Issue 31

R.D.S.G. Campilho et alii, Frattura ed Integrità Strutturale, 31 (2015) 1-12; DOI: 10.3221/IGF-ESIS.31.01 1 Advanced techniques for estimation of the tensile fracture toughness of adhesive joints R.D.S.G. Campilho 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 M.D. Banea, L.F.M. da Silva Departamento de Engenharia Mecânica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal A BSTRACT . Adhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. As a result, adhesives are being increasingly used in industries such as the automotive, aerospace and construction. Thus, it is highly important to predict the strength of bonded joints to assess the feasibility of joining during the fabrication process of components (e.g. due to complex geometries) or for repairing purposes. When using the Finite Element Method with advanced propagation laws, the tensile ( G n c ) and shear ( G s c ) fracture toughness of adhesive joints must be determined with accuracy. Several conventional methods to obtain G n c and G s c exist in the literature, mainly based on Linear Elastic Fracture Mechanics (LEFM). The J -integral technique is accurate to measure these parameters for adhesives with high ductility. In this work, the J -integral is used to obtain G n c by the Double-Cantilever Beam (DCB) test. 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 G n c than the available methods is proposed (by the J -integral technique) and some results are presented regarding joints with different geometry and adherend material. K EYWORDS . Fibres; Fracture toughness; Damage mechanics; Joining. I NTRODUCTION he developments in adhesives technology made possible the use of adhesive bonding in many fields of engineering, such as automotive and aeronautical [1]. However, stress concentrations exist in bonded joints along the bond length owing to the gradual transfer of load between adherends and also the adherends rotation in the presence of asymmetric loads [2]. A large amount of works addresses the critical factors affecting the integrity of adhesive joints, such as the parent structure thickness, adhesive thickness, bonding length and geometric modifications that reduce stress concentrations [3-5]. A large number of predictive techniques for bonded joints are currently available, ranging from analytical to numerical, using different criteria to infer the onset of material degradation, damage or even complete failure. Initially, the prediction was performed by theoretical studies as those of Volkersen [6], which had a lot of embedded simplifying assumptions, by T