Multi‐Frequency Analysis of Glass Transition Behavior in Wet‐Layup Unidirectional Carbon/Epoxy Composites: Insights for Anisotropic Viscoelastic Behavior

This study investigates the time–temperature–frequency dependent viscoelastic response of ambient‐cured wet‐layup unidirectional carbon/epoxy composites using multi‐frequency dynamic mechanical thermal analysis (0.3–30 Hz). Specimens were exposed to elevated temperatures between 66°C and 260°C for durations up to 72 h prior to testing and were evaluated in both longitudinal and transverse directions. Three distinct regimes were identified: post‐cure dominated (66°C–121°C), transitional (149°C–204°C), and degradation dominated (232°C–260°C), reflecting competing crosslinking and thermo‐oxidative mechanisms. Glass transition temperature exhibited a dependence on frequency with sensitivity inversely related to apparent activation energy. Pronounced anisotropy was observed, with specimens tested in the transverse direction demonstrating higher sensitivity to early‐stage post‐curing and degradation compared to those tested in the longitudinal direction. The height of tan δ peak decreases progressively with exposure to temperature and time, showing an inverse trend with activation energy during post‐cure progression and increasing at higher temperatures and long durations, emphasizing higher sensitivity to matrix and interphase degradation. The results emphasize that single frequency characterization may be insufficient and may lead to non‐conservative estimates, whereas testing only in the conventional longitudinal direction may obscure the initiation of matrix‐dominated degradation, highlighting the need for multi‐frequency, direction‐dependent evaluation of thermally exposed composites.