An aircraft composite with various forms of damage was examined using a reflective THz time domain spectroscopy (TDS) system. Indices of refraction and absorption coefficients in the terahertz frequency range for composite were measured using a reflection configuration, however, the addition of a coating reduced the back reflection and prevented the technique from being used for nondestructive analysis. THz TDS reflection imaging on the composite strips was performed first. Burn spots and a hidden circular void could be identified in a THz image, but there was little evidence that areas of damage from mechanical bending stress and simulated hidden cracks could be detected with terahertz TDS reflective imaging. Next, THz TDS reflective configuration was used to image an entire panel, consisting of two composite strips surrounding a honeycomb center. Surface damage, such as puncture holes, burn spots, and paint removal could be detected, but no reflections occurred below the thin composite layer down into the honeycomb. The approximate depth of delamination could be determined in the time domain by measuring the timing of a Fabry-Perot reflection through a thin slice of the composite. Terahertz radiation can exhibit some unique characteristics for nondestructive evaluation from other forms of electromagnetic radiation. Microwaves have the ability to penetrate composites, but their spatial resolution is more limited due to their longer wavelengths. Infrared techniques can have better spatial resolution than terahertz, but less penetration depth. THz can penetrate composite without contacting it, with submillimeter transverse resolution, and can detect surface defects, hidden voids, delaminations, and bending damage in composites. The depth resolution for defects is much better than the spatial resolution, and thin composite samples can be analyzed with THz TDS. There are advantages and disadvantages to using either the TDS transmission or reflection configuration. It was easier to get a smaller THz spot size onto the sample using a transmission configuration. This was primarily due to the large ratio of focal length to diameter in the parabolic mirrors and the cumbersome stages used to steer them. Custom optical parts could alleviate this problem for reflection configurations. It was also easier to measure the material properties of the composite using a transmission configuration. With an exterior coating, the reflective technique for measuring the material properties was ineffective. Reflection mode is certainly a more likely candidate for a maintenance technique, as was mentioned earlier, but transmission setup could aid in the design and quality control of glass fiber composites. Reflection mode would also be a more effective technique in determining the depth of delaminations. This is due to reflection mode signals traveling a shorter distance through the attenuating composite and due to the relative similarity in the amplitude of Fresnel reflections between a void and the initial pulse. This can be easily seen from the plotted calculations. The relative amplitude of the first Fabry-Perot reflection is calculated to be an order of magnitude larger in reflection configuration than in transmission.


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    Titel :

    Nondestructive evaluation of aircraft composites using reflective terahertz time domain spectroscopy


    Weitere Titelangaben:

    Zerstörungsfreie Bewertung von Luftfahrtverbundwerkstoffen mittels reflektierender Terahertz-Zeitbereichs-Spektroskopie


    Beteiligte:

    Erschienen in:

    NDT&E International ; 43 , 2 ; 106-115


    Erscheinungsdatum :

    2010


    Format / Umfang :

    10 Seiten, 14 Bilder, 15 Quellen




    Medientyp :

    Aufsatz (Zeitschrift)


    Format :

    Print


    Sprache :

    Englisch




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