Terahertz NDE for metallic surface roughness evaluation

Abstract

Metallic surface roughness in a nominally smooth surface is a potential indication of material degradation or damage. When the surface is coated or covered with an opaque dielectric material, such as paint or insulation, then inspecting for surface changes becomes almost impossible. Terahertz NDE is a method capable of penetrating the coating and inspecting the metallic surface. The terahertz frequency regime is between 100 GHz and 10 THz and has a free space wavelength of 300 micrometers at 1 THz. Pulsed terahertz radiation, can be generated and detected using optical excitation of biased semiconductors with femtosecond laser pulses. The resulting time domain signal is 320 picoseconds in duration. In this application, samples are inspected with a commercial terahertz NDE system that scans the sample and generates a set of time-domain signals that are a function of the backscatter from the metallic surface. Post processing is then performed in the time and frequency domains to generate C-scan type images that show scattering effects due to surface non-uniformity.

Synthetic aperture imaging extended towards novel THz sensors

“…4 one can observe that for higher frequencies and broader opening angles the resolution improves significantly when applying SA processing. Comparing these theoretical resolutions with the ones obtained with traditional THz imaging (a resolution of 3 mm is obtained with a 0.2 THz CW system [8], using a pulsed system resolutions of 2 to 3 mm are obtained [3], [7], [9], [10]), indicates an improvement of at least a factor of 2.…”

“…Exceptions are published results related to interferometry rather than actual synthetic aperture imaging [2], [3] or small scale SA results based on THz impulse ranging [4] on reflective and opaque objects. This paper will help to fill up this research gap in two ways: by demonstrating the extension from SA(R/S) [5], [6], towards SAT and by exploring the applications reachable combining THz waves and SA imaging in the domain of NDT [7].…”

“…Since most composite materials have a maximum transparency in the subTHz (typically between 0.1 and 0.8 THz), the advantage of using a very short wavelength by going very high in the THz frequency band cannot be used. Actual results show resolutions that are more or less twice the size of the wavelength (a resolution of 3 mm is obtained with a 0.2 THz CW system [8], using a pulsed system resolutions of 2 to 3 mm are obtained [3], [7], [9], [10]). It should be noted that this resolution is only reached for a certain layer within the illuminated object given the beam focusing depth inherent to the used optics.…”

Nondestructive evaluation of aircraft composites using terahertz time domain spectroscopy

“…This paper reported that the resolution was limited to ~ 10 μm. Another paper reported that surface roughness differences below ~ 10 μm were indistinguishable from the metallic background when searching for corrosion under paint [5].…”

Space-time versus frequency domain signal processing for 3D THz imaging

“…Since most composite materials have a maximum transparency in the sub THz (typically between 0.1 and 0.8 THz), the minimum wavelength and thus the minimum opening angle one can use is limited and therefore the achievable azimuth resolution. A set of optics is responsible for keeping the opening angle small and the positioning and fine tuning of these optics need to be done with care [3], [4]. It should be noted that this resolution is only reached for a certain layer within the illuminated object given the so-called beam focusing depth inherent to the used optics.…”