Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection

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

Terahertz Radome Inspection

“…This downgraded the best possible depth resolution by roughly a factor of 2. The overall results of the initial test measurements are in excellent accordance to the outcomes of [1]. On the other hand, the penetration depth in terahertz measurements is, in general, larger at lower frequencies.…”

“…The latter also provides a higher modulation For the initial study, terahertz images of test samples were acquired by raster-scanning the samples in reflection geometry with three different frequency-modulated continuous-wave (FMCW) transceiver units operating at 100 GHz, 150 GHz, and 300 GHz, while providing modulation bandwidths of around 40 GHz, 60 GHz, and 90 GHz, respectively. For the measurements on the test panels, we employed the same quasi-optical system, as described in [1]. Figure 2 shows two exemplary terahertz imaging results at roughly the same depth position around the center in one of the samples.…”

“…Contactless microwave, millimeter wave, and terahertz techniques have proven to be highly suitable for nondestructive testing of glass-fiber reinforced composites [1][2][3][4][5][6]. Respective materials are typically used for the construction of aircraft radomes.…”

“…Besides the required regular functional testing of radomes in service, it is of significant interest to inspect radome structures already during the manufacturing process in order to guarantee the required structural integrity and to ensure proper functionality for relevant radar and communication systems. While different ultrasound methods, as well as optical techniques, for nondestructive radome inspection are widely used [7], the inspection of sandwich structures is extremely challenging and eventually not feasible with these techniques [1,2]. In case of civilian aircraft, up until this day, the only established method for regular inspections of radomes in service are tap tests, which provide only limited information about structural defects and have to be performed manually.…”

Spectroscopic study of terahertz reflection and transmission properties of carbon-fiber-reinforced plastic composites

“…[12][13][14] Many studies have been reported regarding NDE for CFRP by using THz technologies. Redo-Sanchez and Karpowicz 15 and Karpowicz and Dawes 16 applied THz continuous and pulsed wave technology in reflection mode to detect burn damage of CFRP.…”