Effect of Absorbed and Environmental Hydrogen on Short Fatigue Crack Propagation near Threshold in Low Alloy Steel

Abstract

The effects of hydrogen on fatigue crack propagation behavior of short fatigue crack and crack closure behavior were studied using low alloy steel SCM440H. Fatigue crack propagation test using specimen which has a 50µm deep pre-crack was conducted at a frequency of 28Hz in air, vacuum and hydrogen gas. Hydrogen pre-charge was done by cathodic polarization method. The concentration of absorbed hydrogen was 0.83 ppm in hydrogen charged specimen and 0.06 ppm in uncharged specimen. The fatigue crack propagation rate of uncharged material was dependent on environments. The acceleration of fatigue crack propagation rate by absorbed hydrogen was about 2 to 5 times irrespective of environment. As a consequence, the fatigue crack propagation rate of hydrogen pre-charged material tested in air was the highest. The crack opening stress was higher in hydrogen gas and in vacuum compared with that in air. Fatigue crack propagation rate was summarized in two independent bands when plotted against effective stress intensity factor range. This means that the effects of environment (vacuum, air and hydrogen) and stress ratio (R = -1, 0, 0.6) could be explained by crack closure phenomenon. However, the acceleration of crack propagation by absorbed hydrogen could not be explained only by crack closure phenomenon.