IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 56, NO. 5, OCTOBER 2007 1961 Innovative Near-Field Microwave Nondestructive Testing of Corroded Metallic Structures Utilizing Open-Ended Rectangular Waveguide Probes Nasser N. Qaddoumi, Member, IEEE, Wael M. Saleh, and Mohamed Abou-Khousa, Student Member, IEEE Abstract—An innovative method utilizing microwave nonde- structive testing and evaluation (NDT&E) techniques for en- hanced detection and evaluation of rust under paint has been developed. The results are promising, and they show that the use of a layer of dielectric material with known properties to replace the standoff layer (air) yields significant improvements in the measurement sensitivity and resolution for some microwave NDT applications. The goals of this investigation were pursued and achieved using theoretical electromagnetic computer programs describing the interaction of microwave signals radiated out of an open-ended rectangular waveguide with any layered medium. One of the computer programs calculates the aperture reflection coefficient (simulating measurements), and the other computer program calculates the field distribution in any layered media. The experimental results were obtained using an inspection system that was built according to the results of the theoretical investigation, and they confirmed the theoretical results. Index Terms—Near-field microwave nondestructive testing (NDT), open-ended rectangular waveguide, rust detection. I. I NTRODUCTION Q UALITY control is an issue that is of great importance in many real-life applications. Error prevention is one of the main targets that industrial and governmental agen- cies aim for [1]–[3]. Rust detection and thickness evaluation are of great interest in various industrial, civil, and military applications. Microwave imaging and testing techniques are based on studying the magnitude and/or phase data of the transmitted and/or reflected waves to determine the reliability of a specimen under test. The two commonly used param- eters that influence the reflection properties are the standoff distance and the frequency of operation [3]–[11]. In this paper, a new optimization parameter will be introduced to improve the sensitivity of an open-ended rectangular waveguide sensor in detecting rust under paint. Additionally, the capability and potential of utilizing the open-ended rectangular waveguide sensors in detecting and evaluating the thickness of rust under paint will be theoretically and experimentally investigated. Manuscript received July 5, 2006; revised June 24, 2007. N. N. Qaddoumi and W. M. Saleh are with the Microwave Imaging and Nondestructive Evaluation Laboratory, Department of Electrical Engineering, American University of Sharjah, Sharjah, UAE (e-mail: nqaddoumi@aus.edu; wsaleh@aus.edu). M. Abou-Khousa is with the Applied Microwave Nondestructive Testing Laboratory, Department of Electrical and Computer Engineering, University of Missouri—Rolla, Rolla, MO 65409-0040 USA (e-mail: maamc2@umr.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2007.904570 Millions of dollars may be saved when rust detection is con- ducted precisely, consistently, and at its early stages of progress. The savings in maintenance cost, damage minimization, and reduction in repaint cycles in different industrial, civil, and military environments are of great importance. It is desirable to develop an inspection method that is 1) nondestructive and preferably noncontact; 2) fast and provides for spot-checking of a structure; 3) capable of evaluating and estimating the thickness of rust under paint on metallic structures; 4) real time, allowing for on-site evaluation of a structure; 5) small and portable so that the evaluation of a structure can be performed by a single operator; 6) relatively inexpensive; 7) operator friendly while requiring minimal data analysis by the operator. Near-field applied microwave nondestructive testing tech- nique, utilizing the open-ended rectangular waveguide sensors, appears to be one of the most promising techniques in detecting the presence or absence of a certain layer within a layered structure [10]. Accordingly, microwave nondestructive testing and evaluation (NDT&E) methods were considered for the rust- detection application, and they can provide all the features listed earlier [11]. The presence of rust or corrosion may be considered to be an additional thin layer under a coating. Microwave signals, unlike ultrasound signals, can penetrate inside low-loss dielectric materials and interact with their inner structures without suffering from high attenuation. They also undergo total reflection at metal surfaces. Therefore, the signal travels twice through the rust layer, which increases the possi- bility of rust detection. Furthermore, in other applications, opti- mization of the measurement parameters, such as the frequency of operation, has shown measurement sensitivity to thickness variations in the range of a few micrometers at frequencies in the X-band range [10]. When this parameter, which is the frequency of operation, and the other parameter, which is the standoff distance, are optimally chosen using theoretical and/or experimental investigations, significant increases in detection sensitivity may be achieved [9], [10]. Measurement-parameter optimization may be first numerically accomplished using the multilayer computer program. II. BACKGROUND Fig. 1 shows an open-ended rectangular waveguide probe radiating into a stratified dielectric composite that is made 0018-9456/$25.00 © 2007 IEEE