[Show abstract][Hide abstract] ABSTRACT: Dielectric properties of alkali silicates (Na,K)2(SiO2)nO) or ‘water glasses’ are a critical input into the electromagnetic modeling of these materials, which have a broad range of applications. Recent increased interest in understanding geopolymerization of aluminosilicates with water glasses and the potential to improve understanding of the role of moisture in damage due to alkali-silica reaction (ASR) in concrete (where water glass is a suitable analogue for ASR gel) motivates this research. This investigation presents the results of microwave dielectric property measurement of twelve laboratory-produced (synthetic) water glass samples at X-band (8.2–12.4 GHz). Results show an exponential decay of loss factor as a function of increasing silica-to-alkali ratio, suggesting a correlation with increase in bound water in the samples and a decrease in the fluid ionic concentration. The results provide an insight into the temporal changes of the dielectric properties of ASR-affected materials, as well as geopolymers.
[Show abstract][Hide abstract] ABSTRACT: Fiberglass is increasingly used for corrosion-resistant industrial applications. Fiberglass laminate typically consists of a structural layer and a corrosion-resistant layer (commonly referred to as the liner) on the inside of the laminate. Liners are used in all corrosion-resistant fiberglass applications to guard against corrosive chemical attack on the structural layers of the fiberglass. The ability to measure fiberglass thickness from one side of a laminate during equipment operation quickly and easily with high measurement accuracy is extremely important for life assessment and prediction. Microwave nondestructive techniques employing open-ended rectangular waveguides in conjunction with robust and full-wave electromagnetic models have tremendous potential and viability for evaluating complex layered dielectric composite structures for the purpose mentioned here. In this paper, we describe the foundation of this technique, processes involved in optimizing the approach for layered fiberglass evaluation, as well as a number of representative and diverse measurement results demonstrating the efficacy of the technique and the thickness accuracy that can be obtained. In addition, other important issues such as the effect of curvature of the structure and water with different temperatures filling it were also addressed and shown not to adversely affect measurement accuracy associated with liner thickness.
IEEE Transactions on Instrumentation and Measurement 10/2015; 64(10):1-1. DOI:10.1109/TIM.2015.2426352 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Portable millimeter wave imaging systems are desired in many nondestructive testing and imaging applications. Interferometry-based instruments have shown to produce three-dimensional holographic images when proper phase referencing is implemented. This paper introduces the design of an interferometry-based wideband, and low-cost reflectometer capable of producing holographic three-dimensional millimeter wave images. Analysis of sources of image errors, and three-dimensional synthetic aperture radar (SAR) image examples are presented at Ka-band (26.5-40 GHz), V-band (50-75 GHz) and W-band (75-110 GHz).
Conference Record - IEEE Instrumentation and Measurement Technology Conference 07/2015; 2015:925-929. DOI:10.1109/I2MTC.2015.7151393
[Show abstract][Hide abstract] ABSTRACT: Microwave materials characterization techniques have been extensively and successfully used for evaluating important properties of a wide range of cement-based materials and structures. Recent investigations using these techniques for studying properties of mortar with alkali-silica reaction (ASR) gel have also been very promising. In this research, microwave dielectric properties of multiple mortar samples with different compositions and when subjected to different humidity levels are investigated. This paper presents the results of these experiments and the subsequent analysis pertinent to humidity-related issues in the mortar samples.
Instrumentation and Measurement Technology Conference (I2MTC), 2015 IEEE International; 05/2015
[Show abstract][Hide abstract] ABSTRACT: Recent advances in several technological fronts have made it possible to design and develop real-time microwave imaging systems (cameras) for use in a wide range of applications. Proper design of the individual antenna array element in these systems is of critical importance. Furthermore, wideband versions of these imaging systems, which are capable of one-sided operation and in conjunction with advanced synthetic aperture radar imaging algorithms, can produce high-resolution 3-D holographical images. In this paper, an orthogonally fed, small elliptical dual varactor-loaded slot antenna, operating in K-band (18-26.5 GHz), is designed for use in such imaging systems. The feed design uses a coplanar waveguide with a 90° bend without a direct dc connection to the antenna. This feed structure accommodates effective placement of required switches while enabling one-sided measurement. Finally, the slot is loaded with two varactor diodes resulting in 3.5 GHz of bandwidth suitable for high-resolution microwave 3-D holographical imaging. The design of this slot and a (1/4) array of such slots is presented. Two arrays are fabricated and used independently, as well as simultaneously, to form (mono- and bistatic, respectively) imaging results.
