1993 年 42 巻 473 号 p. 163-168
In order to study the physical basis of scatter in fatigue life, fatigue tests were carried out on the smooth specimens of a normalized 0.21% C steel. Fifteen specimens were fatigued at each stress level and the behavior of all cracks which led to the fracture of the specimens was examined. Fatigue data were analyzed statistically by assuming the Weibull distribution. The results show that the scatter in the number of cycles required to attain a crack length 0.2mm is particularly large. There is only slight scatter for the propagation life from 0.2mm up to fracture. The distribution relating to all phases of lifetime can be expressed by the three-parameter Weibull distribution. The values of the coefficient of variance, CV, for the initiation life of a crack length of 0.2mm and fatigue life increase with decreasing stress level. However, CV for crack growing from 0.2mm to failure shows only a slight dependency on the stress level. The scatter in fatigue life is due mainly to the behavior of microcracks. The influence of microstructure on microcrack propagation is especially large at lower stress levels, i.e. near the fatigue limit.