Small fatigue crack closure is observed as a thing with a possibility that the influence of change of the stress level and a stress ratio, etc. to the crack growth behavior can be explained, and until now various researches have been done. Although there is a method using the strain gage as the dynamic measurement method required for the elucidation of a minute fatigue crack, in respect of generating of minute fatigue cracks being directly contacted to a measuring object thing, and there is a problem in durability or measurement of the crack growth behavior, it may not necessarily be effective. Then, its attention was paid to the magnetostriction effect which is one of the magnetic actions of a ferromagnetic substance. Measurement of the opening stress of the minute fatigue crack under dynamic load and crack length performed by measuring the magnetic field change by the magnetostriction effect, without needing the contact to an object with simple equipment. Moreover, it measured by having changed cross-sectional shape of the specimen. Resulting in, opening stress of the minute fatigue crack opening stress was able to measure about specimen regardless of the cross-sectional shape of the specimen. However, at this stage, it is possible to perform a crack length measurement can only specimen having a circular cross-section.
