The experimental determination of the resistance to delamination is very important in aerospace applications as composite materials have superior properties only in the fiber direction. To measure the interlaminar fracture toughness of composite materials, different kinds of specimens and experimental methods are available. This article examines the fracture energy of four-point end-notched flexure (4ENF) composite specimens made of carbon/epoxy and glass/epoxy. Experiments were conducted on these laminates and the mode II fracture energy, , was evaluated using compliance method and was compared with beam theory solution. The crack growth resistance curve (R-curve) for these specimens was generated and the found glass/epoxy shows higher toughness values than carbon/epoxy composite. From this study, it was observed that R-curve effect in 4ENF specimens is quite mild, which means that the measured delamination toughness, , is more accurate. 1. Introduction Owing to their high stiffness and strength combined with low weight, polymer matrix composites have become more appropriate structural materials for aerospace applications. However, the usual laminated nature and the relatively low matrix strength make them particularly susceptible to delamination. The highly anisotropic nature of laminated composite structures causes a mismatch in mechanical properties between individual lamina within the laminate, which in turn can produce interlaminar crack initiation and propagation. For example, low velocity impact can generate relatively large delamination, which is highly detrimental to compressive load because of localized buckling phenomena [1]. Testing of thin skin stiffened panels designed for aircraft fuselage applications has shown that bond failure at the tip of the frame flange is a very important failure mode. Debonding also occurs when a thin-gage composite fuselage panel is allowed to buckle in service [2]. The growing use of composite materials in aircraft and spacecraft applications has motivated researchers to understand their fracture behaviour and damage mechanisms. Hence, fracture characterization of composites structures attains special relevancy. Most of the composites currently in service contain only two-dimensional (in-plane) reinforcement and delamination remains an important failure mode in such composites. The development of standardised test methods to characterize the resistance to interlaminar crack propagation is necessary for two main reasons: (i) such tests offer the possibility to compare existing and new materials on the same
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