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

Development of a Program for Numerical Analysis of Microwave Radio Links under Different Conditions

Numerical analysis of system power budget is performed for radio link design and transmission planning purposes in cellular networks. The behavior of three different frequencies; 10.5 GHz, 23 GHz and 38 GHz is examined under the influence of such parameters like transmitter power output level, rain rate, fog density, distance, polarization type, antenna radius, temperature, foliage depth and etc. Received power levels are measured, calculated and reported for different radio link types and environmental conditions. A new program is developed in order to carry out system power budget analysis and design better microwave radio links. Measured and calculated values are compared, and the reasons for possible differences between them are investigated and interrogated

Measurement of Mode Patterns in a High-Power Microwave Cavity

A wet thermal paper method for measuring of mode patterns and heat distributions in a high-power microwave multimode cavity is developed. The exposure time of the paper is evaluated. It is shown that this method allows measuring and recording mode patterns in the loaded and unloaded cavity, and the heat distribution inside the cavity with a load movement. The mode patterns and heat distributions along horizontal and vertical planes of the cavity are presented. Possible applications of the method in medicine and biology are discussed, and a calibration protocol of a microwave oven for microwave radiation exposure on cell cultures in the cavity is given

Sources of and Remedies for Removing Unwanted Reflections in Millimeter Wave Images of Complex SOFI-Covered Space Shuttle Structures

In the recent years, continuous-wave near-field and lens-focused millimeter wave imaging systems have been effectively used to demonstrate their utility for producing high-resolution images of metallic structures covered with spay on foam insulation (SOFI) such as the Space Shuttle external fuel tank. However, for some specific structures a certain interference -pattern may be superimposed on the produced images. There are methods by which the influence of this unwanted interference can be reduced, such as the incorporation of an incidence .angle and the proper use of signal polarization. This paper presents the basics of this problem and describes the use of the methods for reducing this unwanted influence through specific examples

Non-Destructive Testing of Physical Characteristics of Composite Structures

A method for detecting an anomaly in a composite material comprising directing two transmitted electromagnetic wave signals orthogonally polarized with respect to each other from a probe to the composite material, wherein the probe and composite material are positioned for near-field evaluation of the probe. A related apparatus comprising an open-ended square waveguide probe

Microwave and Millimeter Wave Testing for the Inspection of the Space Shuttle Spray on Foam Insulations (SOFI) and the Acreage Heat Tiles

The utility of microwave and millimeter wave nondestructive testing and evaluation (NDT&E) methods, for testing the Space Shuttle's external he1 tank spray on foam insulation (SOFI) and the acreage heat tiles has been investigated during the past two years. Millimeter wave NDE techniques are capable of producing internal images of SOFI. This paper presents the results of testing several diverse panels with embedded voids and debonds at millimeter wave frequencies. Additionally, the results of testing a set of heat tiles are also presented. Finally, the attributes of these methods as well as the advantageous features associated with these systems are also provided

Numerical Modelling of Whispering Gallery Modes in Parallel-plates Type Cylindrical Anisotropic Dielectric Resonators

A parallel-plates-type cylindrical anisotropic dielectric resonator (DR) with whispering gallery modes (WGMs) has been investigated. Numerical modelling of the resonance frequencies, Q-factors and the electric field filling factors of the isotropic and anisotropic DR are presented. Applications of the results for sensing purposes are discussed

Whispering Gallery Mode Dielectric Resonators for the Millimeter Wave Near Field Sensing Applications

The resonance characteristics of whispering gallery modes (WGMs) in parallel-plate type cylindrical dielectric resonators (DRs) are numerically considered to optimise their characteristics for sensing applications. Results for the dependency of the resonant frequency, Q-factor, slow-down factor and electric energy filling factor of the modes on resonator parameters for both isotropic and anisotropic DR are presented. The possible applications of the results for millimeter wave near field nondestructive measurements are discussed

Application of Frustrated Total Internal Reflection of Millimeter Waves for Detection and Evaluation of Disbonds in Dielectric Joints

Millimeter waves penetrate inside of low loss dielectric materials and they are sensitive to the presence of internal interfaces and nonuniformities. This allows millimeter wave nondestructive inspection techniques to be utilized for inspecting dielectric composite structures. A disbond (a thin and extended airgap) in structures possessing adhesively bonded joints with complex geometries is commonly difficult to inspect. In this letter, we demonstrate the operational principle and the useful features of a millimeter wave technique, employing a frustrated total internal reflection of signals transmitted and received by dielectric waveguide probes for detecting and evaluating disbonds in such joints

Excitation of the High-Q-factor Whispering Gallery Modes in the Hemispherical Dielectric Resonator with the Cylinder Shield

A hollow metal cylinder with a semicircle in the base is used as the shield of a hemispherical dielectric resonator with whispering gallery (WG) modes. It is shown that the spectrum of the WG modes of this resonator is clearer than the spectrum of a similar resonator with a hemispherical shield. It is determined that the high-Q-factor WG modes are excited in the proposed resonator by the coupling slot on the mirror. Their fields are localized in the dielectric. The frequency dependence of the Q-factor value is measured. A good conformity of this experimental dependence with the analytic dependence is obtained in this pape