球状黒鉛鋳鉄の疲労き裂進展特性

抄録

The fatigue crack propagation characteristics at Stage 2 in spheroidal graphite cast iron (s.g. cast iron) have been investigated by fracture mechanics and fractography, and compared with those of steel having almost the same tensile strength. Fatigue tests were carried out at room temperature in laboratory air under stress ratio, R=0.1-0.8.
The relationship between fatigue crack propagation rate, da/dN, and stress intensity factor range, ΔK, in s.g. cast iron was divided into Stage 2a, 2b and 2c in the same manner as in general metallic materials. At Stage 2a, the oxide deposits produced by fretting of fracture surfaces were observed on the fracture surface regardless of the stress ratio. The amount of the deposits depended mainly on the roughness of fracture surface, and the da/dN at Stage 2a was lower than that in steel because the roughness-induced crack closure encouraged the oxide-induced crack closure. On the other hand, the da/dN at Stage 2b was faster than that in steel because of the lower Young's modulus. At Stage 2c, the main fracture pattern was cleavage fracture and the da/dN was accelerated.