Open reflection microwave resonator sensors have been developed for in situ measurement of the bulk moisture absorbed by nonconducting fiber-reinforced composites. These contacting sensors operate at about 4 GHz, and have sensitivities and resolutions of about 0.1% moisture. In application, two independent parameters are measured simultaneously. From these, the two components of the effective complex dielectric constant ε* (ε′= - јε″) as seen by the sensor are determined. Both ε′ and ε″ are highly linear with absorbed moisture.
Both linearly polarized sensors (for measuring anisotropie moisture absorption) and circularly polarized sensors (for measuring the average anisotropy) have been demonstrated. Moisture uptake in the polymer matrix, moisture wicking along Kevlar fibers, and the effects of moisture damage due to blistering, microcracking, etc., all have been tracked in accelerated moisture exposure tests. Where applicable, the experimentally measured profiles have been validated using a one-dimensional Fickian diffusion model.
The sensors are suitable for many applications, e.g., to map the extent of moisture contamination in composite panels, as a portable probe for aid in composite panel repairs, as a cure monitor of moisture in powder or fluid polymers, or as a permanently installed sensor for monitoring the accumulated moisture in radomes. In the latter case, exceptionally improved sensitivities and resolutions are possible.