Available online at www.sciencedirect.com Ciência & Tecnologia dos Materiais 25 (2013) 31–37 http://ees.elsevier.com/ctmat Special Issue on Polymers and Composites Adhesive joints in natural fibre composites: estimation of fracture properties R.D.S.G. Campilho a,* , D.C. Moura b , D.J.S. Gonçalves b a Departamento de Engenharia Mecânica, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal b Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal Abstract Adhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. On the other hand, natural fibre composites have recently gained interest due to the low cost and density. It is therefore essential to predict the fracture behavior of joints between these materials. In this work, the tensile fracture toughness (Gn c ) of adhesive joints between natural fibre composites is studied, by bonding with a ductile adhesive and co-curing. Conventional methods to obtain Gnc are used for the co-cured specimens, while for the adhesive within the bonded joint, the J- integral is considered. For the J-integral calculation, an optical measurement method is developed for the evaluation of the crack tip opening and adherends rotation at the crack tip during the test, supported by a Matlab sub-routine for the automated extraction of these quantities. As output of this work, an optical method that allows an easier and quicker extraction of the parameters to obtain Gn c than the available methods is proposed (by the J-integral technique), and the fracture behaviour in tension of bonded and co-cured joints in jute-reinforced natural fibre composites is also provided for the subsequent strength prediction. Additionally, for the adhesively-bonded joints, the tensile cohesive law of the adhesive is derived by the direct method. © 2013 Sociedade Portuguesa de Materiais (SPM). Published by Elsevier España, S.L. All rights reserved. Keywords: natural fibres; structural adhesives; fracture toughness; bonded joints. 1. Introduction * The developments in adhesives technology made possible the use of adhesive bonding in many fields of engineering, such as automotive and aeronautical, because of higher peel and shear strengths, and ductility. As a result, bonded joints are replacing fastening or riveting [1]. More uniform stress fields, capability of fluid sealing, high fatigue resistance and the possibility to join different materials are other advantages of adhesive bonding. However, stress concentrations exist in bonded joints along the bond length [2]. A large amount of works addresses the critical factors affecting the integrity of adhesive * Corresponding author. E-mail address: raulcampilho@gmail.com (R.D.S.G. Campilho) joints, such as the parent structure thickness, adhesive thickness, bonding length and geometric modifications that reduce stress concentrations [3]. On the other hand, natural fibre composites have recently gained interest, for instance in the construction or automotive industries, due to the low cost and density [4]. Thus, it is highly important the study of adhesive joints between these materials, to assess the feasibility of joining during the fabrication process of components (e.g. due to complex geometries), of joining cured parts of a structure or even for repairing purposes. Natural fibres like flax, henequen, sisal, coconut, jute, palm, bamboo, wood or paper have been used as reinforcement in thermosetting and thermoplastic resin composites [5]. Compared to typical fibre composites, e.g. with glass or carbon fibres, natural fibres benefit from lower density, less machining wear during fabrication, no health hazards,