Microwave Detection Optimization of Disbond in Layered Dielectrics with Varying Thickness

The detection sensitivity optimization of air disbond in layered dielectric composites, using an open-ended rectangular waveguide, is studied both theoretically and experimentally. The sensitivity of the disbond detection is strongly influenced by the proper choice of parameters such as the operating frequency and the layered composite geometry (conductor backed or terminated by an infinite half-space of air). The capability of optimizing the measurement system parameters to detect and estimate the thickness of a disbonded layer independent of some changes in the thickness of the dielectric coating is also demonstrated. The impact of the parameters influencing detection optimization is theoretically investigated and then experimentally verified

Real-Time and On-Line Near-Field Microwave Inspection of Surface Defects in Rolled Steel

The potential and limitations of near-field microwave inspection techniques for detecting various surface defects in rolled steel have been investigated. The focus of this study has been to investigate this potential for tin mill products containing gross and subtle defects including steel induced defects, roll marks, holes, scratches and gouges

Near-Field Microwave Nondestructive Distinction between Surface Height Variations and Defects in Thick Sandwich Composites using Standoff Distance Optimization

Nondestructive inspection of thick composite materials is important for maintaining manufacturing quality and structural integrity. Microwave nondestructive inspection methods are well suited for inspecting such structures, and offer many advantages compared to other nondestructive inspecting approaches. Optimization of microwave measurement parameters offers great versatility in detecting various sub-surface and surface defects while providing a high level of detection robustness. For this study two different thick composite specimens were used, one with a disbond (due to an embedded mylar sheet) having also generated some level of surface height variation in the specimen and the other subjected to an impact damage. An open-ended rectangular waveguide sensor was used to detect and distinguish between the defects and the surface height they may have produced in the specimens. With this probe suspended above the specimen and at a fixed operating frequency, the standoff distance was optimized for detection of the sub-surface defect (the mylar sheet) while nulling out the effect of surface height variations and the impact damage (and vice-versa). Microwave images of sample areas were produced for these optimization exercises effectively showing the surface height variations or the presence of the mylar sheet, respectively. A similar approach was employed to detect the surface height depression caused as a result of the impact damage on the second specimen. The result show that the choice of standoff distance can be optimally used to look for a surface or sub-surface defect while eliminating any influence due to specimen surface height variations (and vice-versa)

Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper

Microwave Method for Locating Surface Slot/Crack Tips in Metals

The detection of exposed (empty and filled) and dielectric covered fatigue/surface cracks on metal surfaces is an important practical issue. Recently, microwave techniques have shown the potential of detecting exposed and dielectric filled cracks and cracks covered with dielectric coatings (i.e. paint). An important issue associated with these investigations is locating the tips of a crack. This is particularly important from the repair point of view in steel bridges and other structures. In this study openended rectangular waveguide probes are used to locate the tip of empty, dielectric filled, and covered cracks. In this paper the results of extensive measurements are discussed, including the accuracy by which the tip location of a crack may be determined. The location of a crack tip can be determined using a two-dimensional crack characteristic signal (image of the crack), and more simply and accurately by using the crack tip characteristic signal. For cracks/slots used in this study, the results indicate that the tip location of exposed (empty and filled) cracks may be identified to within 0.25 mm (0.009 in.) of their actual position, while covered crack tips are located within 2 mm (0.08 in.) of their actual locations. Using optimized measurement parameters and/or a higher order mode detection scheme may result in improved tip location accuracy. Good agreement is obtained between the results of a theoretical model and the measurements, which strongly indicates the possibility of theoretically optimizing (increasing) the accuracy by which a crack tip location may be determined

Influence of Flange, Frequency and Liftoff on Microwave Detection of Stress Induced Fatigue Cracks using Open-Ended Rectangular Waveguides

The influence of the waveguide flange, frequency of operation and liftoff on crack detection sensitivity was studied by using microwave nondestructive inspection methods. The presence of a flange at a waveguide aperture provided two extra signal peaks which increased crack detection sensitivity. The results showed that the optimal choice of these parameters could significantly enhance the crack detection sensitivity in practical applications