A local load ratio approach to fatigue delamination growth simulation in CFRP aeronautical subcomponent

Fatigue delamination is a critical damage mechanism in carbon fibre reinforced polymer (CFRP) laminates subjected to cyclic loading. In most simulations, the so-called envelope load method is adopted to avoid modelling the full load oscillation within each fatigue cycle, applying only the peak load while introducing the applied load ratio (Rapplied=PminPmax) into the damage law. However, this global ratio does not fully capture the local stress variations developing along an evolving delamination front. This study introduces a numerical approach that evaluates the local load ratio directly at each crack-front node. The formulation extends the classical Paris-law approach by incorporating the ratio between the minimum and maximum local energy release rates, expressed as Rlocal=(RminRmax)0.5, and considering the mode-mixity of delamination progression. Here, Rlocal is defined based on the local energy release rates at the crack front, distinguishing it from the global applied load ratio Rapplied, commonly used in envelope-load simulations. The proposed method, named SMART LOOP, accurately captures the delaminated area independently of mesh size through an adaptive load step–time module. Numerical results have been validated against an experimental case from the literature involving a typical stiffened CFRP aerospace panel tested under compression–compression fatigue loading at different global load ratios. The proposed approach has provided a physically consistent description of fatigue damage accumulation in CFRP laminates and has demonstrated strong agreement with experimental trends, confirming its suitability for implementation within standard finite element frameworks for fatigue life assessment. Furthermore, the findings highlight the importance of accounting for the local load ratio when fatigue loading is applied at high global load ratios.