Terahertz nondestructive testing of alumina-based ceramic ballistic protection armor

We prepared an alumina-based ceramic ballistic protection armor sample consisting of hexagonal alumina tiles arranged in a honeycomb structure, glued with Kevlar (R), and covered with a polyurethane-based resin. The sample was shot with four projectiles, strongly hit from the back, and locally covered with water. The terahertz complex refractive index of the materials constituting the sample was measured. A terahertz time-domain spectroscopy (THz-TDS) setup in a reflection configuration was used to raster scan the XY sample. The results in the form of time signals, top views, and cross-sections enabled an understanding the internal structure of the sample. THz-TDS was proved to be suitable for non-destructive testing of the alumina-based armors with depthdiscriminant 3D imaging. Transfer matrix method (TMM) correctly simulated terahertz signals reflected from the sample, calculated the thickness of the sample layers, and help determine the type of hidden defects.