The purpose of this study is to evaluate the effect of mean stress and casting defect on fatigue strength of Ni based castalloys, 246 and 100, using ultrasonic fatigue tests, which were conducted under stress ratios, R = -1, 0 and 0.3, at a cyclic frequency of 20 kHz. The results revealed that fatigue strength decreased by subjecting mean stress. SEM observation for the 246 and 100 alloys near the fracture origin exhibited that cracks initiated from a casting defect near the specimen surface for the majority of the specimens, where a flat fracture surface was formed in the vicinity of the defects. Also conducted were ultrasonic fatigue tests for shot-peened specimens under R = 0 in order to change the location of the fracture origin. The results indicated that the influence of the casting defect near the specimen surface became negligible in the very high cycle fatigue regime. Stress intensity factor range for the casting defect, ΔKCD, which was calculated by the axial projected area of the defect and the stress range, correlated with the fatigue life, Nf. The influence of mean stress and the casting defect was evaluated by the relationship between Nf and the equivalent stress intensity factor range, ΔKeq, calculated by taking account of mean stress. The result suggested that the effect of casting defect could be expressed uniformly by the evaluation of ΔKeq-Nf relationship.
