Terahertz spectroscopy to study the orientation of glass fibres in reinforced plastics

We employ terahertz time-domain spectroscopy (THz TDS), a novel, non-destructive testing method, to study the fibre orientation and fibre content in reinforced plastics. The birefringent properties of plastics filled with differing amounts of short glass fibres are measured at frequencies from 100 GHz up to 1 THz. To predict the permittivity of the experimentally examined composite materials, we use an effective medium theory first introduced by Polder and van Santen. On the basis of the measured data and this model, we deduce the additive content ξ, the preferential orientation of the fibres φ and the fraction of orientated fibres a. Our findings agree well with corresponding mold flow simulations performed with commercially available software.