This paper deals with experimental investigations on syntactic foam and its behavior at outdoor conditions. For the experiments, plate specimens of syntactic foam of a thickness of 3mm are used. Hollow glass microspheres with untreated and silane-coated surface as well as microspheres with a low alkali ion concentration are applied as filler. During a test period of 50 days the specimens are stored in deionized water, which is tempered to 50°C to accelerate the water absorption. The weight of every specimen is measured during the water storage to determine the increase of weight due to water ingress. The dielectric permittivity and the loss factor are measured in constant intervals during the whole test period. As a result of these investigations, a different behavior of the syntactic foam filled with different hollow microspheres is observed. The syntactic foam specimens with untreated hollow microspheres show a significant increase of their permittivity and loss factor during the test period, whereas the dielectric properties of the specimens with silane-coated microspheres and microspheres with low alkali ion concentration are not influenced by water absorption. The results show that syntactic foam can potentially be used as insulation material for outdoor applications if suitable microspheres are used as filler.
Terahertz technologies for non-destructive testing (NDT) are continuing to find their way into the industrial sector in the context of very specific inspection tasks. Part of this development is the capability to adapt terahertz systems in such a way that they can meet the sometimes harsh challenges and requirements of real-world industrial scenarios. One such scenario is the inspection of components with limited available measurement space. In particular, we show here the terahertz NDT inspection of the mica insulation of generator bars of turbogenerators at power plants, where an early on-site detection of defects and cracks in the insulation can be crucial, but where only few centimeters of space between adjacent bars are available. To address this problem, we have developed a measurement system combining a 100 GHz all-electronic terahertz transceiver with a low-loss dielectric waveguide antenna with 90 degree tip. We achieve sub-wavelength image resolution by scanning the waveguide antenna's tip over the surface of the generator bars in a near-field measurement setup. Employing a frequency-modulated continuous wave technique, we obtain depth-resolved, volumetric terahertz images of the objects under test. We discuss here the implementation and performance of the implemented measurement system for terahertz NDT inspection. keywords: terahertz, non-destructive testing, dielectric waveguides, frequency-modulated continuous wave, millimeter waves, power generators
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