Validation of an H cr -based fracture initiation criterion for adhesively bonded joints Angelos Mintzas ⇑ , David Nowell Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK article info Keywords: Adhesively bonded joints Stress singularity Failure initiation criterion Path independent integral abstract The singular stress fields that develop around wedges found in adhesively bonded scarf and double lap joint configurations are fully determined by employing Williams’ eigenfunction expansion method in combination with a path independent contour integral method. The intensities of the near-tip stress fields (H) are then used as fracture initiation parameters in order to predict the strengths of these joints. The predicted strengths are compared to experimental results found in literature and the conditions for the validity of the proposed H cr -based fracture initiation criterion are examined. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Adhesive bonding has gained the attention of various industries such as automotive, marine and aerospace over the last decades. The development of higher performance adhesives as well as the necessity to join efficiently a large variety of mate- rial combinations such as metal to composites, ceramics to composites, etc. has led to a partial shift from mechanical fasten- ing techniques to adhesive bonding ones [1]. Although one of the main advantages of adhesively bonded joints is the more uniform distribution of stresses over the whole joined area, when compared for example to bolted or riveted ones, stress concentrations or even singularities at the interface corner, where the two dissimilar materials meet cannot be avoided. Gi- ven the singular nature of the stresses and strains, conventional failure criteria can be applied only when the values of stres- ses/strains are evaluated at a distance from the singular corner [2] or by averaging them over an area. Unfortunately, this area or distance is found to be a function of the joint geometry and the material properties of the constituent parts and experiments are needed for its determination [3]. A fracture mechanics approach therefore seems more appealing when dealing with such problems. Williams [4] first developed an asymptotic method for the determination of the elastic singular stress field that develops around re-entrant corners in isotropic materials. Since then, a number of researchers have applied the method to bimaterial wedges in isotropic [5–8] and anisotropic materials [9–12] as well as to multi-material wedges [13–15]. A general summary of the main formulae involved is provided by [16]. However, all of the studies reported above focused on the determination of the order of the stress singularity (k  1) and its dependence on the wedge geometry and material combinations. It was only after the 1970s that scientists [17–24] became interested in fully characterising the near-tip stress and displacement fields and thus attempted to calculate their intensities, i.e. the generalised stress intensity factors (H). Over the last years there have been a number of studies that have successfully correlated fracture of specimens containing monolithic [25–27] and bimaterial wedges [28,29] to a critical value of the generalised stress intensity factor (H cr ). Therefore, the idea of using H as a fracture initiation parameter in an analogous way to the use of the crack tip stress intensity factor (K) in 0013-7944/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2011.09.020 ⇑ Corresponding author. Tel.: +44 1865 283475; fax: +44 1865 273906. E-mail address: angelos.mintzas@eng.ox.ac.uk (A. Mintzas). Engineering Fracture Mechanics 80 (2012) 13–27 Contents lists available at SciVerse ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech