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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