Effects of hydrogen gas pressure and test frequency on fatigue crack growth properties of low carbon steel in 0.1-90 MPa hydrogen gas

In order to investigate the effects of hydrogen gas pressure pH2 and test frequency f on the fatigue crack growth (FCG) properties of low carbon steel in hydrogen gas atmosphere, the FCG tests using compact tension specimens were conducted under various combinations of pH2 = 0.1 ～ 90 MPa and f = 0.001 ～ 10 Hz. At pH2 of 0.1, 0.7 and 10 MPa, the FCG rate increased with a decrease in f and then peaked out. In the lower frequency regime, the FCG rate decelerated and became nearly equivalent to the FCG rate in air. Also at pH2 of 45 MPa, the hydrogen-induced acceleration showed an upper limit around f of 0.01 ～ 0.001 Hz. On the other hand, at pH2 of 90 MPa, the FCG rate monotonically increased with a decrease in f, and eventually the upper limit of FCG acceleration was not confirmed down to f = 0.001 Hz. The laser-microscope observation at specimen surface revealed that the hydrogen-induced acceleration always accompanies a localization of plastic deformation near crack tip, i.e. at a low test frequency, the slip localization at crack tip was not observed even in hydrogen gas. These results inferred that the influencing factor dominating the hydrogen-induced acceleration is not the presence or absence of hydrogen in material but is how hydrogen localizes near the crack tip. Namely, a steep gradient of hydrogen concentration can result in the slip localization at crack tip, which enhances the Hydrogen Enhanced Successive Fatigue Crack Growth (HESFCG) proposed by the authors. It is proposed that such a peculiar dependence of FCG rate on test frequency can be unified by using a novel parameter “(pH2∙f)1/2” which represents the gradient of hydrogen concentration near crack tip.