1989 年 38 巻 432 号 p. 1053-1059
The correlation between tensile strength and low cycle fatigue life at elevated temperatures was discussed on a cast Ni base supper alloy (16Cr-3Al-3Ti-9Co-Ni) and a Co base super alloy (11Ni-29Cr-7W-Co). Also, an equation for predicting low cycle fatigue life was developed.
In the low cycle fatigue tests, the Co base super alloy showed similar fracture behavior in a wide range of temperature. However, the Ni base super alloy showed brittle fracture behavior at low temperatures (ex. 500°C) because of poor ductility, but showed ductile fracture behavior at high temperatures (ex. 900°C) because of high ductility. In the Ni base super alloy, the failure life was defined as the number of cycles when the nominal stress value of a specimen began to decrease from a steady state stress. Tensile and low cycle fatigue data obtained at 500-900°C for the Ni base super alloy and 600-900°C for the Co base super alloy were analyzed by the stepwise multiple regression procedure, and the effect of tensile strength characteristics on the low cycle fatigue strength was discussed. The results of this analysis showed that the relationship between the plastic strain range (Δεp) and cycles to failure (Nf) was expressed by a function of εf, while the relationships between the elastic strain range (Δεe) and cycles to failure was given by a function of (σ0.2/E) for both super alloys in a wide range of high temperatures, unlike the results of the Universal Slope method. The life prediction equation obtained was as follows:
Δε=Δεe+Δεp=A(σ0.2/E)lNkf+BεnfNmf
where A, B, k, l, m and n are material constants that are independent of temperature.