In castings for machine structures subjected to severe cyclic loading, estimates of fatigue strength degradation by the casting surface is an important design concern. The casting surface decreases strength by its oxide layer, surface roughness, defects and other factors where the individual effects are not quantitatively understood. In this work, bending fatigue tests were performed on ferritic spheroidal graphite cast iron specimens with four kinds of surface states, as cast and with different degrees of machining, and the primary factor in reducing the strength is discussed. As cast specimens with a surface roughness of 170 μm Rmax showed fatigue strengths about 20% lower than fully machined specimens. Partially machined specimens showed strengths and lives scattered between the two. The SEM observations revealed that most of the fatigue originates from the bottom oj the surface roughness, and the ΔK value calculated form the area of the remaining oxide and inclusions at concavities correspond well to the fatigue strength and life. This indicates that the primary factor in reducing fatigue strength is the surface roughness, and that the fatigue life is mainly determined by the propagation condition of the initiated cracks. Application of Murakami 's root area concept with the extreme roughness depth determined statistically from the surface profile measurements gives a good estimate of fatigue limits of specimens with intact casting surfaces
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