等価欠陥寸法による疲労強度評価法の鋳造欠陥に対する適用限界

抄録

  To discuss the effects of macroscopic casting defects such as porosity on fatigue strength, we addressed the mechanical properties of a crack nucleated from a cavity. The stress intensity factor was calculated and its empirical formula was obtained for the circular ring crack around the spherical cavity. Finite element elastic analyses were carried out to obtain the stress intensity factor for various cavity and crack sizes. The stress intensity factor was formulated by a power function. From this formula, we clarified the effects of cavity radius and crack depth on the stress intensity factor, and also found the equivalence condition between the ring crack and simple circular crack. The fatigue limit prediction using the equivalent defect size is based on the equivalence condition, so if the equivalence is violated, the prediction cannot be applied. When the cavity radius exceeds about twelve times the depth of the ring crack, the equivalence never holds. In general, crack lengths initiated by fatigue do not exceed several ten micrometers, so the equivalence can hold up to the cavity radius of several hundred micrometers. This corresponds to the applicability limit of the prediction method, that is, the equivalent defect size of one millimeter.