In order to investigate the effects of aging structures and humidity on fatigue properties of an extruded bar of age-hardened Al alloy 7075, rotating bending fatigue tests were carried out on T62, T73 and RRA (retrogression and reaging) treated plain specimens in two relative humidity of 25% and 85%. Fatigue strength in low humidity was higher in the order of the RRA, T62 and T73 treated specimens corresponding to the order of static strength. On the other hand, fatigue strength decreased in high humidity in all of the alloys and the decrease was the highest in the RRA treated specimen, which means that the RRA treated alloy is very sensitive to humidity environment. In case of the T73 treated alloy, cracks propagated in shear mode in high humidity and under high stress levels due to the marked texture of the alloy, though tensile mode crack growth is more common in the alloy. Similar crack growth behavior was found in the T62 treated alloy, too. In case of the RRA treated alloy, however, the tendency for a crack to grow in the shear mode was very weak even in high humidity and under high stress levels in spite that the alloy had the same marked texture. The effects of microstructure and humidity on the initiation and propagation of small cracks were marked in all of the alloys.
