Tests are used to verify the hypothesis “the fatigue crack growth (FCG) driving force is the effective stress intensity factor range ΔKeff″. The tests are performed measuring FCG rates in steel and aluminum C(T) and DC(T) specimens under fixed {ΔK, Kmax} loading conditions and during FCG delays induced by single overloads. Crack-opening loads Pop are redundantly measured along the crack path in all tests, using independent near and far field strain-gages, as well as Digital Image Correlation (DIC) techniques. Finally, elastoplastic strain loops within the reverse plastic zone ahead of crack tips are measured using a stereo microscope DIC system.