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The fatigue crack propagation test was carried out in order to clarify the effect of humidity on the characteristics of fatigue crack propagation in spherical graphite cast iron. Spherical graphite cast iron (FCD450) was used as a specimen. Then, two kinds of heat treatments were conducted to the material. One was an austemper treatment (ADI), the other was a quenching and tempering treatment (QT). Moreover, as cast material was added, and with these three materials fatigue crack propagation test was carried out. The experiment conformed to ASTM. Stress ratio R was 0.1, and specimen used was 1CT type, 12.5mm thick. The test was carried out at room temperature and under three or four kinds of humidity, 0, 20, 40, 60, 80%. The relationship between the characteristics of fatigue crack propagation and the crack closure which produced on fracture surface, was investigated. The obtained results were as follows.

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