Influence of Chunky Graphite on Fatigue Crack Propagation Characteristics in Heavy Section Spheroidal Graphite Cast Iron

Abstract

The purpose of this study is to investigate the influence of chunky graphite on the characteristics of fatigue crack propagation in heavy section spheroidal graphite cast iron. Samples containing and not containing chunky graphite were cut from a large ingot of spheroidal graphite cast iron. The fatigue crack propagation test conformed to ASTM. Stress ratio R was 0.1, and the specimen used was of the 1CT type with a thickness of 12.5 mm, and humidity RH = 0%.In the samples containing chunky graphite, the slope of the linear approximation m at the IIb stage of fatigue crack propagation becomes larger and fatigue crack growth rate becomes faster than the spheroidal graphite samples. The reason seems to be that crack propagates along the graphite connected to the chunky graphite. The decrease in the threshold stress intensity factor range ¦K th of the chunky graphite samples was smaller than the decrease in tensile strength. In the chunky graphite samples, the fracture surface roughness became large and the crack closure induced by fracture roughness occurred conspicuously. The threshold stress intensity factor range decreased due to the influence of the chunky graphite. However, it was found that threshold stress intensity factor range does not decrease as much as the tensile strength due to the effect of the crack closure induced by fractured surface roughness.