# Frequency–Direction Coupling in the Glass Transition Response of Thermally Aged Wet-Layup Unidirectional Carbon/Epoxy Composites

## TL;DR

The paper shows how thermal aging affects the glass transition response of carbon/epoxy composites, using multi-frequency DMTA to reveal changes in frequency sensitivity and directionality.

## Contribution

The study introduces frequency sensitivity and directional amplification parameters to quantify frequency-direction coupling in thermally aged composites.

## Key findings

- Thermal exposure alters the glass transition temperature, frequency sensitivity, and directional dependence in composites.
- Degradation at higher temperatures increases transverse frequency sensitivity, indicating matrix and interphase deterioration.
- Multi-frequency DMTA in both longitudinal and transverse directions provides mechanistic insights into thermo-oxidative effects.

## Abstract

Dynamic mechanical thermal analysis (DMTA) is widely used to assess the effects of process- and environment-induced changes in polymer matrix composites, with the glass transition temperature (Tg) often reported from the tan d peak at a single excitation frequency. However, such an approach neglects the inherently kinetic nature of the glass transition and may obscure thermally induced changes in relaxation response. Multi-frequency DMTA was employed to investigate the evolution of glass transition response of a wet-layup unidirectional carbon/epoxy composite subjected to thermal aging at temperatures ranging from 66 °C to 260 °C for periods up to 72 h, using unexposed (23 °C) results as an ambient baseline reference. Tests were conducted using a single cantilever mode in both longitudinal and transverse configurations over a range of excitation frequencies from 0.3 to 30 Hz. Results demonstrate that thermal exposure affects not only the absolute value of the glass transition temperature, but also its frequency sensitivity and directional dependence. A frequency sensitivity parameter and a directional amplification factor are introduced to quantify frequency–direction coupling. While post-cure-dominated aging regimes exhibit relatively stable coupling behavior, degradation-dominated conditions at elevated temperatures and longer periods of thermal exposure lead to pronounced increases in transverse frequency sensitivity, which reflects early evolution of matrix- and interphase-level deterioration. These findings highlight the value of multi-frequency DMTA with tests in both primary directions for the mechanistic assessment of effects of thermo-oxidative response in polymer matrix composites.