Effects of hydrogen pressure, test frequency and test temperature on fatigue crack growth properties of low-carbon steel in gaseous hydrogen

Fatigue crack growth (FCG) tests for compact tension (CT) specimens of an annealed, low-carbon steel, JIS-SM490B were performed under various combinations of hydrogen pressures ranging from 0.1 to 90 MPa, test frequencies from 0.001 to 10 Hz and test temperatures of room temperature (RT), 363 K and 423 K. In the hydrogen pressures of 0.1, 0.7 and 10 MPa at RT, the FCG rate increased with a decrease in the test frequency; then, peaked out. In the lower test frequency regime, the FCG rate decreased and became nearly equivalent to the FCG rate in air. Also, in hydrogen pressure of 45 MPa at RT, the hydrogen-assisted FCG acceleration showed an upper limit around the test frequencies of 0.01 to 0.001 Hz. On the other hand, in the hydrogen pressure of 90 MPa at RT, the FCG rate monotonically increased with a decrease in the test frequency, and eventually the upper limit of FCG acceleration was not confirmed down to the test frequency of 0.001 Hz. In the hydrogen pressure of 0.7 MPa at the test frequency of 1 Hz and temperatures of 363 K and 423 K, the stress intensity factor range, ΔK, for the onset of the FCG acceleration in hydrogen gas was shifted to a higher ΔK with an increase in the test temperature. The laser-microscope observation at specimen surface revealed that the hydrogen-assisted FCG acceleration always accompanied a localization of plastic deformation near crack tip. These results infer that the influencing factor dominating the hydrogen-assisted FCG 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 hydrogen pressure, test frequency and test temperature can be unified by using a novel parameter representing the gradient of hydrogen concentration near crack tip.