Non-Destructive Testing of Glass-Fibre Reinforced Polymers using Terahertz Spectroscopy

We present THz time-domain spectroscopy measurements on glass-fibre reinforced polymers. The experimental set-up consists of a standard free-space THz spectrometer with photoconductive emitter and detector antennas driven by a femtosecond laser source. Single-point measurements to precisely obtain the THz properties of the sample are performed as well as two-dimensional images by scanning the samples perpendicular to the THz beam. We find that absorption coefficient and refractive index of the fibres significantly differ from that of the polymeric matrix material. Furthermore, the THz waves are strongly attenuated due to scattering off the fibres. The refractive index depends on the fibre concentration and orientation with respect to the linearly polarised THz radiation. With these results we can interpret THz images of a fibre reinforced polymer specimen. The THz images show the textures of the fibre network within the specimens and are compared with x-ray images.