Lifetime prediction of adhesive joints subjected to variable amplitude fatigue

Almost all structural applications of adhesive joints will experience cyclic loading and in most cases this will irregular in nature, a form of loading commonly known as variable amplitude fatigue (VAF). This paper is concerned with the VAF of adhesively bonded joints and has two main parts. In the first part, results from the experimental testing of adhesively bonded single lap joints subjected to constant and variable amplitude fatigue are presented. It is seen that strength wearout of bonded joints under fatigue is non-linear and that the addition of a small number of overloads to a fatigue spectrum can greatly reduce the fatigue life. The second part of the paper looks at methods of predicting VAF. It was found that methods of predicting VAF in bonded joints based on linear damage accumulation, such as the Palmgren-Miner rule, are not appropriate and tend to overpredict fatigue life. Improved predictions of fatigue life can be made by the application of non-linear strength wearout methods with cycle mix parameters to account for load interaction effects.