 30 mm. The load level, the displacement and the crack length were recorded during the tests. In order to obtain accurate results the crack length was measured using CCD camera, PCO model PixelFly 270 XS, as shown in Fig. 3. Figure 3 : DBC test apparatus with CCD camera video image. For each condition four samples was tested and the experiments carried out with the crosshead velocity of 0.5 mm/min for loading the samples [14]. In all samples one of the edges was painted with nail varnish and then several marks were made for determination of the crack length. R ESULTS AND DISCUSSION ig. 4a and 4b show the experimental load-displacement curves from the DCB and ENF tests, respectively. Relatively to DCB tests, Fig. 4a, representative curves are presented for the initial cracks (a o ) of 30, 45 and 50 mm. It is possible to observe that longer initial cracks produce curves with lower slopes. For the same displacement, the increase of initial crack length decreases the loads. All of them exhibit oscillatory behaviour, which is explained by the unstably crack propagation. In this case the load drops suddenly at a certain displacement and the crack propagates rapidly, increasing again the load with the displacement until a new drop occurs suddenly. According with several studies [11, 20-21] the unstable crack propagation is consequence of regions with different toughness. When the crack reaches the tougher region, it slows down until the rate of release of elastic stored energy is sufficient to propagate the crack through the tougher region. The release rate of stored energy is then higher than that required for stable growth. The crack then accelerates and unstable fracture occurs [20]. In fact it is possible to observe in Fig. 1 regions with reduced amounts of resin, where the fibres practically contact, while other regions are rich in resin. Therefore there are regions with different toughness which explain the unstable propagation observed. For mode II specimens (ENF), Fig. 4b presents two experimental load-displacement curves for a o = 30 mm, which are reproducible and similar to other reported in literature [9, 14, 17]. An offset of 10 mm is represented between curves in order to obtain more details about their evolution. In this case unstably crack propagation was also observed but less significant. Fig. 5 shows the interlaminar fracture toughness (G I ) against the increment of the crack length (  a). Each value of G I is an average of four tests. According to the ASTM D 5528-01 Standard [16], the MBT data reduction method was used because it promotes the most conservative values of G IC . Therefore, G IC was calculated by the following equation: F