The stress intensity range ΔK is divided according to the sign of the plastic strain excursion. The parameter K_I^G is the stress intensity factor at which the transition from compressive to tensile strain occurs. K_I^G was determined for the alloy 2024-T3 to be 0.5 K_Imax. It is shown that loading conditions which produce only excursions in compressive stresses and strain in the crack tip region do not produce “stage II” fatigue crack growth. Furthermore, it is found that excursions in tensile stresses and strains have to exceed a certain range ΔK^T before fatigue crack growth occurs. ΔK^T so defined is denoted as threshold stress intensity range. The conclusion is reached that the following three elements have to be included in equations predicting “stage II” fatigue crack growth rates: (K_I^G - K_Imin) ΔK^T [(K_Imax - K_I^G) - ΔK^T]

An equation is proposed, containing the three elements, which predicts the observed fatigue behavior and growth rates quite well.