The hydrogen effect on crack growth behaviour in a type 304 austenitic stainless steel was investigated and the following results were obtained. The crack growth rate in hydrogen gas is accelerated compared with that in air. In order to clarify the mechanism of the acceleration, the growth behaviours of a crack propagating in a grain and propagating along the boundary to be a fracture facet were investigated. Slip behaviour, opening displacement and fractography showed that the slip-off mechanism in fatigue crack growth is valid even in hydrogen gas. Hydrogen mainly affects slip behaviour such that slip bands concentrate at a crack tip and result in acceleration of the growth rate. The facets are not significantly responsible for the acceleration. The ratio of facets to the entire area is low, and a crack nearly compensates for the temporary acceleration by the facets with subsequent deceleration.

In order to investigate the brittle-striation formation mechanism of a low carbon steel JIS S10C fatigued in a hydrogen gas environment, fractographic observations of the visualized fracture phenomena during some processes of brittle-striation formation were conducted. The following results were obtained. A striation line is formed during the loading part of the cycle as a trace of blunting by slip. A stable ductile crack then starts growing. These processes are similar to those during the normal ductile fracture from a crack; that is, a ductile tearing process in tension. Based on the experimental results, a brittle-striation formation model, in which hydrogen only enhances the microscopic ductile tearing process just ahead of a crack tip, was proposed. The model rationally explains the peculiar load-frequency effect in the quasi-cleavage range on the fatigue crack growth which reveals a lower growth rate in spite of lowering the load-frequency.

The effects of a hydrogen gas environment on the fatigue characteristics of a type 304 austenitic stainless steel were investigated and the following results were obtained. The hydrogen effect is not clearly seen by judging fatigue life diagram. However, crack initiation retards and crack propagation accelerates in hydrogen gas environment. The retardation seems to be caused by the absence of oxygen and water vapour. The acceleration seems to be caused by the intrinsic hydrogen effect.

Observations by Transmission Electron Microscopy and Electron Backscatter Diffraction corresponding to Fractography were conducted in order to investigate the brittle-striation formation mechanism of low carbon steel JIS S10C in a hydrogen gas environment. The main results are as follows: (1) The quasi-cleavage (QC) facets plane with brittle-striations does not coincide with the (100) cleavage plane. (2) Slip deformation distributions reflecting the brittle-striation formation processes are observed by TEM. One of the conceivable brittle-striation formation mechanisms which can explain these results is as follows. A striation line is formed during the loading part of the cycle as a trace of blunting by slips. A stable ductile crack then starts growing. These processes are similar to those in the normal ductile fracture from a crack; that is, a ductile tearing process in tension.