Issue 30

P.N.B. Reis et alii, Frattura ed Integrità Strutturale, 30 (2014) 431-437; DOI: 10.3221/IGF-ESIS.30.52 432 higher fibre bridging. The fibre direction was studied by Miyagawa et al . [14] and they found that the lowest values were obtained with samples whose fibre direction was 90º. It is evident by the literature that the resistance to crack growth depends of the laminates, in terms of their constituents and set up. Therefore, the present paper describes an experimental study developed to characterize mode I and mode II fracture toughness using DCB and ENF tests, respectively, in carbon/epoxy woven composites. E XPERIMENTAL PROCEDURE omposite laminate sheets were manufactured using twelve woven, balanced, bi-directional, layers of carbon fibres (with 196 g/m 2 ) all of them with the same orientation 0/90º, and an epoxy resin matrix. Fibres and resin were hand placed in a mould. The mould was then put into a vacuum bag producing 0.1 MPa during 8 hours for curing at room temperature. The fibre volume fraction (V f ) was 0.66 and the average plate thickness was 3 mm. Details about the manufacture process of the composites laminates can be found in previous work of the authors [15]. A 100  m PTFE film was introduced into the plates during moulding of the laminates in order to generate the starter crack. Fig. 1 shows the fibre distribution along the longitudinal direction. The fibre angle misalignment was determined using the Designer 6.0 software and an average angle of 5.2º was obtained, with a standard deviation of 1.7º. Figure 1 : Lateral view photo showing the fibre distribution along of the longitudinal direction of the specimen. a) b) Figure 2 : a) DCB specimen (Mode I); b) ENF specimen (mode II). Dimensions in mm. The experimental work involved double cantilever bean (DCB) and end-notched flexure (ENF) tests. The specimens of composite material were cut by a diamond table saw from the original plates, 300 mm long and 100 mm wide, aligned with one fibre direction. The geometry and dimensions of the specimens are shown in Fig. 2. DCB specimens were used with 180 mm length, 25 mm width and initial crack lengths (a o ) of 30, 45 and 50 mm. Two piano hinges were bonded to both surfaces of the specimen at the cracked end for load transmission. The piano hinges and the specimens were grit- blasted with sandpaper before bonding and cleaned with alcohol impregnated soft paper. The adhesive used for bonding was an Araldite 420 A/B bi-component. The DCB tests (Fig. 2a) were performed in tension, according to ASTM D 5528- C