Electromagnetic nondestructive tests are important and widely used within the field of nondestructive evaluation (NDE). The recent advances in sensing technology, hardware and software development dedicated to imaging and image processing, and material sciences have greatly expanded the application fields, sophisticated the systems design and made the potential of electromagnetic NDE imaging seemingly unlimited. This review provides a comprehensive summary of research works on electromagnetic imaging methods for NDE applications, followed by the summary and discussions on future directions.

Electromagnetic Imaging Methods for Nondestructive Evaluation Applications

Microwave holography requires knowledge of the incident field and the Green tensor of the background medium. In near-field imaging, analytical models of these are inadequate and the use of electromagnetic simulations has been previously proposed. In practice, however, the fidelity of such simulations may also be insufficient. Here, we propose a measurement method for the acquisition of this information, thus eliminating the need to estimate it analytically or numerically. We validate the approach through simulated data acquisitions as well as through experiments. It is shown that this approach characterizes the measurement system more reliably than simulations leading to significant improvement of the image quality.

Microwave Holography Using Point-Spread Functions Measured With Calibration Objects

Conventional ultrasound inspection has been a standard non-destructive testing method for a long time. In most cases, though, a liquid coupling medium, such as water or gel, is required because of the huge impedance mismatch at a solid-air boundary. The physical contact with a liquid medium is a drawback because it prevents ultrasound inspection of many important materials. Modern industry, however, has a growing need for new, improved and remote non-destructive testing techniques for the inspection of new kinds of materials. 
This thesis describes the use, advantages, limitations, and applications of a commercially available remote air-coupled ultrasound testing system that has been implemented in an accredited non-destructive testing laboratory (EN ISO/IEC 17025: 2000). One of the objectives of the study, therefore, is to discover whether air-coupled ultrasound conforms to standards of modern quality assurance by comparing requirements of modern standards with the possibilities of this new advanced technique. The results are also compared with other non-destructive methods to discover if air-coupled ultrasound testing can compete with current techniques. First, the possibilities of linear air-coupled ultrasound inspection in through-transmission mode are investigated. In this case the equipment is used in its standard mode and ultrasound images (A-, B-, C-scan images) are performed. Second, the measuring technique is further developed to operate in the non-linear mode. This latter method uses the non-linear behaviour of defects for fast and selective imaging. Therefore, the excitation of a sample by injection of a sinusoidal wave using external or internal sources produces higher harmonics at the damaged area. By using these harmonics, defect-selective imaging is performed in which only the defect itself appears. In the case of non-linear air-coupled ultrasound, the sample is excited at frequencies that have the centre frequency of the air-coupled narrow band receiver as an overtone. When the receiver scans across the sample, it responds selectively to the higher harmonics.
In this study, high performance materials, components, and processes that are close to actual applications and that are difficult or impossible to measure with water are inspected. Results of air-coupled ultrasound imaging performed on fibre-reinforced plastics, water sensitive materials (e.g. C/C-SiC ceramic or wood), and smart structures are presented in order to reveal delaminations, impacts, and growth of internal defects. Further measurements of monitoring the curing process of epoxy resin and of the manufacturing process of the new high temperature resistant C/C-SiC ceramic are presented. In addition, results are shown in which the actuator in shape-adaptive structures is used as transmitter and the receiver of the air-coupled system is used to record the signals. Thus it is possible to detect defect areas of the piezo actuators. 
Analysing the results and by comparing them with other non-destructive testing methods demonstrates that air-coupled ultrasound inspection is a promising new technique that is suitable for modern quality assurance and one that can compete with other non-destructive testing methods. 
Herkommliche Ultraschalluntersuchung ist seit langer Zeit eine Standardmethode der zerstorungsfreien Prufung. In den meisten Fallen wird aufgrund des grosen Impedanzunterschiedes an einer Festkorper/Luft Grenzschicht ein flussiges Koppelmedium (beispielsweise Wasser oder Koppelgel) benotigt. Dieser direkte Kontakt ist jedoch ein Nachteil, da er Ultraschallmessungen an vielen wichtigen Materialien unmoglich macht. Zur Untersuchung neuartiger Werkstoffe hat die heutige Industrie jedoch ein wachsendes Bedurfnis fur neue, weiterentwickelte und beruhrungslose zerstorungsfreie Prufverfahren. 
Diese Arbeit beschreibt den Einsatz, die Vorteile, die Grenzen und die Anwendungen eines kommerziell erhaltlichen luftgekoppelten Ultraschallsystems, welches in ein akkreditiertes Pruflabor (EN ISO/IEC 17025: 2000) eingebunden worden ist. Eine der Zielsetzungen dieser Arbeit ist deshalb herauszufinden, ob luftgekoppelter Ultraschall die Standards der modernen Qualitatssicherung erfullt. Die geschieht durch den Vergleich der Forderungen heutiger Standards mit den Moglichkeiten dieser neuen fortschrittlichen Technik. Die Ergebnisse werden gleichfalls mit weiteren zerstorungsfreien Methoden verglichen, um zu beurteilen, ob luftgekoppelter Ultraschall mit gegenwartigen Verfahren konkurrieren kann. Zuerst werden die Moglichkeiten des linearen luftgekoppelten Ultraschalls in Transmissionskonfiguration untersucht. Dabei wird die Messeinrichtung in ihrem Standardmodus verwendet und Ultraschallbilder (A-, B-, C-Bilder) aufgezeichnet. Dann wird die Messtechnik weiterentwickelt, um sie im nichtlinearen Modus zu betreiben. Diese letztere Methode verwendet das nichtlineare Verhalten von Fehlstellen fur eine schnelle und selektive Darstellung von Defekten. Eine sinusformige Anregung (externe oder interne Anregungsquelle) erzeugt hohere Harmonische in den geschadigten Bereichen. Die Verwendung dieser Harmonischen ermoglicht eine defekt-selektive Darstellung, in der nur die Fehlstellen selbst abgebildet werden. Im Fall des nichtlinearen luftgekoppelten Ultraschalls wird die Probe mit Frequenzen angeregt, die die Mittelfrequenz eines luftgekoppelten schmalbandigen Empfangsprufkopfs als Oberton haben. Beim Abrastern der Probe reagiert der Prufkopf selektiv auf die hoheren Harmonischen. 
In dieser Arbeit werden Hochleistungswerkstoffe, Bauteile und Prozesse untersucht, die nah an praktischen Anwendungen sind und die schwer oder uberhaupt nicht mit Wasser untersucht werden konnen. Es werden Ergebnisse von luftgekoppelten Ultraschall prasentiert, die an faserverstarkten Kunststoffen, wasserempfindlichen Werkstoffen (beispielsweise C/C-SiC Keramik oder Holz) und an formadaptiven Strukturen durchgefuhrt wurden, um Delaminationen, Impactschaden und das Wachstum innerer Fehlstellen zu erfassen. Daruber hinaus werden Messungen dargestellt, bei denen der Aushartevorgang von Epoxidharz und der Produktionsprozess der neuen hochtemperaturbestandigen C/C-SiC Keramik uberwacht wird. Weiterhin werden Ergebnisse gezeigt, bei denen der Aktor einer adaptiven Struktur als Sender verwendet wird und die Signale mit dem Empfanger der Luftultraschallanlage aufzeichnet werden. Auf diese Weise ist es moglich, fehlerhafte Bereiche eines Piezoaktors zu finden.
Durch die Betrachtung der Ergebnisse und deren Vergleich mit weiteren zerstorungsfreien Testmethoden wird gezeigt, dass luftgekoppelter Ultraschall eine vielversprechende neue Technik ist, die fur moderne Qualitatssicherung geeignet ist und die mit anderen zerstorungsfreien Methoden konkurrieren kann.

