Abstract: The experiments on notched specimens of an Aluminium alloy indicat that the notch fatigue life can be increased by a tensile over-pre loading but decreased by a compressive over-pre loading. It is discovered, however, that the beneficial effect of tension lies below that of shot peening and even becomes weak as the level of over-pre loading is too high, and the detrimental effect of compression appears to be more pronounced for specimens with shot peening on the root surface. The shot peening induced increment of fatigue strength can be destroyed completely by compressive over-pre loading. The results are analysed based on the calculation of residual stress and the observation of dislocation configuration around the notch root area. The increase of fatigue life lies in the fact that the crack propagation rate is reduced greatly in a compressive residual stress field. The relaxation of residual stress an the damage in microstructure incorporated with the relaxation result in the decrease of fatigue life.

Key words: Aluminium alloy, notch fatigue, over-pre loading, residual stress