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Most rails in the track cannot avoid suffering defects and notches due to the contact rolling load of the wheels. These rails are often found to be broken by fatigue cracks originating from these defects.The most part of the fatigue lives of the members having sharp defects or notches are occupied by the propagating stage of fatigue cracks.Then, by using the specimens cut out from the experimentally melted rail steels (thick plates) and many kinds of rails, the characteristics of the fatigue crack propagation rate and the effect of the metallurgical structures on them were studied.Especially, the effect of residual stress on the crack propagation rate was investigated, because residual stress is inevitably produced in such a shapes as rails.The main results obtained are as follows;(1) The fatigue crack propagation rate is little affected by the chemical composition, pearlite block size and mechanical properties if the stress ratio is held constant. stress ratio.In addition, the values of C and m can be evaluated immediately from the case of the zero stress ratio.(3) The variation of the crack propagation rate at the constant stress ratio disappears by the residual stress in the specimens.

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It is very difficult to monitor fatigue crack initiation at all parts of machine equipment day after day. In this study, a method of continuous monitoring for fatigue crack initiation is examined which uses the strain waveform of running machine. A notched section of the equipment is taken as the inspection point because many fracture accidents originate in fatigue cracks initiated from such a notch. Fatigue crack initiation can easily be detected by observation of the waveform of a strain function composed of the strains in the vicinity of the notch, because it changes shape on crack initiation. This change of the waveform is brought about by the change of the compliance of the material due to the crack closure behavior. It is found that crack initiation detection is improved by spectral analysis of the waveform.

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