Effect of humidity on growth behavior of a fatigue crack of 7075-T6 extruded Al alloy

In order to investigate the effects of humidity change and loading frequency on growth behavior of a fatigue crack of an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out in relative humidity of 25% and 85% at frequencies of 50Hz and 6Hz. At 50Hz, macroscopic appearances of fractures were a shear mode at high stress levels in high humidity, though those were a tensile mode in other conditions of humidity and stress levels. The crack growth rate was accelerated by high humidity. By changing humidity, both of the growth rate and the growth mode of a crack were immediately changed to those corresponding to the changed humidity. That is, the growth behavior and fatigue life under humidity change may be estimated by those in constant humidity. Fatigue life in high humidity at 6Hz was longer than that at 50Hz in spite of accelerations of both of the crack initiation and its growth rate in the early growth process in high humidity at both frequency, though there was no or little influence of frequency on fatigue life in low humidity. Moreover, the crack propagated in the tensile mode even at high stress levels where the shear mode crack propagated at 50Hz. These results were explained by the effect of hydrogen on the crack growth behavior.