STP942

    Low Cycle Fatigue of Strain Aging Ferrous Alloys

    Published: Jan 1988


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    Abstract

    The fatigue behavior of two high purity Fe alloys, one with 75 ppm (by mass) of both N and C and the other with 0.1% (by mass) C, has been investigated at 5 Hz. These alloys were heat treated to retain different solute concentrations, and it was found that the effect of increasing solute concentration was to increase the fatigue life and to suppress the “knee” in the S-N curve. It is shown that the flat appearance of the S-N curve is due to dynamic strain aging of the alloys. The Coffin-Manson analysis of the data shows that dynamic strain aging hardly affects the fatigue ductility of the material but produces an increase in yield stress sufficient to increase the fatigue life.

    Fatigue predeformation and static aging at 60°C for 16 h produces a precipitation of metastable nitrides, or carbides, both at matrix sites and dislocations and an increase in the yield stress of the alloys. On retesting at the same stress amplitude, the plastic strain is found to have been considerably reduced and the fatigue life greatly increased, with a return of a “knee” in the S-N curve. The Coffin-Manson analysis reveals a great loss in fatigue ductility, but it is found that the cumulative strain to failure remains nearly constant at each stress level. This suggests that the influence of the increased strength by strain aging on fatigue life is more important than the reduction in ductility. The dislocation substructures developed at different amplitudes both before and after aging are correlated with the formation of slip bands and the nucleation of fatigue cracks.

    Keywords:

    fatigue properties, ferrous alloys, interstitial solutes, strain aging, dynamic strain aging, fatigue crack nucleation, dislocation substructures, increase in fatigue life


    Author Information:

    Pickard, SM
    Queen Mary College, London,

    Guiu, F
    Queen Mary College, London,

    Blackie, AP
    Queen Mary College, London,


    Paper ID: STP24521S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP24521S


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