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Today X-ray tomography and Ultrasound Synthetic Aperture Focusing Techniques are representative examples for defect imaging in the field of nondestructive testing. THz time domain spectroscopy (TDS) operates with short electromagnetic pulses. The sensitivity of the pulses to optical material property changes makes THz-TDS attractive for the detection of flaws in dielectric materials. Unfortunately, the X-ray computed tomography (CT) model holds only for THz-TDS reconstruction if minor refraction index differences between the inhomogeneities and the surrounding material matrix exist. A Time Domain SAFT algorithm has been developed to overcome the restrictions. THz time domain measurement on representative sample sets with inherent artefacts were performed to use them for the Image reconstructions. The results will be presented and compared with optical surface images of the used test objects to evaluate the SAFT algorithm in relation to the econstruction quality.
THz applications for NDT
(2017)
Electromagnetic waves in the frequency range between 0.1 and 10 THz are described as THz-radiation (T-ray). The ability to penetrate dielectric materials, to operate contactless and to be non-ionizing as well as be-ing insensitive to ambient temperatures makes THz technologies for NDT applications attractive especially where other techniques are not available or less suitable. Today T-rays are applied in non-destructive testing for quality control and condition monitoring issues. The operation is based on the combination of both transmission and/or reflection measurements and specially developed reconstruction procedures. Firstly, THz-TDS measurements will be presented to demonstrate the opportunities for spectroscopy and imaging of polymer materials. A SAFT algorithm and an Optical Layer-Model will be presented and discussed in more detail to demonstrate the opportunities of image reconstruction for visualization of the inner structures of op-tically and far infra-red opaque polymer composites.
Up to now THz-TDS-systems aren’t considered to be nondestructive testing facilities for large scale industrial applications, despite it was proven that they provide a comprehensive set of quality parameters. A practical approach to bring THz-TDS in addition to already existing testing systems into the industrial mainstream is systematic development of future test procedures and test facilities for dielectrics. For this purpose polyeth-ylene test specimen with introduced artefacts were designed, to evaluate the detection sensitivity of Time of Flight measurements based on dielectrics. SAFT reconstructed tomograms are presented which visualize the sizes and location of artificially introduced flaws.