THz and Thermal Wave Imaging for Material Inspection

Abstract

As one of the major applications of THz imaging is in the field of non-destructive testing, we have applied it to an area of critical need: the testing of aerospace materials. Composite materials such as carbon fiber are widely used in this industry. The nature of their use requires technologies that are able to differentiate between safe and unsafe materials, either due to manufacturing tolerance or damage acquired while in use. In this paper, we discuss firstly the applicability of terahertz (THz) imaging systems to this purpose, focusing on graphite fiber composite materials, carbon silicon composite materials and so on. We apply THz imaging technology to evaluate fire damage to a variety of carbon fiber composite samples. Majority carbon fiber materials have polarization-dependent reflectivities in THz frequency range, and we show how the polarization dependence changes versus the burn damage level. Additionally, time domain information acquired through a THz time-domain spectroscopy (TDS) system provides further information with which to characterize the damage. We detect fuel tank insulation foam panel defects with pulse and CW terahertz system also. The THz TDS system begins with a Ti: Sapphire oscillator (Spectra-Physics Mai Tai), which produces pulses of 800 nm central wavelength, 80 fs duration, and 80 MHz repetition rate with an average power of 800 mW.