On the influence of environment on the load ratio dependence of fatigue thresholds in pressure vessel steel

A study has been made of the influence of load ratio $\text{R}$ on fatigue crack propagation behavior and specifically on the value of the fatigue crack growth threshold, Δ$\text{K}{}_0$, in a bainitic 2.25 Cr-1Mo pressure vessel steel tested at 50 Hz in aqueous, and moist and dry gaseous environments. Data are obtained for crack growth in a distilled water environment and are compared to previously published results in air and hydrogen. It is found that in distilled water the dependence of thresholds Δ$\text{K}{}_0$ values on $\text{R}$ is far less marked than in moist air and dry hydrogen atmospheres where Δ$\text{K}{}_0$ values decrease sharply with increasing $\text{R}$. Furthermore, whereas in air and hydrogen, the threshold condition is characterized by a constant maximum stress intensity at low load ratios, and a constant alternating stress intensity at high load ratios, no such behavior is observed in water. Based on extensive measurements of crack face oxidation products using scanning Auger speetroscopy and on previous crack closure measurements using ultrasonics techniques, the role of load ratio in influencing near-threshold fatigue behavior is ascribed to mechanisms of crack closure specifically plasticity-induced closure and closure arising from crack face oxide debris. The implications of such plasticity-induced and oxide-induced closure to the load ratio-dependence of near-threshold fatigue behavior in various environments are discussed in detail.