Redundant data obtained under quasi-constant {ΔK, Kmax} loading conditions is used to verify if the effective stress intensity factor (SIF) range ΔKeff = Kmax − Kop is indeed the fatigue crack driving force. The crack opening SIF Kop is measured along the entire crack path on DC(T) low carbon steel specimens by a series of strain gages bonded along the crack paths, by a strain gage bonded on their back faces, and by a digital image correlation technique. All such measurements showed a significant Kop decrease as the crack sizes increased, while the fatigue crack growth rates remained essentially constant both in the thin and thick specimens, a behavior that cannot be explained by ΔKeff arguments.