Structural Health Monitoring using a Scanning THz System by Jean-Paul Guillet

Structural Health Monitoring using a Scanning THz System M. Vandewala, E. Cristofania, A. Brooka, W. Vleugelsb, F. Ospaldc, R. Beigangc, S. Wohnsiedlerd, C. Matheisd, J. Jonuscheitd, JP. Guillete, B.Recure, P. Mounaixe, I. Manek Hönningere, P. Venegasf, I. Lopezf , R. Martinezg, Y. Sternbergh a

Royal Military Academy, Brussel (BE), bVerhaert New Products and Services, Kruibeke (BE), cTechnical University of Kaiserslautern, Kaiserslautern (DE), dFraunhofer Institute for Physical Measurement Techniques IPM, Kaiserslautern (DE), eLaboratoire Onde et Matières d'Aquitaine, UMR CNRS 5798, Bordeaux (FR), f Fundación Centro de Tecnologías Aeronáuticas, Vitoria (ES), gApplus+ LGAI Technological Centre S.A., Barcelona (ES), hIsrael Aerospace Industries, Tel Aviv (IL) Abstract—Terahertz waves can provide in-depth information on defects for structural health monitoring of composite materials. This paper describes the technology of a continuouswave and a time-domain terahertz system operating on a 2-D and 3-D motion platform to provide 3-D high spatial resolution. The system as well as the overall detection performance will be described.

I. INTRODUCTION AND BACKGROUND

NE of the domains in which the terahertz (THz) technology can be considered as enabling is short-range in-depth imaging. Using these waves, a contact-free, highresolution inspection becomes feasible for typical composite materials found in aeronautics. This paper will report on the main results obtained with the FP7 project entitled DOTNAC (Development and Optimisation of THz non-destructive testing (NDT) on Aeronautics Composite Multi-layered Structures [1]) with the following objectives: (1) the development of a fibre-coupled Time-Domain System (TDS); (2) the implementation of a Frequency-Modulated Continuous-Wave system (FMCW) with electrical cable coupling; and (3) testing them on a series of calibration and blind samples for evaluation and validation purposes against conventional NDT techniques such as radiographic and ultra sound testing, as well as infrared thermography. II. FMCW THZ SYSTEM

The implemented FMCW system is an all-electronic THz system. It consists of three scanning heads with different frequency ranges (around 100 GHz, 150 GHz, and 300 GHz). They are used to acquire data by scanning a sample placed in front of them in reflection mode. Using a homodyne detection, the amplitude and phase can be measured after demodulating the received signal. Combining this in-depth information with a lateral scanning, a 3-D image can be built up. The in-depth resolution depends on the used bandwidth and the roughness of the sample surface, and varies between 2 mm and 6 mm (with an accuracy between 10 µm and 50 µm). Two configurations have been tested using the FMCW system. The first one is the focused beam configuration ensuring a high across-range resolution through a set of lenses which focus the THz beam inside the material under test along the Rayleigh length. The diameter of the beam waist limits the

across-range resolution which varies between 1 mm and 3 mm. The choice of lenses is based on a trade-off between the beam waist width and length, which should ideally be identical to the depth of the object under test. A second configuration has been created using an unfocused beam (obtained by removing the lenses), leading to an initially very large THz spot on the object under test. The spot overlay created during the scanning in azimuth and elevation direction, allows a coherent collection of the THz signals spread in across-range, at the condition of using a specific processing algorithm referred to as synthetic aperture processing. The across-range resolution is now no longer Rayleigh-limited, which privileges this configuration when inspecting thicker samples. A trade-off, however, is still necessary between across range resolution (improving with higher beam opening angles), and energy spreading (leading to very low signal-tonoise ratios). Typically the across-range resolution for the given frequencies varies between 4 mm and 7 mm, but constant along the entire depth of the object under test. III. PULSED THZ SYSTEM For the construction of the TDS, a pulsed laser system has been implemented in a fibre-optical ECOPS (Electronically Controlled Optical Sampling) pump-probe set-up. The two short-pulse lasers (one for the emitter, one for the detector) are based on Er-doped silica glass fibres and emit around 1560 nm centre wavelength.

