Unified stiffness-strength degradation model for fatigue reliability assessment of polymer matrix composite laminates

Existing linear fatigue cumulative damage (FCD) models for laminates under variable amplitude cyclic loading (VACL) exhibit poor life prediction accuracy due to neglecting the nonlinear damage-rate evolution and the distinction between stiffness and strength degradation, which impedes the reliability assessment of polymer matrix composite (PMC) laminates. This study proposes a fatigue reliability prediction framework addressing these limitations. By considering the different contributions to the FCD under different loading sequences, a loading sequence correction coefficient (LSCC) is introduced into the life prediction framework under VACL conditions. Validation for Gr/PEEK laminates under five typical VACL conditions shows lifespan errors ranging from 3.08 % to 15.4 %. Furthermore, a unified degradation model is proposed, leveraging the intrinsic link between stiffness and strength under equivalent damage, avoiding expensive strength degradation tests. The model shows high correlation (R² > 0.97) for AS-4/PEEK laminates with various lay-up sequences. Finally, a fatigue reliability model that incorporates the fluctuations in material properties and load amplitudes is provided. The 3σ distribution bounds of the fatigue life are validated by published experimental data for GF/epoxy and Gr/PEEK laminates, respectively. This life prediction framework provides a fast and economic evaluation tool for fatigue reliability-based conceptual design of PMC laminates in aircraft, shipbuilding, and vehicle fields.