[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 · 1.71 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 · 2.46 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 · 1.71 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 · 1.71 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 · 1.71 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 · 1.71 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 · 1.71 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.
[Show abstract][Hide abstract] ABSTRACT: Alkali–silica reaction (ASR) is one common cause of concrete deterioration and has been a growing concern for decades. Water, in the presence of reactive aggregates used to make concrete, plays a major role in the formation, sustainment, and promotion of this reaction. In this process, free water becomes bound within ASR gel, resulting in expansion and deterioration of concrete. Devising a test approach that is sensitive to the state of water (free or bound) has the potential to become a method-of-choice for ASR detection and evaluation, since such measures can be used to detect ASR and potentially quantify reaction progression. Microwave signals are sensitive to the presence of water, since the water relaxation frequency occurs in this frequency range. Recently, microwave nondestructive evaluation techniques have shown great potential to evaluate and distinguish between ASR-affected mortar samples and those without ASR gel. Given the complex chemistry of ASR products, their behavior is expected to differ at different microwave frequency bands. To evaluate the sensitivity of different frequencies to the presence of ASR, dielectric constant measurements were conducted at $R$ -band (1.7–2.6 GHz), $S$ -band (2.6–3.95 GHz), and $X$ -band (8.2–12.4 GHz). This paper presents the measured results for mortar samples made with reactive and nonreactive aggregates. The measurement results and subsequent analyses aid in a better understanding of the microwave signals interaction with ASR-affected cement-based materials. Moreover, the results indicate that $S$ -band appears to be the most appropriate frequency band for ASR evaluation in the microwave regime.
IEEE Transactions on Instrumentation and Measurement 01/2014; 64(7):1-1. DOI:10.1109/TIM.2014.2367771 · 1.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, two new techniques for microwave imaging of layered structures are introduced. These techniques were developed to address the limiting issues associated with classical synthetic aperture radar (SAR) imaging techniques in generating focused and properly-positioned images of embedded objects in generally layered dielectric structures. The first method, referred to as piecewise SAR (PW-SAR), is a natural extension of the classical SAR technique, and considers physical and electrical properties of each individual layer and the discontinuity among them. Although this method works well with low loss dielectric media, its applicability to lossy media is limited. This is due to the fact that this method does not consider signal attenuation. Moreover, multiple reflections within each layer are not incorporated. To improve imaging performance in which these important phenomena are included, a second method was developed that utilizes the Green's function of the layered structure and casts the imaging approach into a deconvolution procedure. Subsequently, a Wiener filter-based deconvolution technique is used to solve the problem. The technique is referred to as Wiener filter-based layered SAR (WL-SAR). The performance and efficacy of these SAR based imaging techniques are demonstrated using simulations and corresponding measurements of several different layered media.
IEEE Transactions on Antennas and Propagation 01/2014; 62(1):282-294. DOI:10.1109/TAP.2013.2287024 · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, a real-time and portable 2-D microwave imaging system operating at 24 GHz, incorporating modulation schemes similar to modulated scatterer technique and synthetic aperture radar (SAR) imaging technique, was designed using a heterodyne receiver with superior performance compared with a homodyne receiver. Multifrequency SAR images have the added advantage of providing for: 1) coherent averaging of image data over frequency and hence increasing system signal-to-noise ratio (SNR) and 2) enable volumetric (3-D) image production. This letter describes modifications to this receiver for enabling multifrequency operation through phase uncertainty (PU) analysis. These modifications show significant reduction in PU, thereby allowing the coherent addition of data necessary for SAR image formation with higher overall SNR.
IEEE Transactions on Instrumentation and Measurement 01/2014; 63(1):243-245. DOI:10.1109/TIM.2013.2286651 · 1.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this communication, a new approach for feeding a PIN diode-loaded resonant elliptical slot used in microwave imaging arrays for nondestructive testing applications is considered. The proposed feed is based on coupling the transmitted and received signals into the slot using an out-of-plane (perpendicular) microstrip line. An imaging array prototype with this slot, a 90 °-bent microstrip-line feed and a coplanar waveguide (CPW) to microstrip line transition was designed and experimentally validated. The design readily accommodates placement of the switching and PIN diode biasing networks on the feeding plane using CPWs, enabling one-sided imaging measurements. The relevant design steps and imaging results obtained with this prototype, in mono-static and bi-static modes, are presented.
IEEE Transactions on Antennas and Propagation 10/2013; 61(10):5311-5314. DOI:10.1109/TAP.2013.2271491 · 2.46 Impact Factor