The Influence of Humidity on Fatigue Crack Propagation Properties in Spherical Graphite Cast Iron
Hyoe USAMI, Yoshihiro SUGIYAMA, Naoto SHIRAKI, Masayuki FUNAMOTO and Yoshihito KIMURA
Abstract: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. (1) In the low ĢK region, since the oxide inducement crack closure is generated, the propagation speed lowered, and ĢKth increased. (2) In high ĢK region of QT and ADI, the fatigue crack propagation speed was accelerated by the effect of humidity. (3) ĢKth of As-cast and QT were increased with the rise in humidity. (4) ĢKth of the ADI was highest in the humidity 40% atmosphere. However, it couldn't be clarified that the factor of decreasing ĢKth over the 40%. Key Words:Spherical graphite cast iron, Fatigue crack propagation, Threshold stress intensity factor range (ĢKth), Crack closure, Oxide inducement crack closure