IEEE Transactions on Instrumentation and Measurement 03/2015; 64(3):740-749. DOI:10.1109/TIM.2014.2355451 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Corrosion assessment of embedded steel in concrete structures is generally performed by electrochemical methods that are not fully nondestructive because the device requires connection to the steel. For practical applications, the development of a truly nondestructive technique for the detection of corrosion is desirable. This paper presents an experimental study of a wideband microwave three-dimensional synthetic aperture radar imaging technique applied to reinforced concrete specimens subjected to corrosion. Two orthogonal wave polarization directions were used for this purpose. Two-dimensional microwave image slices were analyzed and compared with the actual loss of steel measured during the destructive testing. As expected, the results indicated that higher-frequency images have higher spatial resolution, while the signal penetration became more limited at these frequencies. Though the relatively high moisture content significantly increased the dissipative properties against electromagnetic propagation, the technique was capable of differentiating between corroded and non-corroded steel bars.
[Show abstract][Hide abstract] ABSTRACT: This paper introduces a general methodical approach for designing frequency reconfigurable antennas. This method was successfully used to design a novel coplanar waveguide (CPW)-fed slot frequency reconfigurable antenna capable of operating at four preselected frequency bands distributed over a wide frequency range from ~ 59.5 MHz to ~ 1000 MHz (i.e., ~ 4 octaves of bandwidth) while keeping its overall size as small as possible. To add reconfigurablility to the antenna, optimally-designed and electronically-controllable PIN diode-loaded slots were used to strategically manipulate the flow of current path and consequently change the characteristics of the antenna. Designing for the lowest operating frequency (59.5 MHz), capacitor-loaded meandered slot lines and reconfigurable matching network were implemented to keep the size of the antenna as small as possible. The resulting overall size of the antenna is only 0.06λL×0.06λL where λL is calculated at 59.5 MHz. The measurement results verified that the antenna successfully operates at 59.25-59.75 MHz, 314-398 MHz, 430-496 MHz, and 792-950 MHz, all with an almost omnidirectional pattern and an acceptable gain.
IEEE Transactions on Antennas and Propagation 12/2014; 62(12):6049-6059. DOI:10.1109/TAP.2014.2364293 · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Detection of surface-breaking cracks in metals is an important issue in many industries (e.g., transportation, aerospace, nuclear). Commonly, eddy current and ultrasonic techniques are used for this purpose. In recent years, a significant amount of work has also been conducted using microwave methods. Consequently, to better understand the interaction between a microwave probe (i.e., open-ended rectangular waveguide or coax) and a crack, a number of electromagnetic models have been developed. For an open-ended coaxial probe, when a crack coincides with the center conductor region of the probe, all previously developed models significantly underestimate the results obtained from measurements. This paper examines the primary reason for this discrepancy, which turns out to be due to a geometrical perturbation in the probe center conductor geometry and its subsequent interaction with a crack.
IEEE Transactions on Instrumentation and Measurement 07/2014; 63(7):1877-1879. DOI:10.1109/TIM.2014.2317295 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nondestructive evaluation of stratified (layered) composite structures at microwave and millimeter-wave frequencies is of great interest in many applications where simultaneous determination of the complex dielectric properties and thicknesses of multiple layers is desired. Open-ended rectangular waveguide probes, radiating into such structures, are effective tools for this purpose. The technique utilizes a full-wave electromagnetic model that accurately models the complex reflection coefficient as a function of frequency and material properties. While the electromagnetic model assumes an infinite waveguide flange (or ground plane), the measurements are conducted using a finite-sized flange. Consequently, the results of the electromagnetic model and those from measurements may not be sufficiently alike for accurate dielectric property and thickness evaluation. This paper investigates the effect of using an open-ended waveguide with a standard finite-sized flange on the error in evaluating the complex dielectric properties of a composite structure. Additionally, we present the design of a novel flange that markedly reduces this undesired effect by producing very similar electric field properties, at the flange aperture, to those created by an infinite flange. Finally, the efficacy of the design for evaluating the dielectric properties of a layered composite structure is demonstrated as well.
