Some components in a hydrogen energy system are subjected to cyclic stress at an extreme low frequency during their operation. Considering that the effect of hydrogen on the fatigue crack growth (FCG) tends to be significant as the frequency decreases, it is important to investigate the FCG properties at low test frequency in the presence of hydrogen. In order to evaluate the hydrogen effect on the FCG at low frequency in an effective manner, we developed a novel testing system with continuous circulation of hydrogen-charging solution into a pipe specimen.
It is known that the frequency dependence of FCG in hydrogen environment is strongly related to hydrogen diffusion behavior to the crack tip. However, in the testing with hydrogen gas to simulate the actual operating condition of hydrogen equipment, the frequency effect depends not only on the hydrogen diffusion inside the material but also on the external hydrogen absorption into the material, which complicates the situation. In contrast, in the newly developed method, hydrogen diffuses to the crack tip from the inside of material. Therefore, the relationship between the internal hydrogen diffusion and FCG can be studied.
In this study, the FCG tests with a carbon steel have been carried out by conducting continuous hydrogen-charging at room temperature and at an elevated temperature. The behavior of hydrogen diffusion to the crack tip under different testing temperatures and frequencies and its influence on the FCG were discussed.