Non-destructive inspection of UHMW-PE based ballistic composites in the THz frequency range

Ultra-high molecular weight polyethylene (UHMW-PE) fiber composites are widely used in ballistic armor plates due to their exceptional strength-to-weight ratio and low density. Despite their advantages, variations in hot-press manufacturing can introduce inhomogeneities such as voids, resin pockets, and surface ridges that compromise ballistic performance. In this study, terahertz (THz) frequency-domain imaging is applied for the non-destructive evaluation of UHMW-PE plates. Transmission measurements were performed on single-ply sheets as well as 5-, 10-, and 15-layer plates. Refractive index and extinction coefficient variations were extracted at different orientations of the THz beam polarization relative to surface ridges. Results indicate that while birefringence is not observed in single- and multi-layer plates due to ply configuration and effective averaging, layer-dependent changes in effective refractive index are evident. Furthermore, surface ridge formations induce measurable anisotropy in transmission, particularly at higher frequencies. These findings demonstrate that frequency-domain THz methods provide a rapid, robust, and non-destructive means of characterizing structural variations in fiber-based ballistic composites and may serve as an effective quality-control technique for optimizing armor manufacturing.