Microwave and Millimeter Wave Nondestructive Testing and Evaluation -- Overview and Recent Advances

This article focuses on three recent applications of microwave and millimeter wave NDT&E techniques that involve novel instrumentation development and measurements, including: 1) disbond detection in strengthened concrete bridge members retrofitted with carbon fiber reinforced polymer (CFRP) composite laminates; 2) corrosion and precursor pitting detection in painted aluminum and steel substrates; and 3) detection of flaws in spray-on foam insulation and the acreage heat tiles of the Space Shuttle through focused and synthetic imaging techniques. These applications have been performed at the Applied Microwave Nondestructive Testing Laboratory (amntl) at the University of Missouri-Rolla

Comparison of Near-Field Millimeter-Wave Probes for Detecting Corrosion Precursor Pitting under Paint

Aircraft structural components such as wings and fuselages are constantly exposed to harsh environments, which make them susceptible to corrosion initiation and growth. To complicate matters, corrosion is normally hidden under paint and primer and cannot be visually detected until significant corrosion has occurred, causing the paint to blister. Corrosion of this type is usually preceded by the presence of corrosion precursor pitting. Hence, early detection of pitting is a critical issue in the maintenance of an aircraft and its structural components. Near-field microwave nondestructive testing techniques have been successfully used for detection of corrosion under paint, including very small laser machined pits. However, it is desirable to improve the spatial resolution associated with these techniques so that pits with dimensions in the range of a few hundreds of micrometers can be effectively detected. In this paper, a comparison between several different millimeter-wave open-ended rectangular waveguide-based probes is made for the detection and evaluation of corrosion precursor pitting at Ka-band (26.5-40 GHz) and V-band (50-75 GHz). A number of laser machined pits with dimensions varying between 150 to 500 μm were produced for this investigation. Using these probes, millimeter-wave images of these pits were produced, indicating that the modified open-ended rectangular waveguide probes, namely, single and double tapered and dielectric slab-loaded waveguide probes, were successful in detecting small pits. The results of this investigation, along with a complete discussion of the results, are presented

Microwave NDT&E using open-ended waveguide probe for multilayered structures

Ph. D. Thesis.Microwave NDT&E has been proved to be suitable for inspecting of dielectric structures due to low attenuation in dielectric materials and free-space. However, the microwave responses from multilayered structures are complex as an interrogation of scattering electromagnetic waves among the layers and defects. In many practical applications, electromagnetic analysis based on analytic- and forward structural models cannot be generalised since the defect shape and properties are usually unknown and hidden beneath the surface layer. This research proposes the design and implementation of microwave NDT&E system for inspection of multilayered structures. Standard microwave open-ended rectangular waveguides in X, Ku and K bands (frequency range between 8-26.5 GHz) and vector network analyser (VNA) generating sweep frequency of wideband monochromatic waves have been used to obtain reflection coefficient responses over three types of challenging multilayered samples: (1) corrosion progression under coating, (2) woven carbon fibre reinforced polymer (CFRP) with impact damages, and (3) thermal coated glass fibre reinforced polymer (GFRP) pipe with inner flat-bottom holes. The obtained data are analysed by the selected feature extraction method extracting informative features and verify with the sample parameters (defect parameters). In addition, visualisation methods are utilised to improve the presentation of the defects and material structures resulting in a better interpretation for quantitative evaluation. The contributions of this project are summarised as follows: (1) implementation of microwave NDT&E scanning system using open-ended waveguide with the highest resolution of 0.1mm x 0.1 mm, based on the NDT applications for the three aforementioned samples; (2) corrosion stages of steel corrosion under coating have been successfully characterised by the principal component analysis (PCA) method; (3) A frequency selective based PCA feature has been used to visualise the impact damage at different impact energies with elimination of woven texture influences; (4) PCA and SAR (synthetic aperture radar) tomography together with time-offlight extraction, have been used for detection and quantitative evaluation of flat-bottom hole defects (i.e., location, size and depth). The results conclude that the proposed microwave NDT&E system can be used for detection and evaluation of multilayered structures, which its major contributions are follows. (1) The early stages (0-12month) of steel corrosion undercoating has been successfully characterised by mean of spectral responses from microwave opened rectangular waveguide probe and PCA. (2) The detection of low energy impact damages on CFRP as low as 4 Joules has been archived with microwave opened rectangular waveguide probe raster scan together with SAR imaging and PCA for feature extraction methods. (3) The inner flat-bottom holes beneath the thermal coated GFRP up to 11.5 mm depth has been successfully quantitative evaluated by open-ended waveguide raster scan using PCA and 3-D reconstruction based on SAR tomography techniques. The evaluation includes location, sizing and depth. Nevertheless, the major downside of feature quantities extracted from statistically based methods such as PCA, is it intensely relies on the correlation of the input dataset, and thus hardly link them with the physical parameters of the test sample, in particular, the complex composite architectures. Therefore, there are still challenges of feature extraction and quantitative evaluation to accurately determine the essential parameters from the samples. This can be achieved by a future investigation of multiple features fusion and complementary features.Ministry of Science and Technology of Royal Thai Government and Office of Educational Affairs, the Royal Thai Embass

Terahertz Time-domain Reflectrometry of Multilayered Systems.

