Issue 30

P.N.B. Reis et alii, Frattura ed Integrità Strutturale, 30 (2014) 431-437; DOI: 10.3221/IGF-ESIS.30.52 431 Focussed on: Fracture and Structural Integrity related Issues Interlaminar fracture in woven carbon/epoxy laminates Paulo N.B. Reis Department of Electromechanical Engineering, University of Beira Interior preis@ubi.pt José A.M. Ferreira, José D.M. Costa CEMUC, Department of Mechanical Engineering, University of Coimbra martins.ferreira@dem.uc.pt , jose.domingos@dem.uc.pt António M. Pereira ESTG/CDRsp, Polytechnic Institute of Leiria mario.pereira@ipleiria.pt A BSTRACT . This paper describes an experimental study developed to characterize the mode I and mode II fracture toughness of carbon/epoxy woven composites, using DCB and ENF tests, respectively. The laminates were manufactured using an epoxy resin and twelve woven balanced bi-directional layers of carbon fibres, all of them with the same orientation (0/90º). Significant instantaneous delaminations were observed particularly for the DCB specimen, which were responsible for an oscillatory behaviour of G I versus crack length. The maximum values obtained for G IC and G IIC were 281 and 1800 J/m 2 , respectively. K EYWORDS . Carbon fibre reinforced composites; Delamination; Fracture toughness; Mechanical testing. I NTRODUCTION mpacts at low velocity are the main cause of in-service delaminations, which are very dangerous because they are not easily detected visually [1, 2] and they can affect significantly the residual properties and structural integrity [3-8]. In this context, for design purpose it is very important to understand the interlaminar fracture toughness properties of fibre reinforced composite materials. If the double cantilever beam (DCB) test is one of the most popular to determine G IC , because it presents simple specimen geometry and a stable crack growth in loading under displacement control, for mode II the most popular tests are the end-notched flexure (ENF), end-loaded split (ELS) and the four-point end- notched flexure (4ENF) [9]. However, according to the Morais and Pereira [9], the ENF specimen combined with the ECM (effective crack method) is the best solution for the characterization of mode II fracture with some advantages like simplicity, negligible friction effects and low tendency for geometric non-linearity. For carbon fibre reinforced epoxies resins, it was found that the typical values of G IC in stable propagation are around 260 J/m 2 [10]. In order to improve the interlaminar toughness of the composites materials the bibliography suggests the application of the thermoplastic resins [10], due their high damage tolerance, or by insertion of ductile resin interleaf between each carbon fibre/epoxy layers [11]. An alternative to the toughened resins is the use of advanced textile technologies with substantially improved delamination resistance [12, 13]. It was demonstrated that G IC values for the knitted composites are about 10 or 20 times higher than for the uniweave composite [6]. Chen et al . [10] observed that the values of G IC , in stable propagation, have a dropping tendency with increasing fibre content in the range 21-39% and I