Thermomechanical fatigue life assessment of polymer-matrix composites via entropy-based damage evolution and stiffness degradation under different frequencies

Abstract

This study assessed the fatigue performance of glass/epoxy composite by exploring the role of loading frequency (20–50 Hz) on fatigue strength, lifespan, and damage evolution. Implementing bilinear thermographic approaches ($\Delta T-\sigma$ and $\dot{q}-\sigma$) highlighted a remarkable fatigue strength reduction as frequency increased. At higher frequencies (40 Hz and 50 Hz), the discrepancies in fatigue strength values resulting from thermographic methods were more pronounced than at lower frequencies. The determined fatigue strengths at higher frequencies were then compared to those obtained from the standard $S-N$ curves as a reference. The analysis demonstrated the closer alignment of $\dot{q}-\sigma$ results with the reference $S-N$ curves. The unfeasibility of bilinear models at higher frequency under high-stress levels necessitated establishing a new trilinear $\dot{q}-\sigma$ model. The developed model aimed to assess the fracture fatigue entropy (FFE), alongside the entropy-based damage index (EDI) as a normalized indicator for damage evolution across low-, intermediate-, and high-cycle-fatigue regimes, facilitating the FFE-based $S-N$ curves validated with the experimental $S-N$ data. The new EDI-based $S-N$ curves were then established at different levels of damage accumulation. Damage evolution was captured via real-time acoustic emission (AE) monitoring synchronized with the registered thermal responses, enabling the identification of critical fatigue cycles, where rapid damage accumulation begins, alongside determining the boundaries that indicate abrupt failure. Correlating the AE-identified critical boundaries with the stiffness reduction enabled the establishment of $S-N$ curves based on various controlled degradation levels, bridging the knowledge gap and establishing a refined methodology for thermomechanical fatigue analysis of polymer-matrix composites (PMCs).