Influence of gaseous environments on rates of near-threshold fatigue crack propagation in nicrmov steel
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INTRODUCTION
MOSTof the available fatigue crack propagation rate data have been generated in the Paris region; consequently, relatively little data are available on near-threshold crack propagation. However, many machine components are often subject to a low-stress and high-cycle fatigue condition, and data regarding the rates of near-threshold crack growth are of vital importance to assure the successful application of a fracture mechanics methodology in fatigue life prediction. Information in this region is also important is formulating a broader view of the mechanisms of fatigue crack growthparticularly for those aspects involving microstructural and environmental effects. The rate of near-threshold fatigue crack growth is more affected by mean stress, microstructure, and environment than crack propagation in the Paris region. Ho It has been reported that load ratios (R = Ormin/O'max) influence the near-threshold crack growth rate, da/dN, with increased R being accompanied by an increase in da/dN. ~'~Grain size strongly affected the near-threshold crack growth behavior in steels, titanium, and aluminum alloys, 2-1~while the crack growth rates in the Paris region were hardly influenced. In reference to the effects of environments on the rates of threshold crack growth, there is little understanding and contradictory results exist. Even though corrosive environments increase the Pads-region fatigue crack growth rates, it is not necessarily true in the near-threshold region. Previous results Lll revealed that a water environment indeed lessened, or had no effect on, threshold crack propagation rates in steels, as compared to ambient air. Nevertheless, with the elimination of the corrosive environments, nearthreshold crack propagation resistance was increased in vacuum for steels and aluminum alloys. 5-7,12-14 Further experiments are necessary to understand the behavior of near-threshold crack growth in corrosive environments. In general, hydrogen environments increase the rates of Pads-region fatigue crack propagation in steels. It has PETER K. LIAW is Senior Engineer with Westinghouse Research and Development Center, Materials Engineering Department. S.J. HUDAK, Jr. is Senior Scientist with Southwest Research Institute, Materials Sciences Department. J. KEITH DONALD is Vice President, Del Research Division of Professional Services Group Incorporated. Manuscript submitted November 25, 1981. METALLURGICALTRANSACTIONS A
been proposed that hydrogen embrittlement is the reason for promoting crack growth. However, relatively limited work on near-threshold crack growth behavior has been carried out in hydrogen environments. ~5-2~ In this investigation, near-threshold fatigue crack propagation experiments were conducted at 93 ~ with an NiCrMoV steel exposed to hydrogen, air, or argon. Crack growth rate data at various load ratios were generated by using an automatically decreasing AK technique. Possible mechanisms for the influence of hydrogen on the rates of near-threshold crack propagation are discussed. Previous
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