Investigation of the mechanism for brittle striation formation in a low carbon steel fatigued in hydrogen gas (fractographic observation on fracture processes visualized by controlling load sequence and testing environment)

In order to investigate the brittle-striation formation mechanism of a low carbon steel JIS S10C fatigued in hydrogen gas environment, fractographic observations were conducted on the visualized fracture phenomena at some processes of brittle striation formation. The results showed as follows. A striation line was formed during the loading part of the cycle as a trace of blunting by slip. Then, stable ductile crack growth started. These processes were similar to those in the normal ductile fracture from a crack, that is, 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 is proposed. The model rationally explains the peculiar loadfrequency effect in quasi-cleavage range on the fatigue crack growth which reveals lower growth rate in spite of lowering the load-frequency.