Mechanistic dissimilarities between environmentally influenced fatigue-crack propagation at near-threshold and higher growth rates in lower strength steels

The role of hydrogen gas in influencing fatigue-crack propagation is examined for several classes of lower strength pressure-vessel and piping steels. Based on measurements over a wide range of growth rates from 10⁻⁸ to 10⁻² mm/cycle, crack-propagation rates are found to be significantly higher in dehumidified gaseous hydrogen compared to moist air in two distinct regimes of crack growth, namely, at the intermediate range of growth typically above ∼10⁻⁵ mm/cycle and at the near-threshold region below ∼10⁻⁶ mm/cycle approaching lattice dimensions/cycle. Both effects are seen at maximum stress intensities K _max far below the sustained-load threshold stress intensity for hydrogen-assisted cracking K _1SCC. Characteristics of environmentally influenced fatigue-crack growth in each regime are shown to be markedly different with regard to fractography and the effect of such variables as load ratio and frequency. It is concluded that the primary mechanisms responsible for the influence of the environment in each regime are distinctly different. Whereas corrosion-fatigue behaviour at intermediate growth rates can be attributed to hydrogen embrittlement processes, the primary role of moist environments at near-threshold levels is shown to involve a contribution from enhanced crack closure due to the formation of crack-surface corrosion deposits at low load ratios.