Terahertz Quality Inspection for Automotive and Aviation Industries

F. Ellrich, M. Bauer, N. Schreiner, A. Keil, T. Pfeiffer, J. Klier, S. Weber, J. Jonuscheit, F. Friederich, D. Molter
2019 Journal of Infrared, Millimeter and Terahertz Waves  
Nondestructive quality inspection with terahertz waves has become an emerging technology, especially in the automotive and aviation industries. Depending on the specific application, different terahertz systems-either fully electronic or based on optical laser pulses-cover the terahertz frequency region from 0.1 THz up to nearly 10 THz and provide high-speed volume inspections on the one hand and high-resolution thickness determination on the other hand. In this paper, we present different
more » ... trial applications, which we have addressed with our terahertz systems within the last couple of years. First, we show three-dimensional imaging of glass fiber-reinforced composites and foam structures, and demonstrate thickness determination of multilayer plastic tube walls. Then, we present the characterization of known and unknown multilayer systems down to some microns and the possibility of measuring the thickness of wet paints. The challenges of system reliability in industrial environments, e.g., under the impact of vibrations, and effective solutions are discussed. This paper gives an overview of state-of-the-art terahertz technology for industrial quality inspection. The presented principles are not limited to the automotive and aviation industries but can also be adapted to many other industrial fields. In the recent years, impressive developments from the terahertz community have shown that terahertz technology is a very attractive tool to complement well-established methods in the field of nondestructive testing [1] [2] [3] [4] [5] [6] [7] [8] . The unique characteristics of terahertz radiation, e.g., being nonhazardous and providing high-contrast images when penetrating dielectrics (compared, e.g., with X-ray techniques) combined with a higher resolution than microwaves, have led to many different applications, especially in the automotive and aviation sectors. Following the study of TEMATYS SARL [9], the market size of nondestructive testing with terahertz radiation in 2012 was comparable with the size of terahertz research at these times. Until 2020, the study predicted a doubling of the industrial market compared with the research volume, while assuming that the overall terahertz market increases with 16% per annum. Most probably, there will not be an abrupt change from existing and reliably working solutions in industrial production lines even when the terahertz solution is able to serve additional values at the same time. Mostly implemented as an additional technology parallel to alreadyimplemented quality inspections systems today, terahertz measurement techniques could gradually lead to fully terahertz-controlled processes. Here, challenges given by rough industrial environments in comparison with research laboratories must be overcome. The largest and most attractive industrial application at the moment is the thickness measurement technology [9, 10] . In cooperation with several industrial partners, we have already solved a number of practical problems, e.g., the vibration of objects during high-precision thickness measurements. Using interferometric-controlled real-time distance information, we were able to eliminate the impact of such vibrations almost completely. Hence, we realized a reliable terahertz-based thickness gauge being able to resolve multilayer systems of, e.g., car bodies in the production line. Terahertz solutions have proven highly attractive in particular for material characterization of novel and complex materials, where state-of-the-art testing technologies often do not work or at least their employment is highly restricted. In the context of the energy revolution, especially, the automotive and aviation industries are in search for new materials, which provide sufficient structural stability and are lightweight in comparison with, e.g., metalbased constructions. Fiber-reinforced composite materials made of multiple layers of carbon, glass, natural fibers, etc. are on the rise. Here, terahertz technology is able to support the quality inspection of such structures in two ways. On the one hand, volumetric inspection of the materials to identify possible inclusions, cracks, or other defects is of high interest. This is done, e.g., for radomes typically constructed out of glass fiber-reinforced plastics (GFRP) forming a protecting shell for radar antennas. A high degree of transparency of radomes is essential to ensure proper signal transmission. We developed a terahertz inspection system for the radome production industries, which inspects this critical part of the radar transmission link for aircrafts. On the other hand, the thickness of the coatings protecting the composite structures has to be controlled. Well-established methods based on eddy-current or photothermic principles often fail for composite substrates, whereas the substrate-independent terahertz technology for thickness measurements meets these demands. The terahertz technologies presented in this paper can be roughly divided into two categories. In the first part of this paper (section 2), we demonstrate electronic, fast-scanning, frequency-modulated continuous-wave (FMCW) technology, employed mainly for applications focusing on volume inspection of composites and foam structures. In addition, thickness information of layer systems in the millimeter range can be gathered with this technology. In Journal of Infrared, Millimeter, and Terahertz Waves
doi:10.1007/s10762-019-00639-4 fatcat:yuzsdv46t5au3d4h5g4nznml6y