Fatigue crack growth rates are generally correlated to the excursion of the stress-intensity factor, ΔK The presently active plastic zone ahead of the crack does not experience the total excursion of the calculated stress intensity for low R-ratios. The lower part of ΔK, for loading conditions with R ≦ 0.35, is cut off due to the influence of the plastic wake zone. The result is the transmission of a reduced stress-intensity range, ΔKred, to the active plastic zone. It is shown that the lower limit of the stress-intensity range transmitted to the active plastic zone, namely KLL, is on the average 0.085 Kmax for the materials and fatigue conditions used. Microstructural features associated with the formation of shear-lips and small flaws in the plane-strain region of the specimen can cause a large rise of KLL.
By making some rough assumptions about the opening stress-intensity factor, Kop, it is shown that there must occur substantial excursions of compressive strains and stresses in the active plastic zone for cycles with low R-ratios.
Arguments are presented that show that the parameter KLL can explain the continuing retardation as a crack propagates through the plastic overload zone.
It is shown that the load to open or close a fatigue crack at certain positions behind the crack front decreases with increasing distance from the crack front.