An experimental study of behavior of low alloy steels in the initial growth stage of creep cracks

In the early stage of creep crack growth, low alloy steels exhibit a transitory behavior by which the expression for the crack growth rate ( da dt or dot a) as a function either of the C^* integral or of the load line displacement rate (δ) becomes anomalous: the data deviate from the linearity describing a nose, thereby forming a part called the tail. In empirically analyzing this phenomenon, which has been taken to indicate the extent of degradation of the material, the following observations are made for its formation mechanism and chronological progress for 1CrMoV steel and 2.25Cr-1Mo steel. The electrical potential method overestimates the crack length while it is small. The times to attain the minimum da dt (T dot a_min) and that to the minimum δ (T δ ̇_min) remain invariable within the scope study, irrespective of the species and state of steel, temperature, specimen configuration, and load at T dot a_min Tf = 0.1-0.2 and T δ ̇_min Tf = 0.2-0.35. The time increase in transition from small-scale creep to extensive creep (Ttr), is less than T dot a_min, so that the transition is completed before T a ̇_min. The creep rupture lifetime (Tf) can be correlated quite well to dot δ_min in a Monkman-Grant expression as log(Tf) + c · log( dot δ_min) = m. This observation is taken to indicate that prediction of Tf an early stage should be possible.