Abstract: Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up- on the aging conditions.The naturally aged and underaged specimens showed the highest re- sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overaged specimen is the lowest and that of the peakaged specimen in between.As compared to air, 3.5% NaCl solution does not change the effect of aging conditions on the fatigue crack growth,but causes a decrease of the resistance to the fatigue crack growth under the same ag- ing condition.Both short cracks from notch and physically short cracks showed much higher rates of fatigue crack propagation in comparison with long cracks under the same aging condi- tion and stress intensity level.The growth behavior of the short crack depends on its type.The growth rate of short crack from notch decreases first to a minimum and then increases with in- creasing △K.However,the physically short crack grows at a progressively increasing rate. The effects of aging conditions are explained in terms of the slip planarity of dislocations and the cyclic slip reversibility.The observed short crack behavior is considered to be dependent on the crack closure and the local plasticity near the crack tip.

Key words: Al-Li alloy, fatigue crack, closure effect, slip planarity