An experimental study of the dynamic molecular state of transient moisture in damaged polymer composites

Abstract Polymeric composites absorb measurable amounts of moisture from their environment in almost all operating conditions. This absorbed moisture exists either in the “free” state, without any interactions, or in the “bound” state—interacting with the polymer matrix via secondary bonding mechanisms. The ratio and distribution of these water states within a moisture‐contaminated polymer composite are sensitive to physical damage. However, the water state distribution is also affected by variations in total water content resulting from humidity or precipitation‐driven fluctuations in the ambient environment, which could confound the ability to detect damage within the polymer matrix using this technique. To understand the effect of moisture content variations on water state distribution, low levels of barely visible impact damage were induced in epoxy/glass fiber composites. Spatial variations in polymer–water interactions were identified by their characteristic dielectric properties, measured using a split post dielectric resonator operating at 5 GHz. Gravimetric moisture uptake and relative permittivity were monitored during the absorption and desorption processes. Results indicate moisture absorption/desorption history has a significant effect on the sensitivity of damage detection using water state variations. Damage‐dependent hysteresis was observed in relative permittivity, highlighting an avenue by which the confounding effects of moisture absorption/desorption history may be mitigated.