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    STP1020

    Gaseous-Environment Fatigue Crack Propagation Behavior of a Low-Alloy Steel

    Published: 01 January 1989


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    Abstract

    Fatigue crack growth behavior of 4340 steel was investigated in gaseous environments: laboratory air, hydrogen, and helium. No significant effect of specimen orientation on the rates of fatigue crack propagation was observed. In each environment, increasing load ratio decreased the threshold stress-intensity range (ΔKth). The effect of load ratio on ΔKth in the air environment was, however, much more significant than that in wet hydrogen and dry environments (dry hydrogen and dry helium). At low load ratios (≲ 0.5), the values of ΔKth in the air environment were larger than those in dry environments. The ΔKth values in wet hydrogen were in between those in air and dry environments. At a high load ratio of 0.8, ΔKth was, however, insensitive to test environment.

    The effects of load ratio and environment on fatigue crack growth rate properties are discussed in light of crack closure and hydrogen embrittlement mechanisms. While the effects of roughness-induced crack closure and hydrogen embrittlement on crack growth rates were found to be negligible, oxide-induced crack closure governed the kinetics of gaseous-environment, near-threshold crack propagation behavior. However, thick-oxide deposits in wet hydrogen did not cause high levels of crack closure.

    Keywords:

    fatigue crack growth, stress intensity, low alloy steel, crack closure, laboratory air, hydrogen, helium, hydrogen embrittlement, environments


    Author Information:

    Liaw, PK
    Westinghouse R&D Center, Pittsburgh, PA

    Leax, TR
    Westinghouse R&D Center, Pittsburgh, PA

    Donald, JK
    Director, Professional Service Group, Inc.Fracture Technology Associates, HellertownPleasant Valley, PAPA


    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP18845S