Using THz waves to diagnose debonding defects in a heat-resistant structure

Abstract

This study proposes a method for finding defects in heat-resistant structures without causing any damage to them, by utilizing THz waves. THz waves, also called Terahertz waves, are a portion of the electromagnetic spectrum with a frequency range of 0.1 to 10 THz, which lies between the microwave and infrared frequencies. These waves have unique properties that make them useful for nondestructive testing of various materials such as plastics, composites, and heat-resistant materials. The proposed method involves mounting a THz wave scanner on a multi-axis stage and using it to detect defects in conical- and cylindrical-shaped heat-resistant structures. Once the scan angle and height are set, the scan path is automatically calculated, and the THz wave scanner follows this path to perform the scan. As an initial step, the time taken for the THz wave to penetrate the heat-resistant structure and reach the adhesive layer was calculated. The time windowed data corresponding to the adhesive layer was then examined. Signal processing was applied to remove noise from the data to achieve more precise defect visualization. The THz signal to which the signal processing was applied was then reconstructed into a three-dimensional matrix, and the result corresponding to the adhesive layer was visualized in the same shape as the planar figure of the actual specimen. Those areas of the visualized image reflecting characteristics that deviated from those associated with a normal signal were identified as suffering from debonding defects. We performed a visual inspection of the adhesive layer of the actual heat-resistant structure and compared it to the images obtained by our method. There was a notable agreement between the two, thereby confirming that the technique presented in this paper is a viable means of inspecting heat-resistant structures for debonding defects.