2010 Volume 82 Issue 2 Pages 86-91
The purpose of this study is to investigate the effects of the distribution of graphite nodules on characteristics of fatigue crack propagation in spheroidal graphite cast iron where carbon content and the distribution of graphite nodules are changed. As specimens, spheroidal graphite cast irons with 3.0, 3.2, 3.4, 3.6 and 3.8mass% carbon content were produced respectively (C3.0%, C3.2%, C3.4%, C3.6% and C3.8% material). The mean diameter of the graphite nodules decreases as the carbon content increases, and the number of graphite nodules per unit area increases. The fatigue crack propagation test was conformed to ASTM standards. Stress ratio R was 0.1, and the specimen used was the 1CT type of 12.5mm in thickness. The test was carried out at room temperature atr the humidity of 40%. The relationship between the characteristics of fatigue crack propagation and crack closure produced on the fracture surface was also investigated.
Threshold stress intensity factor range, ΔKth was almost constant in C3.0 to C3.6% materials, but in the C3.8% material in which the graphite was close to each other, ΔKth decreased. In all materials, fatigue crack propagated connecting the graphite nodules. In the C3.8% material, the spacing between graphite nodules was small. Therefore, fatigue crack propagation rate increased in comparison with other materials, thereby decreasing the ΔKth of the C3.8% material as a result.
