Characterization of small crack growth in 12% CrMoV steel under high temperature, low cycle fatigue conditions

Abstract

Cylindrical specimens of 12% CrMoV steel were tested under high temperature, low cycle fatigue (HTLCF) conditions to investigate the growth of microcracks which nucleate at the surface. Results are presented which indicate that the microcracks maintain an equilibrium shape under HTLCF conditions, making it possible to estimate crack depth from a measure of the crack lenght along the surface. The individual growth of these microstructurally small cracks was monitored using a surface replication technique during periodic interruptions of the experiments. Crack growth rate data for different microcracks are compared on the basis of the cyclic J-integral ΔJ to examine the use of fracture mechanics for predicting HTLCF microcrack growth. A determination of ΔJ is shown to be successful in characterizing the growth of fatigue microcracks in 12% Cr steel at elevated temperatures despite differences in overall orientation at the surface. Good agreement with fatigue crack growth data for through-cracks in CrMoV steels at high temperatures further demonstrates the usefulness of ΔJ for predicting HTLCF microcrack growth. The results also suggest that large extrapolations of the observed correlation between ΔJ and crack growth rate could result in unacceptable errors owing to the relative contribution of oxidation damage at the crack tip.