球状黒鉛鋳鉄の疲労特性に及ぼす黒鉛の効果

抄録

  High-strength high-toughness austempered spheroidal graphite cast iron has been put into practical use, and begun to be applied to parts requiring high-fatigue strength as an alternate material for forgings. The design of strength-requiring parts, on the other hand, has been changing to one using the parameters such as stress intensity factors and fatigue crack growth rates based on the fracture mechanics.
  However, the effect of graphite on the fatigue behavior of spheroidal graphite cast iron, especially on the fatigue crack growth rate are not clarified yet. Accordingly, a series of fatigue tests were carried out for 4 kinds of ferrite and pearlite base substrate spheroidal graphite cast iron with different graphite particle size and for ferrite base substrate SS 41 and pearlite base substrate SK 5 by changing stress ratios in three stages-i. e. 0.1, 0.3, and 0.5.
  The testing was performed in the second region of fatigue in which fatigue cracks grow stably. The relation between the fatigue crack growth rate (da/dN) and stress intensity factor range (ΔK) were examined and arranged by using the effective stress intensity facter range (ΔKeff) proposed by Elber. The dimensions of the plastic area generated at the ends of cracks were also measured to study the effects of the graphite particle size and substrate structure on the fatigue crack growth rate of spheroidal graphite cast iron.
  The results indicated that in the stress ratio ranging from 0.1 to 0.5, the effect of stress ratio in the case of spheroidal graphite cast iron could be explained by applying the concept of ΔKeff. It was also clarified that the crack growth rate of spheroidal graphite cast iron depended on the graphite particle size and the size of the plastic deformation area formed at the end of a crack namely, with the value of ΔK where the dimension of plastic area is equal to the particle size of graphite being the boundary, graphite reduces the crack growth rate like the key hole for stress relieving in the low ΔK region in which the plastic area is smaller than the graphite particle size, whereas graphite enhances brittle fracture to increase the crack growth rate in the high ΔK region in which the plastic area is greater than the graphite particle size.