Damage and defect inspection with terahertz waves

We present recent results in applying pulsed and continuous-wave (CW) terahertz (THz) imaging and sensing techniques on damage and defect assessment in insulating foam and carbon fiber materials. The comparison between the optical inspections versus the THz image of the defects in the foam shows a very good agreement. For the carbon fiber samples, both THz imaging at 0.6 THz and time-domain data are used to evaluate the degree of heat damage. The carbon fiber has a polarization dependent reflectivity in the THz frequency range, which can be related to burn damage level. Introduction In the past years, terahertz (THz) technology has received more interest and attention because of its unique properties and capabilities that make it very attractive as a non-destructive evaluation (NDE) tool. The THz frequency region represents an important intersection between spatial resolution and penetration depth and many dry, non-metallic materials show little THz absorption which allows imaging their internal structure with THz. These materials (such as plastics, ceramics, clothes, etc) are usually also transparent to microwave radiation, but terahertz radiation has the potential for a higher spatial resolution due to its shorter wavelength. THz radiation is non-ionizing and presents a low risk to biological tissue, which makes THz imaging also very attractive for biological applications. On the other hand, THz radiation cannot penetrate metal objects or materials with significant water content. There are basically two types of THz radiation technology: pulsed [1] and continuous-wave [2-5] (CW). A pulsed system is based on the use of electromagnetic wave pulses in the range of picosecond duration. This pulse is sent to a sample and is the resulting waveform is coherently recorded in time-domain. This waveform can be analyzed later in frequency by means of a Fourier Transform. CW systems work at single frequencies and do not provide spectroscopic information but they can be faster, more compact and simpler to operate. In the past years, the inspection of the sprayed-on foam insulation (SOFI) used on the external tank of the Space Shuttle has been a major driving force to use THz technologies as a NDE tool. SOFI is a good subject for THz imaging because it has a low absorption coefficient and index of refraction in this range [6-7]. The samples, provided by Lockheed Martin Space Systems and NASA Marshall Flight Center, are sprayed layer-by-layer onto an aluminum substrate. Both pulsed and CW techniques can be used to image the samples. So far, CW systems are simpler and faster than pulsed system and quite a few results have been published already [8-9]. The effort to implement THz as an effective NDE tools continues and, indeed, THz technology is in NASA’s phase 4, which means that THz imaging may become space certificated and implemented as a standard verification stage in the inspection procedure. The 4 International Workshop on Ultrasonic and Advanced Methods for Nondestructive Testing and Material Characterization, June 19, 2006 at UMass Dartmouth, N. Dartmouth, MA – Proceedings published in www.ndt.net 68 Beside the SOFI inspection, THz technology has also great potential in the inspection of other materials. In this paper the inspection of carbon fiber composites is explored. Carbon fiber composites are widely used in aerospace industry and their use requires technologies that are able to differentiate between safe and unsafe materials according to manufacturing tolerance or damage suffered in their use. In particular, the damaged caused by heat is the interest of study for the carbon fiber materials. Actually, the evaluation of such damage is performed by visual and destructive techniques, thus, the availability of a NDE tool is very attractive. In more detail, the carbon fiber samples inspected in this paper are used as a structural component in the fuselage of an aircraft. The applications reported in this paper use different THz technologies and setups. In particular, the SOFI foam inspection is performed by CW systems while the carbon fiber and electrical insulator rods use both CW and pulsed wave techniques for the inspection. As for the CW systems, the sources are Gunn diodes and a gas THz laser. Gunn diodes are particularly attractive at sub-THz range because of its compactness and simple operation but are difficult to obtain them commercially above 0.8 THz. A THz gas uses a CO2 laser to pump a gas cavity filled with the proper gas at the proper pressure. The THz gas laser offers several lines from 1 THz to 6 THz by changing the pumped gas. The detectors used are either Schottky diodes or pyroelectric. Schottky diodes are narrowband and have a very fast response and are used along the Gunn diodes while pyroelectric are broadband and slow and are used with the THz gas laser. Measurements The CW systems use Gunn diodes at 0.2 and 0.38 THz and the THz gas laser (Coherent SIFIR-50) at 1.63 THz with a power of 20 mW, 5 mW, and 180 mW respectively. Schottky diodes are used at 0.2 and 0.38 THz, and a pyroelectric at 1.63 THz. The beam is focused by aspherical (hyperbolic) lenses and three geometries can be implemented: transmission, normal reflection, and small angle reflection. The spot size is diffraction limited and it goes from 2.7 mm at 0.2 THz to 0.5 mm at 1.63 THz. Because all these systems are point emitters and receivers, the image is obtained by raster scan of the sample [8-9]. The THz time-domain system used to inspect the carbon fiber samples implements 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. It uses a p-type InAs wafer as emitter and a ZnTe crystal as detector via electro-optical rectification [10]. The measurements are performed in N2 environment to remove the effects of water vapor absorption and in small angle reflection geometry. SOFI samples are imaged with CW systems at 0.2 and 0.38 THz in normal reflection geometry. CW systems in normal reflection geometry show a common problem related to the generation of an interference pattern due to the interference between the incoming beam and reflected beams from the sample, lens and other surfaces. In order to reduce such effect, the system incorporates a compensating mirror with the intention to send part of radiation reflected from the beam splitter again to the detector in order to cancel the reflection coming from the lens, which is the main reflective surface besides the sample itself. One SOFI sample is imaged in order to check the difference in resolution and dynamic range. 0.38 THz images are expected to give a better resolution but the power available is less and the absorption is higher, which may cause a worse image quality. Another SOFI sample is imaged at 0.2 THz with the intention to The 4 International Workshop on Ultrasonic and Advanced Methods for Nondestructive Testing and Material Characterization, June 19, 2006 at UMass Dartmouth, N. Dartmouth, MA – Proceedings published in www.ndt.net 69 identify all defects and later slice it in small section to verify visually whether the defects are real or not. Also, this will allow verifying if some important defect was present buy not detected. Carbon fiber-based materials are conductive and therefore exhibit a high THz reflectivity. As a result, the measurements are uniformly performed in a reflection imaging geometry, which has the advantage of more accurately simulating the type of measurement that could be performed in a real-world setting. The majority of carbon fiber materials have polarization-dependent reflectivity in the THz frequency range, which may provide the mechanism to evaluate the damage degree because the heat may change the local reflectivity by resin alteration and/or fiber distortion.