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, ΔK_red, to the active plastic zone. It is shown that the lower limit of the stress-intensity range transmitted to the active plastic zone, namely K^LL, is on the average 0.085 K_max 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 K^LL.

By making some rough assumptions about the opening stress-intensity factor, K_op, 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 K^LL 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.