On the influence of fatigue underloads on cyclic crack growth at low stress intensities

The influence of variable-amplitude loading in the form of block underload cycling is examined for (plane strain) near-threshold fatigue crack propagation in a bainitic 2.25Cr-1Mo pressure vessel steel (ASTM A542 class 3), tested in ambient temperature moist air at low load ratios (R = 0.05). It is shown that the application of fatigue underloads with stress intensity ranges below the threshold ΔK0 can result in significant transient retardations in initial growth rates when cycling is subsequently resumed at baseline levels above the threshold. The magnitudes of the retardation are found to be dependent on the baseline ΔK B and underload ΔK u stress intensities. Using electron spectroscopy for chemical analysis and scanning Auger spectroscopy, the observations are shown to be consistent with concepts of oxide-induced crack closure, which arises from extra corrosion debris (Fe2O3) generated within the near-threshold crack during the underload cycling period. Such oxide deposits, which are of the order of pulsating crack-tip-opening displacements ΔCTOD, can lead to increased closure loads and reduced effective stress intensity ranges, thereby retarding subsequent initial growth rates at baseline levels. It is shown that sub-threshold fatigue underloads have negligible effects on baseline near-threshold growth rates when the underload ΔCTOD appears to be smaller than the existing (baseline) excess oxide thickness. In this instance the crack remains effectively closed during underload cycling, thus restricting further oxide growth. The relevance of these observations to current procedures for the determination of threshold values and slow crack growth rates at low stress intensities is discussed in detail.