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

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

Application of Millimeter Wave, Eddy Current and Thermographic Methods for Detection of Corrosion in Aluminum Substrate

Aluminum structures exposed to the elements are susceptible to corrosion. Corrosion may cause various mechanical and structural deficiencies such as material thinning. It is desirable to rapidly detect and evaluate the properties of an aluminum substrate early in the corrosion process to avoid costly maintenance actions later. There are several nondestructive testing methods for this purpose. To investigate capabilities of millimeter wave, conventional eddy current, and flash thermography techniques for detection of large corrosion areas in aluminum substrates, two corroded samples were inspected with and without dielectric coating (appliqué). This paper presents the results of the c-scan imaging of these samples using the methods mentioned above. The attributes of these methods for detection and evaluation of large, severe and non-uniform corrosion areas with and without a dielectric coating are discussed

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

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

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

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

Radio frequency non-destructive testing and evaluation of defects under insulation

PhD ThesisThe use of insulation such as paint coatings has grown rapidly over the past decades. However, defects and corrosion under insulation (CUI) still present challenges for current non-destructive testing and evaluation (NDT&E) techniques. One of such challenges is the large lift-off introduced by thick insulation layer. Inaccessibility due to insulation leads corrosion and defects to be undetected, which can lead to catastrophic failure. Furthermore, lift-off effects due to the insulation layers reduce the sensitivities. The limitations of existing NDT&E techniques heighten the need for novel approaches to the characterisation of corrosion and defects under insulation. This research project is conducted in collaboration with International Paint®, and a radio frequency non-destructive evaluation for monitoring structural condition is proposed. High frequency (HF) passive RFID in conjunction with microwave NDT is proposed for monitoring and imaging under insulation. The small-size, battery-free and cost-efficient nature of RFID makes it attractive for long-term condition monitoring. To overcome the limitations of RFID-based sensing system such as effective monitoring area and lift-off tolerance, microwave NDT is proposed for the imaging of larger areas under thick insulation layers. Experimental studies are carried out in conjunction with specially designed mild steel sample sets to demonstrate the detection capabilities of the proposed systems. The contributions of this research can be summarised as follows. Corrosion detection using HF passive RFID-based sensing and microwave NDT is demonstrated in experimental feasibility studies considering variance in surface roughness, conductivity and permeability. The lift-off effects introduced by insulation layers are reduced by applying feature extraction with principal component analysis and non-negative matrix factorisation. The problem of thick insulation layers is overcome by employing a linear sweep frequency with PCA to improve the sensitivity and resolution of microwave NDT-based imaging. Finally, the merits of microwave NDT are identified for imaging defects under thick insulation in a realistic test scenario. In conclusion, HF passive RFID can be adapted for long term corrosion monitoring of steel under insulation, but sensing area and lift-off tolerance are limited. In contrast, the microwave NDT&E has shown greater potential and capability for monitoring corrosion and defects under insulation

Detection of Surface Cracks in Metals using Microwave and Millimeter-Wave Nondestructive Testing Techniques-A Review

Integrity Assessment of Metallic Structures Requires Inspection Tools Capable of Detecting and Evaluating Cracks Reliably. to This End, Many Microwave and Millimeter-Wave Nondestructive Testing and Evaluation (NDT&E) Methods Have Been Developed and Applied Successfully in the Past. Detection of Fatigue Cracks with Widths Less Than 5 Μ M using Noncontact Microwave-Based Inspection Methods Was Demonstrated in the 1970s. Since their Introduction, These Methods Have Evolved Considerably Toward Enhancing the Detection Sensitivity and Resolution. Undertaking Key Application Challenges Has Attracted Considerable Attention in the Past Three Decades and Led to the Development of the Near-Field Techniques for Crack Detection. to Address a Need that Cannot Be Fulfilled by Other NDT&E Modalities, Innovative Noncontact Microwave and Millimeter-Wave NDT&E Methods Were Devised Recently to Detect Cracks of Arbitrary Orientations under Thick Dielectric Structures. While the Reported Methods Share the Same Underlying Physical Principles, They Vary Considerably in Terms of the Devised Probes/sensors and the Application Procedure. Consequently, their Sensitivity and Resolution as Well as their Limitations Vary. This Article Reviews the Various Crack Detection Methods Developed To-Date and Compares Them in Terms of Common Performance Metrics. This Comprehensive Review is Augmented with Experimental Comparisons and Benchmarking Aimed to Benefit NDT&E Practitioners and Researchers Alike