Abstract
The multilayer structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material was investigated in the terahertz (THz) spectral range by means of time-domain spectroscopy (TDS) technique. Such structures consist of many alternating layers of fibers, each being perpendicular to the other and each having a thickness of about 50 μm. Refractive indices of two composite samples and of a sample composed of four single layers (plies) having the same fiber orientation were determined for two orthogonal orientations of the electric field in a transmission TDS system. The birefringence of a single layer was measured, and the origin of this phenomenon is discussed. Using the TDS system in reflection, the formation of many pulses shifted in time was observed originating from reflections from interfaces of successive layers caused by the periodic modulation of the refractive index along the propagation of the THz radiation. This phenomenon is theoretically described and simulated by means of a transfer matrix method (TMM). A time-domain fitting procedure was used to determine thicknesses of all layers of the composite material. The reconstructed waveform based on the optimized thicknesses shows very good agreement with the measured waveform, with typical differences between measurements and simulations between 3 and 7 μm (depending on the sample). As a result, we were able to determine the thicknesses of all layers of two multilayer (~200 plies) structures by means of the reflection TDS technology with high accuracy.
Highlights
Terahertz (THz) radiation in the range between 0.1 and 3 THz has been used to analyze the internal structure of a large number of materials and elements
Krimi et al have recently proposed a time-domain fitting procedure for a structure composed of few thin layers of different thickness, useful in inspection of automotive paints [22]. This approach is based on the transfer matrix method (TMM) and requires prior knowledge of the material parameters to determine the thicknesses of the layers
To describe the interaction between the polarized THz radiation and the ultra-high molecular weight polyethylene (UHMWPE) fiber, we utilized the Polder and van Santen approach [32], adopted to the THz range by Jordens et al
Summary
Terahertz (THz) radiation in the range between 0.1 and 3 THz has been used to analyze the internal structure of a large number of materials and elements. For thin layers, roughly about 50 μm, pulses reflected from adjacent layers overlap and cause errors in determining thicknesses of consecutive layers To solve this problem, van Mechelen et al [21] assume that the investigated material that composed of a few layers is a stratified system, and for each layer, both its thickness and its refractive index are optimized. Krimi et al have recently proposed a time-domain fitting procedure for a structure composed of few thin layers of different thickness, useful in inspection of automotive paints [22] This approach is based on the transfer matrix method (TMM) and requires prior knowledge of the material parameters to determine the thicknesses of the layers.
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