Edge debonding in FRP strengthened beams: Stress versus energy failure criteria

In the present paper we analyse the edge debonding failure of a beam strengthened by a fibre reinforced polymer. As well known from the literature, a stress concentration is found at the edge of the reinforcement which triggers the debonding of the fibre reinforced polymer strip when the load reaches a certain critical threshold. Two failure criteria are proposed to study the debonding mechanism. The former is a stress assessment criterion, i.e. failure takes place whenever the maximum shearing stress reaches a limit value (the interfacial bond strength). The latter is an energy, fracture mechanics criterion, i.e. failure takes place as the strain energy release rate reaches a critical value (the interfacial fracture energy). It is argued that the energy criterion is more effective to address the edge debonding failure mode. However, under the assumption of shear lag behaviour for the adhesive layer between the beam and the reinforcement, a general rule linking the two approaches is set, thus providing the key to bypass the rather complicated energetic analysis. The final part of the paper is devoted to the crack instabilities that may occur after the debonding initiates, i.e. snap–back and snap-through phenomena. The size effect is then investigated by means of a dimensional analysis and a simplified formula providing the critical load is proposed that could be useful in engineering practice.