Fig. 1. (a) Schematic of the SENT specimen (unit: mm); (b) an exemplary image showing the pre-crack grown from the notch root (from IA700); (c) and (d) the pre-crack tip indicated by white arrows and the surrounding microstructure for IA555 and IA700, respectively.

Fig. 2. Fatigue crack growth rate (FCGR) diagram of the tested IA555 and IA700 specimens in Vac and H conditions. Fitted lines are shown as solid and dashed lines for IA700 and IA555, respectively. Black lines show the data in vacuum condition and red lines show the data in hydrogen-charged condition. (digital version in color).

Fig. 3. Crack morphologies at a low ΔK level: (a, b): IA555 crack grown in Vac and H, respectively; (c, d): IA700 crack grown in Vac and H, respectively. The magnifications for c and d are NOT the same. The yellow arrows indicate the crack growth path for the last loading of 100 cycles. The information in the figures shows the environmental condition, ΔK level, crack growth length and the material. (digital version in color).

Fig. 4. Long-term crack morphology observation of IA555. The crack growth data from the presented region is shown in the bar chart. The global FCG direction is from left to right. The figures are stitched to show the crack growth history.

Fig. 5. Long-term crack morphology observation of IA700. The crack growth data from the presented region is shown in the bar chart. The global FCG direction is from left to right. The figures are stitched to show the crack growth history.

Fig. 6. Fractographs in an overview of the tested specimens: (a) IA555; (b) IA700. The global FCG direction is from left to right. The white arrows indicate the fracture features in alternative Vac and H environments. The yellow arrows indicate the dimple/ void structures. (a1) and (b1) are showing the typical magnified features in the regions fractured in the Vac environment of a and b, respectively. (a2) and (b2) are showing the typical magnified features in the regions fractured in the H environment of (a) and (b), respectively. (TG: transgranular; IG: intergranular; “QC”: “quasi-cleavage”) (digital version in color).

Fig. 7. Fractography of the IA555 specimen. (a) in Vac; (b) in H; (c) and (d) are the magnified areas marked in (a) and (b), respectively. The global FCG direction is from left to right. The environmental condition and the ΔK level are marked accordingly. (MV: microvoid; TG: transgranular) (digital version in color).

Fig. 8. Fractography of the IA700 specimen. (a) in Vac; (b) in H; (c) and (d) are the magnified areas marked in (a) and (b), respectively. The global FCG direction is from left to right. The environmental condition and the ΔK level are marked accordingly. More proof of the boundary-like fracture features can be found in supplementary files. (MV: microvoid; TG: transgranular; IG: intergranular) (digital version in color).

Fig. 9. ECC images of the areas immediately connecting the crack growth path in the IA555 specimen. (a) and (b) overview in Vac and H, respectively. (c) and (d) are the magnified view of (a) and (b), respectively. The global FCG direction is from top to bottom. The global ΔK level is about 26 MPa m1/2 for all subfigures.

Fig. 10. ECC images of the areas immediately connecting the crack growth path in the IA700 specimen. (a) and (b) Overviews in Vac and H, respectively. (c) and (d) are the magnified view of (a) and (b), respectively. The global FCG direction is from top to bottom. The global ΔK level is about 20 MPa m1/2 for all subfigures.

Fig. 11. Proposed FCG mechanism of the studied medium-Mn steels in different environmental conditions.