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Abstract—Terahertz waves can provide in-depth information on defects for structural health monitoring of composite materials. This paper describes the technology of a continuous- wave 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, high- resolution 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-to- noise 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. 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 aRoyal 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), fFundación Centro de Tecnologías Aeronáuticas, Vitoria (ES), gApplus+ LGAI Technological Centre S.A., Barcelona (ES), hIsrael Aerospace Industries, Tel Aviv (IL) Structural Health Monitoring using a Scanning THz System O 978-1-4673-4717-4/13/$31.00 ©2013 IEEE The key elements of the set-up are a repetition rate stabilisation, synchronisatio deliberately de-tune the repetition frequency respect to the other laser, and a polarisation- delivery to the remote THz emitter/detector m The in-depth resolution is significantly be obtained with the FMCW THz system at t lower penetration capacity for the material tested within the scope of the project. T resolution depends again on the beam focus a to the one of the FMCW system usi configuration. The main trade-off that needs for the TDS is the one involving measure signal-to-noise ratio. As for the FMCW system, an alternative c been used. The applied tomosynthesis approa source moving along a linear trajectory, howe the sample under different incidence angles (the range of the antenna pointing angles is k and +30°). A limited number of so-called obtained; a dedicated processing algorithm wi and shift the acquired projections giving a pla IV. IN- SITU NDT SYSTEM CONFIGU In DOTNAC the FMCW system and TDS out first using a planar 2-D scanning syste front of the object under test. The latter havi (with a depth up to 1 cm), the scanning took parallel to the front side of the object under phase, a 5-axis (3 linear and 2 rotational) syn stage has been developed to enable the insp objects (such as the radome in Fig. 2 perpendicular illumination to the sample su sensor-surface distance. For the performed measurements, the positional repeatabilit microns and the pointing accuracy of the s Scan speeds of up to 1 m/s have been targete the FMCW sensor integrated on the motion inspecting the radome. Fig. 2-b shows a clos sensors installed on the same 5-axis platform. (a) Fig. 2 Picture of the integrated THz system: (a) F illuminating a radome, (b) close-up of the T piezo-controlled n electronics to of one laser with maintaining fibre odules. tter than the one he expense of a s and structures he across-range nd is comparable ng the focused to be considered ment speed and onfiguration has ch also implies a ver, illuminating during scanning ept between -30° projections are ll then superpose ne of focus. RATION have been tested m positioned in ng a boxed shape place in a plane test. In a second chronised motion ection of curved -a), maintaining rface at a fixed reflection-mode y equalled 50 ensor was ±0.2°. d. Fig. 2-a shows platform while e-up of the TDS (b) MCW transceiver DS sensors. V. RESULT A series of 20 flat Glass Fibre Re samples (solid laminates and san Carbon Fibre Reinforced Plastics ( artificial defects such as inserts, stu to create well-controlled and well-k validate the THz systems and algo evaluation as a THz NDT method h blind samples (12 GFRP and 3 embedded defects such as delamina by an intentional miss-process. The FMCW THz system has potential with respect to NDT performances were obtained for san cases outperforming the convention shows a measurement result of an structure with different inserts at var (a) Fig. 3 GFRP A-sandwich honeycomb photograph, (b) THz FMCW measurem configuratio The TDS has demonstrated good various types of samples. Prom delamination detection have bee regarding quality inspection of coati (a) Fig. 4 GFRP A-sandwich honeyco between skin and adhesive sheet: (a) ph THz TDS measur CONCLUS THz NDT can outperform con sandwich structures using a Rohace good potential has been observed equal performances have been materials/structures and this for a va systems have demonstrated to c capacity of the conventional NDT te REFERENCE [1] (June 2013) The DOTNAC project p http://www.dotnac-project.eu/ S inforced Plastics (GFRP) dwich structures), and 5 CFRP) were modified by cks, water inclusions, etc., nown samples in order to rithms. In a next step the as been performed on 50 8 CFRP samples) with tions and debonds caused shown an overall high applications. Promising dwich structures in some al NDT techniques. Fig. 3 A-sandwich honeycomb ious depths. (b) structure with inserts: (a) ent obtained in focused beam n. defect detection results for ising results regarding n obtained as well as ng on CRFP substrates. (b) mb structure with debond otograph, (b) close-up of the ement. ION ventional techniques for ll or a honeycomb core; a for thick solid laminates, noted for many other riety of defects. Both THz omplement the detection chniques. S ublications. [Online]. Available:
