The Multiaxial creep-fatigue failure mechanism of Mod. 9Cr-1Mo steel under non-proportional loading; Effect of strain energy on failure lives

Ogawa, Fumio*; Nakayama, Yuta*; Hiyoshi, Noritake*; Hashidate, Ryuta ; Wakai, Takashi ; Ito, Takamoto*

The strain energy-based life evaluation method of Mod. 9Cr-1Mo steel under non-proportional multiaxial creep-fatigue loading is proposed. Inelastic strain energy densities were calculated as the areas inside the hysteresis loops. The effect of mean-stress has been experimentally considered and the relationship between inelastic strain energy densities and creep-fatigue lives was investigated. It was found from the investigation of hysteresis loops, the decrease in maximum stress leads to prolonged failure life, while stress relaxation during strain holding causes strength reduction. The correction method of inelastic strain energy density was proposed considering the effect of maximum stress in hysteresis loop and minimum stress during strain holding, and strain energy densities for uniaxial and non-proportional multiaxial loading were obtained. Based on these results, the mechanisms governing creep-fatigue lives under non-proportional multiaxial loading have been discussed.