Loading frequency effect on fatigue crack growth rate in low-pressure hydrogen gas environment in the case of 6061-T6 aluminum alloy

In order to clarify the loading frequency effect on fatigue crack growth rate in hydrogen gas environment, fatigue crack growth tests were carried out on a 6061-T6 aluminum alloy at several loading frequency levels in a pure dry hydrogen gas, in air and in a pure dry nitrogen gas. Hydrogen enhances the fatigue crack growth rate. Moreover, the growth rate increases as the loading frequency decreases not only in hydrogen but also in nitrogen. The main cause of the loading frequency effect relate to thermal activation in plastic zone at the tip of a crack growing across grains by slipoff. The loading frequency effect concerning hydrogen itself could not be revealed. There exists a transition loading frequency below which the crack growth rate is practically independent of loading frequency.