Ceramic materials have excellent corrosion, wear, and heat resistant properties etc. However, due to their brittleness, it is essential for all ceramic products to be checked during the manufacturing process in order to maintain safely. Therefore, it is important that detecting cracks, which exist on the surface or inside the ceramic materials, by means of an accurate and speedy non-destructive evaluation (NDE) technique. Moreover, if the crack can be evaluated quantitatively, it is identified whether the crack is permissible or not. However, the NDE technique that is capable of quantitatively evaluating cracks for the ceramic materials has not been found yet.
This paper shows that high-frequency electromagnetic wave transmission characteristics of the ceramics can provide a new NDE technique with the possibility of quantitatively evaluating cracks on the ceramic surfaces. This technique is hereafter called the High-Frequency Transmission (HFT) technique. The HFT technique is one of the NDE techniques by means of electromagnetic wave such as an eddy current testing. The electromagnetic wave signal with higher than 1 GHz frequency can penetrate easily through the ceramic materials. Accordingly, the HFT technique can detect defects more sensitively than the other electromagnetic techniques.
Specifically, experiments have been performed with specimens made of alumina, containing an artificial and a natural crack. As a result of the detection of the artificial crack, whose width is 0.5 mm and depth is 1 mm, the signal changed approximately 18 dB. From measuring the different depth crack, it was clarified that this change of the electromagnetic wave signal was caused by the depth of crack on the alumina ceramic. A numerical analysis was applied to discuss about the factors, which affect the variation in the electromagnetic wave signal. It was found that electric field was changed by existence of the crack. In addition, the naturally closed crack was measured by the HFT technique. As a result, it could also be found that the naturally closed crack changed the signal.