Strength wearout of adhesively bonded joints under constant amplitude fatigue

The behaviour of adhesively bonded lap joints subjected to fatigue loading is still not well understood. In this paper strength degradation of joints during fatigue cycling is measured experimentally and related to damage evolution. Strength wearout (SW) measurements carried out under constant amplitude fatigue loading of single lap joints are presented and correlated with in situ measurements of back-face strain (BFS) and estimations of damage progression from fracture surfaces and sectioning of partially fatigued samples. Residual strength was found to decrease non-linearly with respect to the number of fatigue cycles and this corresponded to non-linear increases in the BFS and damage measurements. In particular it was noted that fatigue damage accelerated very quickly towards the end of the fatigue life of a joint. A non-linear SW model is proposed and was found to agree well with the experimental results. This model can be used to predict the residual strength of a joint after a period of fatigue loading once a single empirical constant has been determined.