Department of Physics, Capital Normal University , Beijing 100037 CHINA

Cunlin_zhang@mail.cnu.edu.cn

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. The laser beam is split into pump and probe beams, the former being incident on a ptype InAs wafer, which produces the THz pulses and the latter is recombined with the THz signal after it is focused onto and returned from the sample. A pulsed system sends a short (typically less than one picosecond) transient onto or through a sample, and coherently records the resulting waveform, which can be analyzed in both time and frequency domains. The CW system, a Gunn diode oscillator and frequency multipliers are used as the source in the imager. The output THz radiation is focused by an (or a pair) aspherical (hyperbolic) polyethylene lens(es) on to the substrate of the target. The reflected beam is collected by the same lens and is steered into a Schottky diode detector by a high resistivity silicon wafer. The working distances are 8 inches for both 0.2 THz and 0.38 THz imager. CW imaging works with a single frequency, which in the absence of rapid tuning may inhibit spectroscopic imaging, but has the advantages that it is simpler to reduce into a compact imager and can be simpler to operate since the signal does not need to be sought in the time domain.Thermal wave imaging is another effective technique for evaluation defects of composite material, is based on the concept that after applying a uniform heat pulse to a sample surface, a localized disruption of the heat flow will occur when defects and/or flaws are present. The change in heat flow translates into temperature differences on the material surface, which can be used to create thermographic images in terms of either temperature difference or thermal diffusivity. In this study, thermal wave was employed to map the through-thickness thermal diffusivity of specimens.Thermal wave provides the required fully integrated thermographic inspection system. This system has been approved as a nondestructive inspection method for use on the carbon composite. We study defects in metal, carbon fiber, glass fiber, carbon silicon composite and so on. The two image technologies are discussed in terms of nondestructive testing applications to the defense and aerospace industries.

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