Presented in this work are applications of terahertz pulse ranging, spectroscopy and imaging to the nondestructive evaluation of three disparate multilayer systems for the detection and measurement of hidden layers, as well as the extraction of system information that will aid in its maintenance, repair or replacement. Thermal protection systems for turbine engine components were investigated. Thermal barrier coatings (TBC) and thermally-grown oxide (TGO) thicknesses were determined with 10 micron resolution using time-of-flight and refractive index calculations. Two alternative methods of monitoring TGO growth using reflection amplitudes and spectral shifts were proposed for the prediction of TBC failure. Laser-machined defects as narrow as 50 microns were resolved in one- and two-dimensional images. The light and dark rings of trees, which reflect the changes in tree growth density over the course of a year, are measurable using pulsed terahertz beams. Tree-rings of bare and painted wood specimen were laterally and axially tomographically imaged in order to facilitate the dendrochronological cross-dating of artifacts. Comparisons were made between photographs and terahertz images to demonstrate the reliability of the technique. Historically, numerous unique artworks have been lost through the act of being covered over time. Samples of paintings, drawings and mosaics were imaged beneath layers of paint and plaster using pulsed-terahertz techniques to demonstrate the efficacy of the technique for art history and restoration. Sketch materials and pigments were measured, between 0.05 and 1.0 THz, to help identify colors in spectroscopic images. Other computational and processing methods were used to optimize the distinction between color domains. Additional time-domain terahertz applications for the examination of artwork and other artifacts were proposed.Ph.D.Applied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/61752/1/jbjz_1.pd

Development of active microwave thermography for structural health monitoring

Active Microwave Thermography (AMT) is an integrated nondestructive testing and evaluation (NDT&E) method that incorporates aspects of microwave NDT and thermography techniques. AMT uses a microwave excitation to generate heat and the surface thermal profile of the material or structure under test is subsequently measured using a thermal camera (or IR camera). Utilizing a microwave heat excitation provides advantages over traditional thermal excitations (heat lamps, etc.) including the potential for non-contact, selective and focused heating. During an AMT inspection, two heating mechanisms are possible, referred to as dielectric and induction heating. Dielectric heating occurs as a result of the interaction of microwave energy with lossy dielectric materials which results in dissipated microwave energy and a subsequent increase in temperature. Induction heating is a result of induced surface current on conductive materials with finite conductivity under microwave illumination and subsequently ohmic loss. Due to the unique properties of microwave signals including frequency of operation, power level, and polarization, as well as their interaction with different materials, AMT has strong potential for application in various industries including infrastructure, transportation, aerospace, etc. As such, this Dissertation explores the application of AMT to NDT&E needs in these important industries, including detection and evaluation of defects in single- or multi-layered fiber-reinforced polymer-strengthened cement-based materials, evaluation of steel fiber percentage and distributions in steel fiber reinforced structures, characterization of corrosion ratio on corroded reinforcing steel bars (rebar), and evaluation of covered surface cracks orientation and size in metal structures --Abstract, page iv

Fiber Optic Sensor Fused Additive Manufacturing

This dissertation research establishes the foundation for converging disciplines to fiber optic sensors and additive manufacturing for smart part fabrication for energy system applications. Through innovation in numerical designs, thorough studies of layer-by-layer additive manufacturing procedures, and innovation in high-temperature fiber optic sensor development, this dissertation presents fiber optic sensor embedding in metals for smart component manufacturing. In this dissertation, standard telecom-grade single-mode optical fibers were metalized by nickel sulfamate electroplating method. Through electroplating process optimization, residual strain of fiber coating induced on optical fiber were controlled to ensure metal integrity of fiber sensors. Using Laser Engineered Net Shaping (LENS) process, metalized fiber sensors were embedded into objects with flat surfaces and curved surfaces to fabricate smart components. Using Rayleigh optical frequency domain reflectometry technology, the embedded fiber optic sensors were used to perform accurate and distributed temperature and strain sensing with 5 mm spatial resolution. Finite element analyses were performed to study additive manufacturing process. Plastic and elastic residual strain distributed incurred by the process were calculated and compared with measurement results obtained by embedded sensors. Both temperature and strain measured by fiber sensors are in excellent agreement with numerical simulations. Using embedded fiber sensor measurement results are cue, various laser processes were applied to further temper properties of metal components. This dissertation explores potentials on using adaptive optical technology to perform rapid and high precision laser shock peening to mitigate residual strain induced by additive manufacturing. Using embedded fiber sensors, laser shock peening induced strain modifications were measured with high spatial resolution to improve properties and accuracy of 3D manufactured metal components against corrosion. Research discussed in this dissertation has advanced both fiber optic sensing technology and additive manufacturing. By incorporating advanced optical sensing technology directly into the component’s design and additive manufacturing processes, this research results in new manufacturing techniques to produce a wide array of smart parts for advanced energy systems. The seamless incorporation of multi-functional fiber optic devices into components and parts common to advanced energy system will enable and facilitate condition-based monitoring of key components and systems of fossil, renewable and nuclear power systems to improve safety and efficiency of fossil-fuel energy power generation