IEEE Transactions on Instrumentation and Measurement 06/2014; 63(6):1524-1534. DOI:10.1109/TIM.2013.2291952 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Millimeter wave near-field nondestructive testing (NDT) and imaging techniques are well-suited for the inspection of exposed (bare) and coated metallic surfaces for detecting surface-breaking discontinuities such as cracks and corrosion under coatings. However, one drawback of near-field imaging is the long scan time due to the dense grid requirements at these high frequencies. This paper presents a novel imaging technique for NDT of metallic surface for detecting surface-breaking cracks. This technique incorporates wideband measurements, synthetic aperture radar (SAR) based imaging algorithms, and surface wave propagation. The proposed technique vastly reduces the required scan time to produce an image. This paper introduces the imaging technique and studies its efficacy using full-wave numerical electromagnetic simulations. Results of measurements at K-band (26.5-40 GHz) and V-band (50-75 GHz) are provided illustrating the feasibility of the proposed technique for rapid detection of metal surface discontinuities.
2014 IEEE International Instrumentation and Measurement Technology Conference (I2MTC); 05/2014
[Show abstract][Hide abstract] ABSTRACT: Recent technological advancements have made it possible to produce synthetic aperture radar (SAR) based microwave images in real time by using a variety of microwave imaging (array) systems. However, depending on the imaging array construction and the data collection scheme, the pertinent data of interest may be corrupted by undesired signals resulting from array element mutual coupling and overall poor isolation. Poor isolation associated with array elements may result from imperfect RF switching, internal coupling mechanisms inherent in a measurement system, or imperfect signal tagging schemes (i.e., multiple elements modulating when using the modulated scatterer technique). Images produced from such corrupted data are blurred or have artifacts that tend to mask the desired indications. This paper demonstrates the extension of a well-known correlation canceling technique for the purpose of preprocessing the data to remove such undesired coupling effects. To demonstrate its effectiveness, it is applied to a recently developed 2-D high-resolution and real-time microwave imaging system (camera). This camera is composed of 576 array elements, which are susceptible to the type of signal degradation mentioned above. Three correction estimates of the preprocessor are performed and compared. Two of the correction estimates directly address coupling, and the third, which does not consider coupling, is used for comparison purposes. Simulation results show the efficacy of this method, which is then corroborated by experiments.
IEEE Transactions on Instrumentation and Measurement 05/2014; 63(5):1310-1319. DOI:10.1109/TIM.2013.2283550 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Small, portable, and wideband millimeter-wave imaging systems are used in many nondestructive testing and imaging applications, as these systems are capable of producing high-resolution images of the interior of composite structures. Typically, systems capable of producing holographic 3-D images incorporate expensive and bulky wideband heterodyne coherent reflectometers or commercial vector network analyzers. In many nondestructive testing applications, evaluation of electrical property distribution of an object is not of interest; instead, the geometrical distribution of the object is studied. In such cases, the use of coherent reflectometers that provides referenced information about the magnitude and phase of a reflected signal is not required. Consequently, simpler reflectometers, capable of producing 3-D holographical images of objects are of great value since they reveal significant information about the object. Moreover, 3-D holographical images enable production of image slices at different depths. This paper presents a novel wideband, small, and low-cost reflectometer capable of producing holographic 3-D millimeter-wave images. The design of the reflectometer, as well as several examples of produced images in diverse materials, is provided.
IEEE Transactions on Instrumentation and Measurement 05/2014; 63(5):1328-1336. DOI:10.1109/TIM.2014.2298618 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Previously, a Wiener filter-based synthetic aperture radar (SAR) technique was developed to successfully image embedded objects in a general layered structure. The results of the imaging technique were then verified through performing extensive measurements. Here, the sensitivity of this technique to different critical parameters is investigated using a full-wave electromagnetic simulation software. These parameters include those related to the sample being imaged (e.g., electrical properties of layers), those related to measurements (e.g., electromagnetic wave polarization), and those associated with the modeling process (e.g., electrical properties of layers used in the image reconstruction procedure).
