Abstract
The rotor axes in low pressure turbines and generators in service are occasionally 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 give 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 overloads on the fatigue damage in the crack initiation and growth stages was investigated in alternating two-load level fatigue tests. The base block consisted of axial reversed strain cycling Δep=0.5% as overloads and constant stress cycling σa=421MPa (above the fatigue strength) or 372MPa (below the fatigue strength) as a baseline 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 the crack 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.
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