Studies on crack growth rate under high temperature creep, fatigue and creep-fatigue interaction-II. On the role of fracture mechanics parameter a_effσ_g

For high temperature creep, fatigue and creep-fatigue interaction, several authors have recently attempted to express crack growth rate in terms of stress intensity factor K_I = α aσ_g, where a is the equivalent crack length as the sum of the initial notch length a₀ and the actual crack length a^*, that is, a = a₀ + a^*. On the other hand, it has been shown by Yokobori and Konosu that under the large scale yielding condition, the local stress distribution near the notch tip is given by the fracture mechanics parameter of aσ_gf{hook}(σ_g), where a is the cycloidal notch length, σ_g is the gross section stress and f{hook}(σ_g) is a function of σ_g. Furthermore, when the crack growth from the initial notch is concerned, it is more reasonable to use the effective crack length a_eff taking into account of the effect of the initial notch instead of the equivalent crack length a. Thus we believe mathematical formula for the crack growth rate under high temperature creep, fatigue and creep-fatigue interaction conditions may be expressed at least in principle as function of a_effσ_g, σ_g and temperature. In the present paper, the geometrical change of notch shape from the instant of load application was continuously observed during the tests without interruption under high temperature creep, fatigue and creep-fatigue interaction conditions. Also, the effective crack length a_eff was calculated by the finite element method for the accurate estimation of local stress distribution near the tip of the crack initiated from the initial notch root. Furthermore, experimental data on crack growth rates previously obtained are analysed in terms of the parameter of a_eff σ_g with gross section stresses and temperatures as parameters, respectively.