Abstract: Fatigue test with a large number of underloading were performed. Fatigue crack growth delay after the action of underloading was found. The delay was correlated to the development of shear lips on the crack surface. It was found that in stage one (during underloading) and in stage two (after underloading) the effect of shear lips on fatigue crack growth was different. The effect of the shear lips was much stronger in stage two than in stage one. A calculation model of fatigue crack growth including the effe...