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 ductile cast iron (ADI), the other was a quenching and tempering treatment (QT). And, as cast material (As-cast) was added, and with these three materials fatigue clack 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 four or five kinds of humidity, 0,20,40,60,80%. The relationship between the characteristics of fatigue crack propagation and the crack closure produced on fracture surface was investigated. The obtained results were as follows. (1) In the low ΔK region, since the oxide-induced crack closure is generated, the propagation speed lowered, and ΔK_<th> increased. (2) In high ΔK region of QT and ADI, the fatigue crack propagation rate was accelerated by the effect of humidity. (3) ΔK_<th> of As-cast and QT were increased with the rise in humidity. (4) ΔK_<th> of the ADI was highest in the humidity 40% atmosphere. However, the factor of decreasing ΔK_<th> over the 40% couldn't be clarified.
