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 ci) as a function either of the C* integral or of the load line displacement rate (6) 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-IMo steel.
The electrical potential method overestimates thecrack length while it is small. The times to attain the minimum do/dt (IX,,) and that to the minimum 6 (TS,,) remain invariable within the scope of this study, irrespective of the species and state of steel, temperature, specimen configuration, and load at Td,,/Tf = 0.14.2 and TG,,/Tf = 0.2435. The time increase in transition from small-scale creep to extensive creep (Ttr), is less than Tri,, , so that the transition is completed before Tti,, The creep rupture lifetime (Tf) can be correlated quite well to 6,, in a MonkmanGrant expression as log( Tf ) + c . log@,& = m.
This observation is taken to indicate that prediction of Tf at an early stage should be possible.