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Optical Engineering

Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection
Author(s): Edison Cristofani; Fabian Friederich; Sabine Wohnsiedler; Carsten Matheis; Joachim Jonuscheit; Marijke Vandewal; René Beigang
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Paper Abstract

The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.

Paper Details

Date Published: 21 March 2014
PDF: 11 pages
Opt. Eng. 53(3) 031211 doi: 10.1117/1.OE.53.3.031211
Published in: Optical Engineering Volume 53, Issue 3
Show Author Affiliations
Edison Cristofani, Royal Belgian Military Academy (Belgium)
Fabian Friederich, Fraunhofer-Institut für Physikalische Messtechnik (Germany)
Sabine Wohnsiedler, Fraunhofer-Institut für Physikalische Messtechnik (Germany)
Carsten Matheis, Fraunhofer-Institut für Physikalische Messtechnik (Germany)
Joachim Jonuscheit, Fraunhofer-Institut für Physikalische Messtechnik (Germany)
Marijke Vandewal, Royal Belgian Military Academy (Belgium)
René Beigang, Fraunhofer-Institut für Physikalische Messtechnik (Germany)

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