Detection Optimization of Disbond in Layered Composites with Varying Thicknesses using an Open-Ended Rectangular Waveguide

The detection of air disbond in layered dielectric composite, which is an important practical issue in many industries, is studied both theoretically and experimentally. Sensitivity of disbond detection depends on certain parameters, like the frequency of operation, the distance between the sensor and the first dielectric layer, and the layered composite geometry (conductor backed or terminated by an infinite half-space of air). The impact of all these parameters is investigated theoretically and then verified experimentally

Calibration and Measurement of Dielectric Properties of Finite Thickness Composite Sheets with Open-Ended Coaxial Sensors

The application of open-ended coaxial sensors for dielectric measurement of finite thickness composite sheets is studied. Expressions for calculation of the complex aperture admittance for two geometries are presented. These expressions are used to calculate the dielectric constant of infinite half-space as well as finite thickness slabs. A more efficient method of such calculations, using a personal computer, for low to medium loss dielectrics is demonstrated. The question of when a dielectric layer may be considered as infinitely thick is also addressed, and examples are presented. A different calibration technique (compared to the conventional ones) is described and successfully implemented. This calibration technique utilizes a dielectric sheet with known dielectric properties and thickness. Measurements for different airgaps between the open-ended coaxial line and the dielectric sheet are used to perform and enhance the calibration. The results of this calibration technique and several subsequent measurements are presented and discussed

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

Microwave Noncontact Examination of Disbond and Thickness Variation in Stratified Composite Media

Numerical and experimental results of a microwave noncontact, nondestructive detection and evaluation of disbonds and thickness variations in stratified composite media are presented. The aperture admittance characteristics of a flange mounted rectangular waveguide radiating into a layered, generally lossy dielectric media backed or unbacked by a conducting sheet is modeled. The theoretical implementation is based on a Fourier transform boundary matching technique to construct the field components in each medium, coupled with a stationary form of the terminating aperture admittance of the waveguide. The model can serve as a reliable test bed for real-time examination of layered composite media. Experimental results for several cases are presented which show good agreement with the theoretical findings. This is a versatile technique for near-field in situ interrogation of stratified composite media which provides for high resolution measurements

Microwave Diagnosis of Rubber Compounds

The results of a study investigating the dielectric properties of rubber compounds and their constituents in the frequency range of 5 to 24 GHz are presented. A completely filled short circuited waveguide technique was used to conduct these dielectric measurements. The influence of carbon black content in rubber was investigated for carefully prepared rubber samples. The results showed that for all frequencies, the dielectric constant increases as a function of increasing carbon black content. The variation of dielectric constant values decreases as frequency increases. The presence of curatives in uncured rubber samples was also detected, which indicates the sensitivity of microwaves to the chemical reaction triggered by curatives

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

Microwave Detection of Corrosion under Paint and Composite Laminate Coatings

Microwave and millimeter wave nondestructive methods using open-ended sensors have shown great potential for detecting minute thickness variations in laminate structures, in particular those backed by a conducting plate. Similarly, slight variations in the composition of dielectric materials may also be detected using a set of optimal parameters which include the standoff distance and the frequency of operation. In this paper the potential of using an open-ended rectangular waveguide for detecting the presence of rust under common paint and composite laminate coatings will be demonstrated. An experimental scanned image of a steel specimen with an area of induced corrosion will also be shown

Preliminary Study of the Influences of Effective Dielectric Constant and Nonuniform Probe Aperture Field Distribution on near Field Microwave Images

Near field microwave imaging is based on transmitting a wave into a dielectric structure, which is located in the near field of a sensor, and using a signal proportional to the magnitude and/or phase of the transmitted or reflected wave to create a two or three dimensional image of the structure under investigation. To analyze the features and properties of an image, it is important to understand the mechanism by which the incident electric and magnetic fields interact with the structure. In this study, an effective dielectric constant formula is used to model the reflection properties of two and three dielectric half spaces arranged side by side when scanned by on open ended rectangular waveguide probe. Importantly, the influence of the nonuniformity associated with the electric field distribution at the waveguide aperture is investigated as well. To this end, linear and nonlinear volume fraction calculations are incorporated to calculate the effective dielectric constant of the specimens made of two and three dielectric half spaces. Consequently, the phase and magnitude of the reflection coefficient at the waveguide aperture are calculated. Theoretical and experimental results are presented and compared. In addition, a discussion related to the relative influences of the effective dielectric constant and aperture field distribution is presented

Microwave Scattering Characteristics of Defective Dielectric Cylinders

Microwave scattering properties (radar cross-section, RCS) of inhomogeneous dielectric (composite) cylinders with finite lengths and radii are used to determine the presence of defects of various shapes, sizes and dielectric properties. These cylinders are assumed to be irradiated by a uniform plane wave with arbitrary linear polarization and incidence angle. The induced and scattered fields are numerically obtained using the method of moments. This technique is used to optimize defect detection parameters such as frequency of operation, polarization and incidence angles. The results show the potential of detecting very small voids embedded in various cylinders. The role of frequency as a detection sensitivity enhancer is also demonstrated. The angle of incidence proves to be an important parameter for increased detection sensitivity. Defect size determination is also shown to be possible. This technique shows promise as a test bed for examining the properties of dielectric (composite) cylinders

Microwave Detection of Rust under Paint and Composite Laminates

Accurate, reliable, and early detection of rust/corrosion under paint and laminate composites in many industrial and military applications is of great concern and interest. Although there are several methods for detecting rust under paint, they all have their respective limitations. Microwave nondestructive and noncontact inspection techniques are well suited for the early and reliable detection of rust under various paints and composite laminates. In this paper, the potential of utilizing open-ended rectangular waveguide sensors for detecting rust under paint and laminate composites is investigated theoretically and experimentally. An electromagnetic model describing the interaction of microwave signals radiating from an open-ended rectangular waveguide with a stratified dielectric structure is used to model the presence of rust under paint and a composite laminate coating. For the experimental investigations, we used a rusted steel specimen that was painted over by common spray paint to as much as ten coats of paint. These experiments are conducted at 24 and 10 GHz. The issue of detecting rust unambiguously while a certain paint thickness variation may exist is also addressed