低圧タービンロータ材の両振引圧疲労における塑性ひずみ繰返しの影響

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The rotor axes in low pressure turbines and generators in service are occationary subjected to such a loading that a few overloads happen to be overlapped to the steady state repetition of low level stresses. Unless understanding the influence of overloads, therefore, the fatigue design of turbine rotors will not be completed. Such loadings supposedly given rise to an extreme acceleration of fatigue crack growth. In this paper, by employing the same material as usual low pressure turbine rotors', heat-treated 3.5Ni-Cr-Mo-V steel, the influence of ovweloads on the fatigue damage in the crack initiation and growth stages was investigaated in alternating two-laod level fatigue tests. The base block consisted of axial reversed strain cycling Δε_p=0.5% as overloads and constant stress cycling σ_a=421 Mpa (above the fatigue stregth) or 372 Mpa (below the fatigue strength) as a baseling loading. Contrasting to the previous investigation on an annealed S40C steel, the material exhibited cyclic softening in the low cycle fatigue test whereas the induced plastic strain during the low cycle fatigue stage disappeared gradually in the course of the following base line loading ; the application of overloads always reduced thecrack initiation life. The failure life was also reduced shorter than the predicted from Manson's double linear damage rule with using the fatigue properties of virgin material. The crack growth retardation, which may be induced by the application of overloads in the crack growth stage, was not observed on this material unlike on the previous carbon steel.
1985-09-15

低圧タービンロータ材の両振引圧疲労における塑性ひずみ繰返しの影響

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