A Unified Equivalent Driving Force for Fatigue Crack Growth Analysis

ABSTRACT This paper challenges the use of the effective stress intensity factor range, Δ K eff = K max − K op , as a universal fatigue crack driving force, which has dominated fatigue crack growth research for more than five decades. The fundamental deficiency of the empirically based Δ K eff formulation is that it combines intrinsic and extrinsic parameters, thereby obscuring the true physical driving force governing fatigue crack propagation. In contrast, the proposed framework establishes a physically consistent intrinsic description of fatigue crack growth based on fundamental energy considerations and the principle of equivalent displacement. The resulting driving force inherently accounts for load‐ratio ( R ‐ratio) effects without relying on crack‐closure assumptions or closure measurements, thereby providing a unified and rigorous foundation for fatigue crack growth analysis across multiple length scales. Furthermore, the nondimensional ratio Δ K /Δ K 0 , evaluated at a constant crack‐growth rate (da/dN), provides insight into the identification of single or multiple crack‐growth mechanisms governed by the applied stress‐intensity range, Δ K , and/or load ratio, R . The validity and broad applicability of the framework are demonstrated using fatigue crack growth data for relevant engineering alloys widely used in structural applications.