Fig.1 Set-up of the TDS using ECOPS.

The key elements of the set-up are a piezo-controlled repetition rate stabilisation, synchronisatioon electronics to deliberately de-tune the repetition frequency of one laser with respect to the other laser, and a polarisation-maintaining fibre modules. delivery to the remote THz emitter/detector m The in-depth resolution is significantly beetter than the one obtained with the FMCW THz system at tthe expense of a lower penetration capacity for the materialls and structures tested within the scope of the project. T The across-range resolution depends again on the beam focus aand is comparable to the one of the FMCW system usiing the focused configuration. The main trade-off that needs to be considered for the TDS is the one involving measureement speed and signal-to-noise ratio. As for the FMCW system, an alternative cconfiguration has been used. The applied tomosynthesis approaach also implies a source moving along a linear trajectory, howeever, illuminating the sample under different incidence angles during scanning (the range of the antenna pointing angles is kkept between -30° and +30°). A limited number of so-calledd projections are obtained; a dedicated processing algorithm wiill then superpose and shift the acquired projections giving a plaane of focus.

V. RESULT TS A series of 20 flat Glass Fibre Reeinforced Plastics (GFRP) samples (solid laminates and san ndwich structures), and 5 Carbon Fibre Reinforced Plastics (CFRP) ( were modified by artificial defects such as inserts, stu ucks, water inclusions, etc., to create well-controlled and well-k known samples in order to validate the THz systems and algo orithms. In a next step the evaluation as a THz NDT method has h been performed on 50 blind samples (12 GFRP and 38 3 CFRP samples) with embedded defects such as delaminaations and debonds caused by an intentional miss-process. The FMCW THz system has shown an overall high potential with respect to NDT applications. Promising performances were obtained for san ndwich structures in some cases outperforming the convention nal NDT techniques. Fig. 3 shows a measurement result of an n A-sandwich honeycomb structure with different inserts at varrious depths.

IV. IN- SITU NDT SYSTEM CONFIGU URATION In DOTNAC the FMCW system and TDS have been tested out first using a planar 2-D scanning systeem positioned in front of the object under test. The latter having a boxed shape (with a depth up to 1 cm), the scanning tookk place in a plane parallel to the front side of the object under test. In a second phase, a 5-axis (3 linear and 2 rotational) synnchronised motion stage has been developed to enable the insppection of curved objects (such as the radome in Fig. 22-a), maintaining perpendicular illumination to the sample suurface at a fixed sensor-surface distance. For the performedd reflection-mode measurements, the positional repeatabilitty equalled 50 microns and the pointing accuracy of the sensor was ±0.2°. Scan speeds of up to 1 m/s have been targeteed. Fig. 2-a shows the FMCW sensor integrated on the motionn platform while inspecting the radome. Fig. 2-b shows a closse-up of the TDS sensors installed on the same 5-axis platform.

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Fig. 3 GFRP A-sandwich honeycomb b structure with inserts: (a) photograph, (b) THz FMCW measurement obtained in focused beam on. configuratio

The TDS has demonstrated good defect detection results for mising results regarding various types of samples. Prom delamination detection have beeen obtained as well as regarding quality inspection of coatiing on CRFP substrates.

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Fig. 4 GFRP A-sandwich honeyco omb structure with debond between skin and adhesive sheet: (a) ph hotograph, (b) close-up of the THz TDS measurrement.

CONCLUSSION THz NDT can outperform con nventional techniques for sandwich structures using a Rohaceell or a honeycomb core; a good potential has been observed for thick solid laminates, equal performances have been noted for many other materials/structures and this for a vaariety of defects. Both THz systems have demonstrated to complement the detection capacity of the conventional NDT teechniques. REFERENCE ES (a)

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Fig. 2 Picture of the integrated THz system: (a) F FMCW transceiver illuminating a radome, (b) close-up of the T TDS sensors.

[1]

(June 2013) The DOTNAC project publications. p [Online]. Available: http://www.dotnac-project.eu/