Terahertz frequency-domain characterization of UHMW-PE ballistic armor plates

Ultra-high molecular weight polyethylene (UHMW-PE) fiber-based ballistic armor plates are analyzed in the terahertz (THz) frequency region to better understand their structural properties. Various thickness plates are produced by hot pressing 5, 10 and 15 layer four-ply cross-weaved UHMW-PE fiber sheets whose refractive index and absorption characteristics are then examined for three different frequencies using time-domain imaging techniques. The obtained transmission images reveal significant structural information, including surface texture and potential defects, such as air bubbles or voids. It is found that with an increasing number of layers in each plate there is an increase in the refractive index approaching the value of the UHMW-PE fiber sheet. The change in the effective refractive index is attributed to inhomogeneities such as bunching up of the fibers as well as voids because of the manufacturing processes with a larger inhomogeneity in the 5-layer plates as compared to the 10-and 15-layer plates. Furthermore, a significant change in refractive index is observed for different orientations of the armor plate with respect to the input beam polarization due to the presence of periodic surface ridge formations. A detailed analysis of the extinction coefficient with respect to the refractive index shows groupings in the range of values for different thickness plates which can serve as an indicator of the inhomogeneities inside the structures. The purely frequency domain analysis performed here suggests that such methods in the THz region can offer a new, rapid way for non-destructive analysis of ballistic armor plates.