Millimeter-Wave Differential Probe for Nondestructive Detection of Corrosion Precursor Pitting

Critical aircraft structural components, such as wings and fuselages, are exposed to harsh environments that vary considerably in temperature and moisture content. In most cases, the corrosion is hidden under paint and primer and cannot be visually detected. The initiation of corrosion is preceded by the presence of corrosion precursor pitting. Near-field millimeter-wave nondestructive testing (NDT) methods have been successfully used for detecting corrosion precursor pitting in exposed as well as painted aluminum substrates. However, near-field millimeter-wave measurements are susceptible to clutter that may mask indications of small defects such as pitting. Standoff distance variation produces an unwanted intensity gradient on an image and may be considered the most undesired clutter-producing effect. This paper presents a differential millimeter-wave probe consisting of a pair of radiating apertures. It is shown that the differential nature of this probe tends to significantly reduce the undesired effect of standoff distance variation, thereby enhancing probe detection sensitivity. Furthermore, when this probe is used for the purpose of millimeter-wave imaging, it produces defect indications with unique features that help in distinguishing the defect from noise. This dual differential probe was used for detecting corrosion precursor pitting. The design of the probe and the results of detecting various pittings are presented in this paper

Novel Near-Field Microwave and Millimeter Wave Differential Probe using a Dual-Modulated Single Aperture

A novel differential probe design is introduced in this paper for near-field microwave and millimeter wave non-destructive testing (NDT) and imaging applications. In such applications, the variations in the distance between the probing antenna and the structure under inspection, i.e., standoff distance, can potentially mask the signal of interest, and hence, adversely impact the detection capability of the probe. Differential near-field probes and compensation methods were developed in the past to null out the standoff distance variation effect from the measured signal. The available methods, however, suffer from some limitations such as using two balanced apertures or offering limited range of compensation. While the differential probe proposed here exhibits an excellent immunity against standoff distance variation, it overcomes the limitations of the aforementioned methods. The proposed probe is based on electronically modulating the aperture of a rectangular waveguide using PIN diode-loaded dipoles placed symmetrically in the aperture region. It will be shown that the adverse effect of standoff distance variation can be eliminated, or otherwise, significantly reduced by non-coherently subtracting the signals measured at two diferent aperture modulation states

Dielectric Properties Determination of a Stratified Medium

The method of detection of variation in dielectric properties of a material covered with another material, which requires nondestructive measurement, has numerous applications and the accurate measurement system is desirable. This paper presents a dielectric properties determination technique whereby the dielectric constant and loss factor are extracted from the measured reflection coefficient. The high frequency reflection coefficient shows the effect of the upper layer, while the dielectric properties of the lower layer can be determined at the lower frequency. The proposed technique is illustrated in 1-11 GHz band using 5 mm-thick water and 5% saline solution. The fluctuation of the dielectric properties between the high frequency and the low frequency, results from the edge diffraction in the material and the multiple reflections at the boundary of the two media, are invalid results. With the proposed technique, the dielectric properties of the lower layer can be accurately determined. The system is validated by measurement and good agreement is obtained at the frequency below 3.5 GHz. It can be applied for justifying variation of the material in the lower layer which is important in industrial process

High Resolution Near Field Microwave Imaging using Loaded Circular Aperture Probe

Circular aperture probes have been successfully used for high-resolution near-field microwave imaging. It has been established that circular aperture probes could conceivably provide higher image fidelity compared to rectangular aperture probes used conventionally for near-field imaging. In this paper, it is proposed to further enhance the near-field imaging resolution and sensitivity by loading the circular aperture with a resonant iris. The proposed probe herein operates in the X-band frequency range and exhibits very localized near-field distribution at the opening of the iris. Consequently, its imaging resolution and sensitivity are enhanced compared to the conventional aperture probes operating over the same frequency band. The imaging capability of the proposed probe is analyzed using 3D electromagnetic simulation, and its performance is validated experimentally. The efficacy of the proposed probe for high-resolution imaging is demonstrated by practical imaging dielectric and metallic samples. Furthermore, the obtained images using the proposed probe are compared to those acquired using conventional circular and rectangular aperture probes. It will be demonstrated that the proposed probe provides higher sensitivity and resolution compared to the conventional aperture probes

Microwave detection of surface breaking cracks in metallic structures under heavy corrosion and paint

