Both crack initiation problem and crack propagation problem in creep fracture are studied. In the crack initiation problem, an applicability of some fracture parameter such as stress intensity factor (K), $C^*$ integral, net stress, and crack tip opening displacement (CTOD) as a creep crack initiation parameter is investigated. Crack initiation time is measured as a function of various crack initiation parameters. The crack tip blunting process under creep conditions is also investigated. In order to estimate the CTOD size a relation between the CTOD and a parameter Cb proposed here is proposed. In the crack propagation problem, the applicabilities of both the C integral and the CTOD rate as a fracture parameter which governs the crack growth behavior under extensive creep conditions are investigated. A modified $C^*$ integral, Cm, as a creep fracture parameter is also proposed considering the effect of crack growth. It is shown that the parameter does not depend on crack velocity.
By performing experiments using STS 304 stainless steel and STS 316 stainless steel at 600'C and 625'C respectively, the following results are obtained; (1) the CTOD size is well predicted by the CTOD-Cb relation; (2) crack initiation time for the stainless steels can be correlated with $C^*$ integral, however, the critical value of CTOD at the instant of crack initiation is not constant. A crack initiation model based on the variation of creep ductility with strain rate proposed here is shown to give reasonable agreement with the experiment; (3) creep crack growth rate of the stanless steels can be correlated with the $C^*$ integral, the CTOD rate, and the Cm parameter.