Composite Aircraft Lightning Strike Protection Damage Evaluation Using Microwave Microscopy

RÉSUMÉ Au cours des 30 derni`eres anne´es, les concepteurs d’avions se sont tourne´s vers les mat´eriaux composites pour fabriquer leurs v´ehicules plus légers, plus économes en carburants et plus confortable pour les passagers. Les matériaux composites présentent un certain nombre d’avantages substantiels par rapport au métal, traditionnellement utilisés dans l’industrie. L’une de leurs principales propriétés est un rapport rigidité/poids très élevé. Des fibres très rigides (généralement des fibres de carbone ou verre) sont incorporées dans un matrice (généralement de la résine epoxy). Un deuxième avantage par rapport au métal est leur tolérance a` la corrosion. Les métaux sont sensibles à la fois à la corrosion et a` la fatigue. Les composites ne se corrodent pas, et ils ne sont pas soumis a` des dommages de fatigue dans l’e´tendue des structures métalliques. Par contre, les composites ne sont pas conducteurs (par exemple la fibre de verre) ou sont nettement moins conducteurs que les métaux (par exemple la fibre de carbone), et ne peuvent donc pas évacuer les courants intenses développées lors d’un événement de foudre. Les constructeurs d’avions ont mis au point un certain nombre de techniques spécialisées de protection contre la foudre (LSP en anglais). Bien que les LSP soient devenus d’un intérêt primordial au fil des ans comme moyen de protéger les structures composites, aucune technique n’a été trouvée pour diagnostiquer les LSP en fonction des exigences des fabricants. Les travaux de cette thèse ont ´ete´ partiellement financés par Bombardier Aérospatiale. Elle se concentrera sur une forme particulière de LSP, a` savoir la feuille de cuivre expansée (ECF en anglais) qui est largement utilisée dans la plupart des avions de long courrier de Bombardier. Le premier chapitre de cette thèse consiste en une revue des techniques les plus répandues actuellement utilisées dans l’évaluation des polymères renforcés de fibres de carbone (CFRP en anglais). Il compare ces techniques et indique leurs limites et contraintes pour résoudre le problème formule´. Une introduction a` la microscopie micro-onde en champ proche est également présentée dans ce chapitre. Elle représente le noyau de tous les développements de cette thèse. Il y a de nombreuses variables qui interviennent dans la technique de microscopie micro-onde et qui doivent être adaptées `a l’objectif de ce travail. Le Chapitre 2 est une exploration des sondes électriques en champ proche ainsi qu’une analyse du dispositif de mesure correspondant. Il fixe les limites, les avantages et les inconvénients de l’applicabilité de la microscopie a` micro-ondes pour l’´evaluation des CFRP. Une sonde `a guide d’onde coplanaire est utilisée ici pour démontrer que des défauts peuvent être trouvés dans le maillage LSP.----------ABSTRACT The last decades have seen a surge in the amount of composite materials used by the aircraft industry. This tendency is mainly due to a key advantage of composites compared with traditional aluminum alloys. They have strength while lowering the overall weight of the aircraft, which saves fuel consumption and improve efficiency and performance. A second advantage over metal is their tolerance to corrosion. Metals are susceptible to both corrosion and fatigue. Composites do not corrode, and they are not subject to fatigue damage to the extent of metal structures. Conversely, composites are either not conductive (e.g. fiberglass) or significantly less conductive than metals (e.g., carbon fiber), so they can not conduct away the intense currents developed during a lightning event. Aircraft manufacturers have developed a number of specialized lightning strike protection materials (LSP). Even though LSP has become of paramount interest over the years as a means to protect composite structures, no techniques have been found to diagnose LSP according to manufactures requirements. Since this work was partially supported by Bombardier Aerospace, it will be concentrated in one particular form of LSP, namely expanded copper foil (ECF) which is widely used in most Bombardier long courier aircraft.The first chapter of this thesis reviews the most established techniques currently used in carbon fiber reinforced polymer (CFRP) evaluation. It compares these techniques and it states their limitations and constraints to solve the formulated problem. An introduction of near field microwave microscopy is also presented in this chapter which will be the core of all developments in this dissertation.There are many variables that intervene in microwave microscopy technique that must be adapted to the purpose of this work. Chapter 2 is an exploration of near filed electric probes and analysis of the related measurement set-up. It sets the limits, pros and cons of the applicability of microwave microscopy for scanning CFRP. A coplanar waveguide probe is used here to demonstrate that defects can be found in LSP mesh. The limits of this design prompts the development of a magnetic probe.A new application of microwave microscopy technique is experimentally demonstrated in Chapter 3. A new magnetic probe design is introduced capable of revealing flaws as small as 100 µm in the LSP, even under 350 µm thick paint layer. A potential application consisting of measuring the paint thickness is identified as well. Experimental measurements of the paint layer thickness with a resolution better than 2 µm have been demonstrated