Terahertz based non-destructive testing (NDT)

Abstract

Established methods based on X-rays, ultrasound, thermography, eddy current, and optics are very effective in fault detection and structural analysis. However, these methods are limited for some applications, for example, the non-destructive testing of fiber-reinforced plastics, foams, and sandwich or hollow structures. In these cases, terahertz technology offers an innovative method to overcome these limitations. Tremendous advances have been made in this new technology in terms of their industrialization in recent years. This paper presents techniques for use in industrial applications.

Fast FMCW Terahertz Imaging for In-Process Defect Detection in Press Sleeves for the Paper Industry and Image Evaluation with a Machine Learning Approach

“…Terahertz technologies, and in particular their application in nondestructive testing (NDT) for quality control and/or defect recognition, are on their way into industrial markets and real-world applications in production environments, maintenance tasks, and other areas of quality assessment [ 1 , 2 , 3 , 4 ]. The terahertz terminology in these contexts commonly refers to electromagnetic radiation in the region of the electromagnetic spectrum with frequencies between 0.1 and 10 THz corresponding to free-space wavelengths of 3 mm to 30 μm.…”

“…At the same time, the small terahertz wavelengths of few millimeters down to several tens of micrometers constitute an ideal premise for imaging techniques [ 17 , 18 , 19 , 20 , 21 ] with image resolutions on the order of typical, relevant defect sizes in components produced from the above materials. Compared to other established NDT technologies such as ultrasound inspection, X-ray screening and computed tomography, terahertz waves offer the unique combination of low photon energies and low radiation power - making it harmless to biological tissue and safe to be used in industrial contexts - and, due to the electromagnetic nature of the waves, the possibility to be employed contact-free with no need for a coupling medium to penetrate the materials under investigation [ 3 , 22 ]. With suitable materials for quasi-optical components, e.g., the polymers PE and PTFE, the radiation can be easily guided and focused according to the specific context of application with quasi-optical lenses [ 23 ], typically produced of low-cost materials such as the polymers PE and PTFE, and diffractive elements [ 24 ], or with simple metallic mirrors.…”

Towards Practical Terahertz Imaging System With Compact Continuous Wave Transceiver

“…THz imaging techniques can image not only the precise two-dimensional (2D)-distribution of the target of interest but also three-dimensional (3D)-information such as layers, cavities, and adhesive joints. The precise THz image is crucial for a variety of applications ranging from nondestructive testing (NDT) and security to medicine, food inspection and agriculture [1]- [10]. However, the technical difficulties and high costs involved in scaling up these techniques for industrial This work is financially supported by Japan Society for JSPS Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists (19K14995) and the Core Research for Evolutional Science and Technology (CREST) program of Japan Science and Technology Agency (#JPMJCR1534).…”

“…However, the technical difficulties and high costs involved in scaling up these techniques for industrial This work is financially supported by Japan Society for JSPS Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists (19K14995) and the Core Research for Evolutional Science and Technology (CREST) program of Japan Science and Technology Agency (#JPMJCR1534). use have restricted them to laboratory demonstrations [5], [10]. In order to improving the THz imaging technique, many significant works have been proposed for summarizing the related topics from different angles such as imaging speed [7], high-resolution imaging [1], [10], THz array integration [8], [11], [12] and specific techniques [3], [6], [13], while this article mainly focuses on the techniques that will lead to a practical THz imaging system, which includes a compact transceiver and cost-effective spatial sampling method.…”

“…THz waves can be generated via both photonic and electronic techniques. Moreover, the singlefrequency or narrow-band THz imaging systems have been demonstrated in real-time for security and/or inspection applications with a sub-millimeter resolution [1], [3], [6], [10], [15]- [18]. In terms of 3D imaging, modulated CW signals with frequency sweep can be used with a coherent detection scheme to generate synthetic pulses of high power [19].…”