The crack tip opening displacement (CTOD), δp, in the plastic zone at a crack tip has been determined according to the dislocation-free zone (DFZ) theory. From this, the total CTOD, δT, can be obtained. It follows that the total CTOD range, ΔδTunder an applied cyclic stress can be achieved through a transposition from monotonic loading to cyclic loading. Under the assumption that when Δδr≥ Δδcrita fatigue crack can propagate, where Δδcritis the critical CTOD range which corresponds to the fatigue crack propagation threshold ΔKth, then a very simple relationship between ΔKthand the yield strength, σ, is obtained, namely