Fatigue life prediction of bonded joints in composite materials

An extensive experimental activity proved that the fatigue life of bonded joints in composite materials can be divided into two distinct phases: an initiation or nucleation phase followed by the crack growth up to a critical length. The fraction of the life spent in the two phases depends on many parameters like joint geometry, stress distributions, stress ratio, adhesive type and thickness, environmental conditions and others. The fatigue cracks nucleate and grow mainly in the adhesive or at the adhesive/adherent interface. It is in the authors’ opinion that the most efficient way to model the fatigue behaviour and assess the fatigue life of these bonded connections is to account for the actual evolution of the damage and a model was developed accordingly. The first part of the fatigue life, up to the initiation of a technical crack (about 0.3 mm), is estimated using a generalised stress intensity factor approach. The duration of the propagation phase is later assessed by integrating a suitable power law relating the crack growth rate to the maximum strain energy release rate. This work deals with the first part of the model and presents the results of several numerical two-dimensional analyses carried out with the aim to investigate the stress fields in bonded composite joints, to evaluate the static strength and the crack onset under cyclic fatigue loading. The generalised stress intensity factors for the different joint geometries were evaluated by means of geometrically linear analysis on uncracked 2D FE models.