Experimental study of the CTBN effect on mechanical properties and mode I and II fracture toughness of a new epoxy resin

Abstract

Due to the high strength and adhesiveness of epoxies, these materials are widely used in various industries such as aerospace. But because of their inherent brittleness and poor fracture toughness, they do not have enough resistance against crack propagation. In adhesive joints, maximum stresses near the ends of the overlaps resulted in premature failure of the joints. In this study, for toughening the epoxy, carboxyl-terminated butadiene acrylonitrile (CTBN) as a liquid rubber was used. Blends of a new epoxy resin (R510) and the CTBN in several weight fractions were produced and characterized. In this article, four tests have been done to find the mechanical properties and mode I and II fracture toughness of the adhesives. The tensile and shear tests were performed to achieve Young’s modulus (E), maximum tensile stress (σf), shear modulus (G), maximum shear stress (τf) and Poisson’s ratio (ν) of the adhesives. Also, the double-cantilever beam (DCB) and end-notched flexure (ENF) tests were used to obtain the mode I fracture toughness (GIc) and mode II fracture toughness (GIIc), respectively. The results from each of the DCB and the ENF tests were analyzed by three different methods and compared. Results showed that by adding CTBN to epoxy, E, G, ν, σf and τf are reduced, but toughness, the resistance to crack growth and as a result, the GIc and GIIc increased efficiently. These data can be used to predict the adhesive joint strength and simulating the damage initiation and its evolution until failure with cohesive zone modeling (CZM).