On the Crack Characteristic Signal from an Open-Ended Coaxial Probe

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

Microwave microfluidic resonant sensors and applicators

Microwave sensors and applicators are of major interest in applications where no physical contact is possible or the use of active devices is impractical. Microwave sensors offer numerous advantages compared to traditional techniques, not least in terms of convenience and speed, since they do not require any markers. Furthermore, such microwave sensor methods can be designed to be fully compatible with lab-on-a-chip approaches. In this work, the interaction between the microwave electric field and a microfluidic dielectric sample using resonant microwave sensors has been studied, and therefore the dielectric constant for sample materials can be measured by using perturbation theory when the sample is placed in the electric field. Two forms of novel resonator for microfluidic sensing are proposed: a re-entrant microwave cavity (RMC) and split-ring resonator (SRR). The RMC is one of the most useful forms of cavity for this purpose due to its simple geometry, wide frequency range tuning and high quality factor. It has been designed, machined, and evaluated experimentally with common liquids and different mixtures based on water and dielectric microspheres, in both static and flow situations. Furthermore, we present a new approach for microfluidic sensing and microfluidic heating using a novel split ring resonator (SRR) for high sensing sensitivity and efficient heating of lossy dielectrics. The designed SRR shows very good performance experimentally in microfluidic sensing (pure liquids, chemical solutions, and saline concentration level), as well as in microfluidic heating where it is demonstrated how nearly all the microwave power is delivered to the sample under test. Owing to its compact size and high efficiency, the SRR has been utilized in an important microbiological applications for rapid DNA release using low power levels (< 1 W). It is envisaged that this system is now suitable for incorporation within a rapid, hand-held, point-of-care detector for bacterial infections such as Clostridium difficile

Application of millimeter-wave differential probe for crack detection on riveted structures

This thesis proposes the use of a W-band wideband differential probe for crack detection on riveted structures, such as aircraft fuselage lap joints. Detection of crack at its early stage (surface-breaking) is particularly important to prevent the development of a fatigue crack. This probe utilizing millimeter-wave frequencies serves as a viable candidate on detecting surface-breaking cracks, as it can be employed in measurements in a non-contact fashion, while allowing for high spatial-resolution images and the abilities to penetrate through dielectric materials (paint), making it attractive for detecting small cracks. In previous works, a V-band differential probe has shown promise for detecting surface breaking crack near a fastener head. This work is extended in this thesis by investigating the surface crack detection capability of the W-band differential probe. The W-band probe is tested with various intentional misalignments of the probe, as well as a paint layer covering the crack, the variations in its crack detection capability are then observed. The measurement results indicate that this fabricated probe is capable of detecting a surface crack at a length of 1.27 mm (adjacent to a fastener head), and the detection is not significantly affected by a slight misalignment, although consequently some undesired signals may also be registered. In addition, a layer of thick paint over crack introduces uncertainties to the detection signals and complicates the evaluations. To reduce the significance of these undesired signals, SAR filter is applied to the results. The outcome demonstrates an enhanced crack detection and weakened undesired signals, but the influence of a thick paint layer cannot be completely removed. A more in-depth analysis regarding the influence of paint on crack detection may be desired to fully understand the crack detection capabilities of the W-band differential probe --Abstract, page iii