2014 IEEE International Instrumentation and Measurement Technology Conference (I2MTC); 05/2014
[Show abstract][Hide abstract] ABSTRACT: Microwave materials characterization techniques based upon dielectric property measurements are well-suited for detection and evaluation of physical and chemical changes in cement-based materials. In this investigation, microwave dielectric properties of several mortar samples were measured at S-band (2.6-3.95 GHz) and X-band (8.2-12.4 GHz) at two different times nearly one year apart. It was found that during this period, while the samples remained in ambient environment conditions, their masses remained essentially constant over time. However their dielectric properties underwent a relatively substantial change. To investigate the reason(s) behind this phenomenon, both pH indicator test and thermogravimetric analysis were conducted and the results confirmed carbonation in the samples. In this paper, the results of these investigations are presented. Additionally, a first-order dielectric mixing model capable of carbonation depth estimation is described.
2014 IEEE International Instrumentation and Measurement Technology Conference (I2MTC); 05/2014
[Show abstract][Hide abstract] ABSTRACT: Wideband waveguide-based couplers have many utilities in microwave and particularly millimeter wave frequency ranges. This paper presents the design for a versatile full waveguide band nondirectional coupler used in conjunction with noncoherent single-port reflectometers, and for other pertinent applications. The design offers several parameters that once simultaneously optimized can provide a constant coupling level throughout the desired frequency band. The design procedure and simulation results for a Ka-band (26.5–40 GHz) coupler are presented. Such a coupler was then designed and constructed to operate in X-band (8.2–12.4 GHz) and the measured and simulated coupling levels show good agreement.
IEEE Transactions on Instrumentation and Measurement 04/2014; 63(4):984-986. DOI:10.1109/TIM.2013.2297671 · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we evaluate the utility of microwave and mechanical wave nondestructive testing techniques to detect delamination in reinforced concrete bridge deck mock-up samples. The mechanical wave tests comprise air-coupled impact-echo measurements, while the microwave measurements comprise three-dimensional synthetic aperture radar imaging using wideband reflectometery in the frequency range of 1-4 GHz. The results of these investigations are presented in terms of images that are generated from these data. Based on a comparison of the results, we show that the two methods are complementary, in that provide distinct capabilities for defect detection. More specifically, the former approach is unable to detect depth of a delaminated region, while the latter may provide this information. Therefore, the two methods may be used in a complementary fashion (i.e., data fusion) to give more comprehensive information about the 3D location of delamination.
[Show abstract][Hide abstract] ABSTRACT: Robust detection of vertical cracks in high-density polyethylene (HDPE) pipes is a challenging task for the majority of nondestructive testing (NDT) techniques. Vertical cracks are specifically referred to those whose largest planar view is parallel to the signal direction of propagation, leaving very little signal to be scattered for detection. In such pipes this commonly occurs between two pipes sections when thermally or adhesively joined. This work presents the utility and efficacy of three-dimensional (3D) millimeter wave holographical imaging based on synthetic aperture radar (SAR) algorithm for imaging such cracks. Such a 3D millimeter wave image can readily represent the type, size, and location of various flaws within a structure. Two-dimensional (2D) slices of the 3D image, at different orientations, can also be readily produced showing the cross-sectional views of the structure and flaws, further aiding in identifying, and sizing a flaw or vertical crack. Imaging results for planner and curved (pipe section) specimen with machined flaws are presented. These images are produced using a novel field-portable, small, and low-cost wideband millimeter-wave reflectometer capable of rapid 3D image production.
[Show abstract][Hide abstract] ABSTRACT: The presence of reactive aggregates combined with sufficient moisture and concentration of alkalis are the three basic requirements for damage due alkali-silica reaction (ASR) gel formation and expansion in concrete. For health-monitoring of concrete structures, and in order to investigate the potential for detecting ASR gel formation in existing structures, one potential technique involves studying changes in the temporal complex dielectric constant of concrete structures. In this paper, a microwave nondestructive testing approach is proposed which involves soaking two hardened mortar samples and measuring the change in their temporal complex dielectric constant in order to distinguish between the sample containing ASR gel and the one devoid of it. Part of the free water becomes bound in the sample containing ASR gel and since a portion of the microcracks in this sample contain ASR gel, the rate of evaporation of water of the two samples is expected to be different. The complex dielectric constant of the samples is significantly dependent upon the volumetric level and movement (in and out of the samples) of free water. Therefore, studying the relative different temporal rate of change in this parameter is expected to yield information about the presence or absence of ASR gel.