Characteristic Dependency of Hydrogen-Affected Fatigue Crack Growth and Crack Tip Plasticity on Low Loading Frequency in α-Iron

Aiming to investigate the dependency of hydrogen-affected fatigue crack growth (HAFCG) on the loading frequency f, this study experimentally characterized the HAFCG in a commercially pure iron as a function of f ranging from 0.02 to 20 Hz as well as a stress intensity factor range ΔK and hydrogen gas pressure $$ P_{{{\text{H}}_{2} }} $$. The crack tip plasticity was analyzed by interferometric microscopy and transmission electron microscopy in addition to the fracture surface observation. As a result, the amount of FCG rate acceleration by hydrogen increases as f decreases until it reaches a critical value (f = 2 Hz at $$ P_{{{\text{H}}_{2} }} $$ = 3.5 MPa, f = 0.2 Hz at $$ P_{{{\text{H}}_{2} }} $$ = 35 MPa), while the HAFCG rate greatly decreases once f decreases below the critical value. At the same time, the fracture mode at $$ P_{{{\text{H}}_{2} }} $$ = 3.5 MPa changes from brittle quasi-cleavage to a more ductile manner showing no clear hydrogen influence. On the other hand, the size of cleavage-like facets becomes large in case of $$ P_{{{\text{H}}_{2} }} $$ = 35 MPa suggesting that the number of lattice planes causing cleavage is reduced because of less hydrogen effect. The crack tip plasticity reduction by hydrogen is clearly confirmed while the HAFCG enhancement occurs above the critical value of f. In contrast, at f below the critical value, the crack tip plasticity is recovered to almost the same level as in the inert environment. Based on a theoretical estimation of hydrogen diffusion from the crack tip, the critical value of f causing this reversal f dependency is likely determined by the hydrogen concentration gradient in the vicinity of the crack tip. Although the associated mechanism is still unclear, it is suggested that the dynamic interaction between mobile dislocations and diffusing hydrogen atoms is important in clarifying the mechanism of HAFCG.