Abstract
The crack growth rates and threshold stress intensities,K TH, for a 3 1/2 NiCrMoV steel (0.2 pct proof stress 1200 MPa) have been measured in a hydrogen environment at various temperatures and hydrogen pressures. Fractographic evidence and the observation of alternating fast and slow crack growth nearK TH suggests that the crack advances by the repeated nucleation of microcracks at microstructural features ahead of the main crack. Transient crack growth is observed following load increases just belowK TH. Using the idea, from unstable cleavage fracture theory, that for fracture a critical stress must be exceeded over a critical distance ahead of the crack, and assuming that this critical stress is reduced in proportion to the local hydrogen concentration (in equilibrium with the external hydrogen atK TH), a theoretical dependence ofK TH on hydrogen pressure is derived which compares well with the experimental evidence.
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