Detailed non-destructive evaluation of UHMWPE composites in the terahertz range

We report on the terahertz analysis of an internal structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material, which is based on the HB10-tape from Dyneema. This type of composite is very hard and resistant and therefore it is often used to manufacture personal armors such as bulletproof vests and helmets. The multilayer structure of the UHMWPE composite was investigated by means of a raster scanning time domain spectroscopy technique in a reflection configuration. The mechanism of the formation of many shifted in time THz pulses (reflected from the internal layers of the sample) originates from the periodic modulation of the refractive index along the propagation of the radiation. This modulation is connected with alternate layers of fibers, each having different direction (perpendicular to each other). As a result we obtained the detailed three dimensional profile of the 3.3-mm thick sample with all 74 layers clearly visible. Thicknesses of all layers, having around 45 m each, were determined. Moreover, it is also possible to identify internal defects i.e. delaminations in the internal structure of this composite material.