Abstract
In order to investigate the extremely slow fatigue crack propagation characteristics of JIS SCM440 CT specimen in 9MPa hydrogen gas environment, ΔK decreasing test by in-situ observation with microscope camera were carried out. Fatigue crack growth rate in hydrogen gas did not show saturating behavior but in air showed saturating behavior clearly. In addition, the fatigue crack growth rate in hydrogen gas showed oscillating phenomenon in the extremely low late region. The fatigue crack in hydrogen gas showed sudden increase in growth by coalescing with a new micro-crack initiated in front of crack tip. Fractographic analysis showed the existence of intergranular facets at the micro-crack. Intergranular facets are seen on the fracture surface tested in hydrogen gas, though the sizes and the area fraction decreases with decreasing the ΔK, while there is no facets in helium and in air in the extremely low late region.