Air-coupled ultrasound inspection as a new non-destructive testing tool for quality assurance

This paper shows that the high-frequency eddy-current (HFEC) measurement devices can be used not only for characterizing conductivity and magnetic permeability related properties of electrically conductive materials, but also for permittivity characterization of insulators. Maxwell’s equations, finite-element method simulations, and experimental research are applied to support this hypothesis. An industrial HFEC device is used to measure the change of dielectric properties during the curing process of the epoxy resin L20. The measurement results are in good agreement with the expected behavior of the parameters relative permittivity and tan $\delta $ during cure. Using a capacitive reference device, similar characteristics regarding the change of the complex permittivity of the resin can be observed. In addition, HFEC imaging results on polymethyl methacrylate are presented, discussed, and compared with capacitive imaging. HFEC permittivity mapping benefits from a high spatial resolution with a sensitivity and penetration depth that is at least comparable with those of capacitive imaging technology.

Measuring and Imaging Permittivity of Insulators Using High-Frequency Eddy-Current Devices

Thermal barrier coatings have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. To monitor thermal barrier coating (TBC) integrity over its lifetime, the detection of top coat (TC) and thermally grown oxide (TGO) thicknesses was carried out using a microwave non-destructive technique. The results showed a good resolution of 1° change in phase for 15 μm TC thickness when a rectangular waveguide operating at relatively high frequency was applied. The phase of the reflection coefficient at the interface of TC and waveguide varies for different TGO and TC thicknesses. Therefore, measuring the phase of the reflection coefficient enables us to accurately calculate these thicknesses. Finally, a theoretical analysis was used to evaluate the reliability of the experimental results.

Non-destructive microwave detection of layer thickness in degraded thermal barrier coatings using K- and W-band frequency range

The results of an experimental study of microwave imaging with an openended waveguide at 30 GHz for NDE of dielectric materials are presented. Using a hybrid measurement method, the near-field point spread function is investigated. Improvement of sensitivity and resolution via deconvolution also is discussed. A comparison of results obtained with more prominent NDE methods as ultrasonics, X-rays, and thermal waves demonstrates the performance of the technique.

L. Diener

Papers

Microwave near-field imaging with open-ended waveguide—Comparison with other techniques of nondestructive testing