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 t ransitory behavior by which the expression for the crack growth rate ( da/dt or a) as a function either of the C integral or of the load lin e displacement rate (delta) becomes anomalous: the data deviate from t he linearity describing a nose, thereby forming a part called the tail . In empirically analyzing this phenomenon, which has been taken to in dicate the extent of degradation of the material, the following observ ations are made for its formation mechanism and chronological progress for 1CrMoV steel and 2.25Cr-1Mo steel. The electrical potential metho d overestimates the.crack length while it is small. The times to attai n the minimum da/dt (Ta(min)) and that to the minimum delta (Tdelta(mi n)) remain invariable within the scope of this study, irrespective of the species and state of steel, temperature, specimen configuration, a nd load at Ta(min)/Tf = 0.1-0.2 and Tdelta(min)/Tf = 0.2 0.35. The tim e increase in transition from small-scale creep to extensive creep (Tt r), is less than Ta(min), so that the transition is completed before T a(min). The creep rupture lifetime (Tf) can be correlated quite well t o delta(min) in a Monkman-Grant expression as log(Tf) + c . log(delta( min)) = m. This observation is taken to indicate that prediction of Tf at an early stage should be possible.