A study of adhesively bonded joints subjected to constant and variable amplitude fatigue

Double lap joints were tested under constant and variable amplitude loading. Palmgren–Miner’s rules were applied to predict the constant amplitude fatigue life using variable amplitude loading. These significantly overestimated the fatigue life, indicating a high crack growth acceleration due to the load transitions in the variable amplitude fatigue spectrum. Crack growth acceleration was mainly attributed to mean stress variations in the spectrum but overloads were shown to be important in crack initiation. A model incorporating a ‘cycle mix’ factor has been proposed as an improved method of predicting the fatigue life of bonded joints subjected to variable amplitude fatigue and this model has been shown to result in significantly improved fatigue lifetime predictions.