Issue 43

D. Gentile, Frattura ed Integrità Strutturale, 43 (2018) 155-170; DOI: 10.3221/IGF-ESIS.43.12 156 start from regions of the composite with an elevated void volume fraction. Alternatively, these flaws can be initiated by post-production handling or by the action of the external load during the service life. Damage tolerance design approach to composite structures requires to establish the critical conditions, in terms of maximum allowable flow size and design loads, which may be sustained without the risk of unstable flaw propagation or the drop of the nominal design material resistance below acceptable limits. From a conceptual point of view, interlaminar flaws are bidimensional discontinuities that can be treated as cracks with a non-straight front, in general. Conditions for stable or unstable crack propagation may be reached during the application of thermo-mechanical loads when the intensity of the stress field at the crack tip exceeds a critical value typical for the material under investigation. Due to the similarity to planar cracks, delaminations can be correctly analyzed with fracture mechanics tools. The appropriate parameter that characterizes composite laminate resistance to the propagation of an interlaminar flaw is the unitary strain energy release rate, G. Moreover, interlaminar cracks might develop in many of the load cases that are likely to develop under normal use of aerostructures, e.g. at concentrated loads at joints and due to un-avoidable impact loads. These cracks are very difficult to detect and their presence may severely reduce the load carrying capacity of the component. Thus, modelling of the interlaminar behavior is crucial for safe design of an advanced structural component; especially when initiation of cracks is studied, [6]. This is means that in order to develop a robust model to predict components behavior it is very important to characterize composite laminate resistance to the propagation. Experimentally Double Cantilever Beam (DCB) and End Notched Flexure (ENF) testing mainly measured the fracture toughness for Mode I and Mode II. In the present work, the fracture resistance of carbon fiber based laminates under mode I and mode II loading have been experimentally determined. Mixed mode configuration has not been considered at this time. At present, experimental testing and the procedure for the determination of fracture resistance in composite laminates is coded in the international standards such as ASTM only for unidirectional laminates [0]n with propagating delamination along the fiber direction. In multidirectional laminates, delamination crack may branch through the oriented plies invalidating the fracture mechanics assumption of planar propagation. In addition, secondary effects, such as edge effects, crack tip oscillation, etc., which may strongly affect the estimation of G, are very difficult to be avoided, [7, 8]. As far as concern composite laminates, the procedure for the determination of the laminate fracture resistance under mode I and mixed mode I+II loading conditions are coded in the ASTM D-5528-01 and ASTM D-6671-01, respectively. The test procedure for the determination of G under mode II loading condition followed in the present work, is available in the AECMA book of standards (prEN 6034), [9]. Preliminary considerations and fundamental relationships for laminate fracture resistance under mode I Laminate fracture resistance under mode I loading configuration can be easily determined on double cantilever beam (DCB) specimen geometry. The reference geometry for the specimen is given in Fig. 1 together with the definition of the fundamental dimensions. Figure 1: DCB geometry definition Even though experimental data reduction is not based on the classical beam theory, as initially proposed by Wilkins et al., [1], the simplicity of the theory allows to derive some of the fundamental relationships for this geometry. In this approximation, the geometry compliance is given by: