The effect of environment on the fatigue of bonded composite joints. Part 2: fatigue threshold prediction

Abstract

In Part 2 of this paper, the fatigue strength of adhesively bonded joints is analysed using stress analysis and fracture mechanics. Composite lap strap joints are considered for this study. Substrates made of unidirectional and multidirectional composite laminates are bonded together using epoxy film adhesive. Non-linear stress and fracture analyses are performed in order to predict the strength of the joints in different hostile environmental conditions. The use of several threshold criteria is investigated. Criteria based on the principal stress provide good threshold prediction for small plastic deformation. The maximum principal strain, von Mises strain, shear stress and von Mises stress predict with good accuracy the fatigue thresholds of the joints that undergo large plasticity. The plastic zone size does not show correlation at the fatigue threshold load in the different joint types. Furthermore, elastic and elasto-plastic fracture parameters are calculated for two different inherent cracks. The first one is initiated at the adhesive/adherend interface, while the second is in the centre of the adhesive layer. The J-integral that accounts for plasticity and the elastic strain energy release rate have been shown to correlate with the threshold load for the different joints.