*3.3. E*ff*ect of Hydrogen Pressure*

The effect of hydrogen on the embrittlement of steels has received significant attention [34]. Recently, Adlakha and Solanki [35] analyzed this problem extensively using both atomic simulations and continuum-based models. They show that hydrogen segregation around the crack tip enhanced both dislocation emission and cleavage behavior. Figure 13 shows the discrete dislocation analysis of elastic-plastic cracks in the presence of hydrogen [14]. The results indicate that for a given μ, the ratio of lattice friction to applied stress, the slope of log(stress) vs. log(crack length) increases as the hydrogen pressure increases, due to the increased cleavage component, and the curve moves closer to the Griffith crack.

**Figure 13.** Effect of hydrogen pressure on crack growth for a given μ ratio. With the increase in H pressure, the lines move closer to the elastic Griffith crack behavior.

Experimental support for the analysis is provided in Figure 14, which is taken from Vehoff and Rothe [36], and shows that the cleavage component increases with increasing H-pressure, thereby reducing the crack mouth angle, α. Attached micrography shows the reduction in the crack mouth angle with increasing H-pressure. Further analysis (Figure 15) shows that the hydrogen effect saturates at high pressures. Thus, we have a negligible effect at shallow pressures and a saturation effect at high pressures. In the intermediate range, the presence of hydrogen reduces the mechanical driving force for crack growth, making the material more brittle. Experimental data are shown in Figure 16, taken from ref. [37], on the hydrogen embrittlement in alloy steels, which supports the above results.

**Figure 14.** Experimental results from Vehoff and Roth, 1983, on Fe-3%Si showing the effect of hydrogen pressure on crack growth. (**a**) With the increase in the hydrogen pressure cot(α) increases (α decreases) due to the increase in the cleavage component. (**b**) The associated micrograph indicates the crack mouth angle decreases when hydrogen pressure increases.

**Figure 15.** Results of discrete dislocation analysis showing that the stress-intensity factor of an incipient crack shows a saturation effect with H coverage.

**Figure 16.** Experimental results showing the saturation effect with the embrittling species on the crack growth threshold.
