A modified Progressive Fatigue Damage Model (PFDM) is presented on the Shokrieh-Lessard model by replacing its fourth-degree stiffness degradation law with a cubic formulation. This modification corrects an unjustified increase in residual stiffness up to 60% of the fatigue life. A cubic degradation law was developed through further research into the early stages of fatigue life, introducing a continuous and physically consistent trend with the stiffness degradation that could reflect the multistage fatigue micro-mechanisms in cross-ply GFRP laminates. The resulting third- degree polynomial equation showed an acceptable statistical agreement with experimental measurements (R2 = 0.86), providing a revised stiffness degradation model applicable over the entire fatigue life. Modal analysis was performed on fatigued specimens before and after cycling. Modal analysis established the relationship between residual tensile elastic modulus and the first-mode natural frequency for different fatigue damage levels. The modified degradation model was implemented into the PFDM framework through a UMAT subroutine in ABAQUS to simulate natural frequency under the same boundary conditions used in modal tests. The difference between the simulated and measured natural frequencies remained within 2% for all data. This demonstrates that the modified PFDM, when coupled with modal analysis, could non- destructively predict the stiffness degradation and provide a practical approach for life prediction and structural-health monitoring of composite structures.

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