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
O
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.
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