Nondestructive Testing of Defects in Polymer–Matrix Composite Materials for Marine Applications Using Terahertz Waves

Fibreglass components employed in the marine environment are susceptible to moisture ingress, resulting in a degradation of mechanical properties. The nondestructive testing and evaluation of such materials using acoustic methods is possible under certain conditions, but the detection of internal voids and delaminations is masked by the presence of water in such internal flaws. Herein, we present an investigation into the use of terahertz technology to overcome these limitations, for the detection of damage and water ingress in thick woven glass-fibre composites. This investigation is facilitated by terahertz time-domain spectroscopy, coupled with physical raster scanning to realize object-penetrating imaging. Air-filled defects within the fibreglass volume are clearly identified using this technique. A spectroscopic investigation on the alteration of the terahertz-range dielectric properties of the fibreglass material due to water ingress is performed and a small change is measurable, although likely to be obfuscated by the interaction of terahertz-frequency radiation with the internal fibreglass structure. Moisture-ridden laminates do not impede propagation of terahertz radiation, and therefore wetted materials may be inspected for volumetric defects. A simulation of water-filled volumetric defects shows promise for practical application.