Predicting degradation in bonded composite joints using a semi-coupled finite-element method

In this paper, a method of predicting failure in bonded composite joints subjected to combined mechanical loading and environmental degradation is described. The technique is based on a coupled mechanical-diffusion finite-element analysis combined with an appropriate failure criterion. The method is evaluated by predicting the fatigue thresholds of epoxy-CFRP lap-strap joints preconditioned and tested in dry and wet environments. The coupled stress-diffusion method was shown to be capable of predicting correctly the effect that different environments would have on the fatigue resistance of the joints. Both elastic and elasto-plastic fracture criteria were then used to quantitatively predict the fatigue thresholds. The elastoplastic fracture criterion was the most suitable when moisture was encountered at elevated temperatures and was capable of predicting fatigue thresholds within the experimental scatter range. It is concluded that the technique described is a powerful and flexible way to predict failure in bonded joints subjected to a wide variety of loads and environmental conditions. The method does, however, rely on material data not commonly available, and reasonable simplifying assumptions should be made to make the method cost effective.