Prediction of Creep-fatigue Life by Use of Creep Rupture Ductility

Abstract

It was clarified that tension strain hold reduced creep-fatigue life of many engineering materials in different degrees depending on material, temperature and test duration. However the reduction in the life due to holding for various durations could be correlated to the fraction of intergranular facets on fracture surfaces which was considered to be an index of the damage introduced during strain hold. This fraction of intergranular facets by creep-fatigue failure exhibited a direct relation to the creep rupture ductility of the material tested at the same temperature and for the same creep-fatigue life-time.
From these results an empirical equation has been derived as follow;
Δεi/D_cN_h^α=C,
where Δεi is inelastic strain range, Dc is the creep rupture ductility for the same duration as creep-fatigue life time, N_h is the creep-fatigue life under tension strain hold conditions, and α and C are constants depending on the material and testing temperature. From the equation the life prediction is possible for a given inelastic strain range Δεi if the constants α and C, and D_c are known.
The value of α was found to be 0.62 and 0.74 for various austenitic stainless steels and NCF800 at 600°C and 700°C, respectively, and 0.69 for 1¹/₄Cr-1/2Mo steel at 600°C. The value of C was found to be 0.50 and 0.59 for various austenitic stainless steels and NCF800 at 600°C and 700°C, respectively, and 0.49 for 1¹/₄Cr-1/2Mo steel at 600°C. The creep rupture ductility D_c is available in the NRIM Creep Data Sheets up to 10⁵ h for multi-heats of many kinds of heat resistant alloys.