We live in the world of aging infrastructures . In this environment, critical and heavily utilized infrastructure, i.e. ships, planes, bridges, etc., are operating at or beyond their designed age. Replacement is no longer an option and retirement for cause is the current approach to maintenance and replacement. Consequently, there is an ever-increasing demand for efficient and robust nondestructive evaluation (NDE) methods that can determine the physical health of these structures. Large structures, which are primarily made of metals, either steel or aluminum, are susceptible to high-stress cracking and corrosion. Stress-induced cracks in heavily corroded steel, used in bridges, railroads, storage tanks, etc., are extremely difficult to detect. Current methods have limitations that render inspection to take longer either than it should or risk not detecting an existing crack. Microwave signals readily penetrate through dielectric materials such as paint and corrosion byproducts (i.e., rust), while conducting materials (i.e., metals) strongly reflect microwave signals. Therefore, interrogating a metal surface for surface-breaking cracks is readily possible even in the presence of a relatively thick layer of corrosion or paint. Normally, surface-breaking cracks are very small and the perturbations they cause to an irradiating microwave signal are small in amplitude unless the detection is performed very close to the surface. In this thesis, the implementation of a microwave imaging system that utilizes a synthetic aperture radar (SAR) approach to detect surface-breaking cracks in metallic structures under heavy corrosion and corrosion preventive paints is investigated. The resulting SAR images were analyzed and compared to numerical simulations to identify real-world capabilities and theoretical limitations --Abstract, page iii

Review and comparison of shearography and active thermography for nondestructive evaluation

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Depth evaluation of damage to aircraft fuselage skins using microwave and millimeter wave methods

The depth of damage to metal surfaces is a particularly useful, sought after, and difficult to obtain piece of information, and is used to guide repair decisions. For this investigation such damage is represented as rectangular slots (cracks) or cylindrical pits. Several millimeter wave methods are presented to evaluate depth. The quarter-wavelength resonant response of cracks, excited by a probing rectangular waveguide, allows evaluation of depth using the phase of reflection coefficient. A theoretical derivation is also supplied, modeling the system as the junction of two rectangular waveguides, the probing waveguide and the crack. If the crack is filled with a dielectric material, shallower cracks may be evaluated and the magnitude of reflection coefficient may be used instead of phase. This approach has the advantage of low sensitivity to crack width and probe position, but has some limitations in the minimum depth and the smallest openings which can be evaluated. The depth of pits and shorter cracks may be evaluated using the phase in a non-resonant approach, by comparison with reference curves. A dielectric slab-loaded waveguide probe is also developed which theoretically would allow measurement of smaller damages, of both opening and depth, as a resonance perturbation. Sensitivity to very small damage is apparent, but high sensitivity to probe position is also evident. Extensive simulation results are presented for each approach, with supporting measurements. Evaluation of the depth of filled cracks, using the quarter-wavelength resonance approach, is, in particular, demonstrated in measurements for depths from 0.7- 2.7 mm for three frequency bands and three filling materials --Abstract, Page iii

Hardware architectures for compact microwave and millimeter wave cameras

Millimeter wave SAR imaging has shown promise as an inspection tool for human skin for characterizing burns and skin cancers. However, the current state-of-the-art in microwave camera technology is not yet suited for developing a millimeter wave camera for human skin inspection. Consequently, the objective of this dissertation has been to build the necessary foundation of research to achieve such a millimeter wave camera. First, frequency uncertainty in signals generated by a practical microwave source, which is prone to drift in output frequency, was studied to determine its effect on SAR-generated images. A direct relationship was found between the level of image distortions caused by frequency uncertainty and the product of frequency uncertainty and distance between the imaging measurement grid and sample under test. The second investigation involved the development of a millimeter wave imaging system that forms the basic building block for a millimeter wave camera. The imaging system, composed of two system-on-chip transmitters and receivers and an antipodal Vivaldi-style antenna, operated in the 58-64 GHz frequency range and employed the ω-k SAR algorithm. Imaging tests on burnt pigskin showed its potential for imaging and characterizing flaws in skin. The final investigation involved the development of a new microwave imaging methodology, named Chaotic Excitation Synthetic Aperture Radar (CESAR), for designing microwave and millimeter wave cameras at a fraction of the size and hardware complexity of previous systems. CESAR is based on transmitting and receiving from all antennas in a planar array simultaneously. A small microwave camera operating in the 23-25 GHz frequency was designed and fabricated based on CESAR. Imaging results with the camera showed it was capable of basic feature detection for various applications --Abstract, page iv