Impact of Waveguide Filling Material on Near-Field Microwave Inspection of Carbon-Loaded Composites

The advent of carbon loaded composite materials gave a boost to many industries. This is because of their light weight, durability and strength. As new structures utilizing carbon loaded composites are built, the need for a reliable nondestructive testing technique increases. A carbon-loaded composite testing poses a challenge to most nondestructive testing and evaluation (NDT&E) techniques. Microwave NDT&I techniques main challenge is the lossy nature of carbon, especially at high microwave frequencies. Lower frequencies penetrate deeper in carbon-loaded composites, however, to operate at lower frequencies the size of the waveguide probe increases significantly which degrades the resolution rapidly. Open-ended rectangular waveguide sensors filled with a dielectric material will be used to inspect carbon-loaded composites. The filling of the waveguide reduces the frequency of operation and keeps the small size of the waveguide (i.e. increases the penetration depth and maintains the resolution). However, varying the waveguide filling material dielectric properties will have an impact on the measurement parameters optimization process and consequently on the detection sensitivity. In this paper, the use of the waveguide filling material as an optimization parameter will be investigated. Carbon-loaded composites with disbonds will be inspected and the variation of the dielectric properties of the loading material of rectangular waveguide probes for carbon loaded composites inspection will be assessed.     

Microwave diagnosis of low-density fiberglass composites with resin binder

Dielectric properties of low-density fiberglass composites are studied at microwave frequencies and are related to the resin binder content and state of cure. A completely filled short-circuited waveguide method is used to study the dielectric properties of fresh and slightly cured resin binder (used in the production of low-density fiberglass composite materials) in the frequency range of 4–18 GHz. The preliminary results indicate that the dielectric properties of resin binder may be used to determine its state of cure. The same measurement approach is used to determine the dielectric properties of fiberglass materials containing uncured resin binder, no resin binder, and three different levels of resin binder. It is found that the dielectric properties of these low-permittivity and low-loss materials are very similar. Consequently, to distinguish among real fiberglass products with different resin binder levels, an open-ended rectangular waveguide measurement approach, is used. This technique allows for noncontact and on-line inspection. The standoff distance is utilized as an optimization parameter increasing the sensitivity of detection to slight dielectric property variations. Two microwave images, at 24 and 10 GHz of a panel containing five different fiberglass specimens are presented.     

Microwave Diagnosis of Low-Density Fiberglass Composites with Resin Binder

Abstract

Dielectric properties of low-density fiberglass composites are studied at microwave frequencies and are related to the resin binder content and state of cure. A completely filled short-circuited waveguide method is used to study the dielectric properties of fresh and slightly cured resin binder (used in the production of low-density fiberglass composite materials) in the frequency range of 4–18 GHz. The preliminary results indicate that the dielectric properties of resin binder may be used to determine its state of cure. The same measurement approach is used to determine the dielectric properties of fiberglass materials containing uncured resin binder, no resin binder, and three different levels of resin binder. It is found that the dielectric properties of these low-permittivity and low-loss materials are very similar. Consequently, to distinguish among real fiberglass products with different resin binder levels, an open-ended rectangular waveguide measurement approach is used. This technique allows for noncontact and on-line inspection. The standoff distance is utilized as an optimization parameter increasing the sensitivity of detection to slight dielectric property variations. Two microwave images at 24 and 10 GHz of a panel containing five different fiberglass specimens are presented.     

A Two Coupler Method for Measurement of Complex Permittivity of Dielectric Materials at Microwave Frequencies

This paper describes two coupler method for measurement of dielectric properties of materials, at microwave frequencies. Two waveguide sections, fed from a common source, are coupling power into a waveguide filled with dielectric. The phase and attenuation constants are computed from the values of phase and attenuation adjustment required for cancellation of the signal in the dielectric filled waveguide. The two coupler method is particularly applicable for measurements of dielectric properties of aerosols and gases.     

热透波材料技术研究进展

热透波材料技术是高超声速飞行器实现通讯与精确导航的关键技术,文章从热透波材料体系、热透波材料热电行为和高温电性能测试技术等方面对热透波材料及其相关技术的发展现状进行了简要介绍。在材料体系方面,石英陶瓷及二氧化硅基复合材料是目前应用的主要材料品种,多孔氮化物陶瓷及陶瓷基复合材料是未来发展的重要方向。在热电行为研究方面,对典型氧化物、氮化物、氮氧化物材料热电行为规律及杂质离子对材料热电行为的影响等方面的研究获得重要进展,并获得试验验证。在高温电性能测试方面,近年来突破了1 600℃高温宽频测试关键技术,并获得了氧化硅熔融态介电性能实测数据,国外和国内已实现8 MW/m2热透波实时测试。     

Uncertainty Estimation in Simultaneous Measurements of Levels and Permittivities of Liquids Using TDR Technique

Time-domain reflectometry (TDR) based on instruments are commonly used for several monitoring methods, particularly in soil moisture and volumetric water-content evaluation. Furthermore, significant advantages of TDR methodology, mostly related to the possible determination in real-time and to a nondestructive approach of the spatial location and nature of various objects, make this technique an appealing candidate for a variety of environmental and industrial applications. In this paper, we show that the suitable combination of TDR-detecting functionalities can lead to a joint quantitative and qualitative monitoring method for liquid-control purposes, so that, in one shot, the analysis of TDR data allow the measurement of liquid levels, the determination of multiple interfaces in layered media, as well as the evaluation of dielectric properties, thus opening challenging perspectives for several monitoring applications, particularly in fluid-processing-related industry. For such purposes, a detailed analysis of the uncertainty of the proposed measurement method is mandatory; hence, a metrological characterization of the method is carried out, demonstrating that the presented technique is definitely valid for simultaneously measuring levels and dielectric constants of liquids, with uncertainties under 2%. Results obtained for different liquid samples validate the approach on a wide range of dielectric materials and demonstrate the robustness and reliability of the proposed TDR technique.     

Automatic measurement of permittivity and permeability at microwavefrequencies using normal and oblique free-wave incidence with focusedbeam

A free-wave method for determining the dielectric and magnetic properties of materials from reflection measurements made at normal incidence and transmission measurements made at normal and oblique incidence is proposed. The method combines frequency domain measurements and time domain (TD) analysis and uses polarization to avoid typical ambiguities in the results. Varying the incident angle and the polarization, measurements were made in the X-band. The technique was validated by comparing the results obtained with those from well-established waveguide techniques. A focusing assembly makes it possible to measure relatively small samples, thus avoiding diffraction problems. It also improves the ambiguity-solving procedure proposed for the technique. The measurement procedure is fully automated by using the HP8510 network analyzer controlled by an HP362 computer, which also processes the data. Results for low-loss dielectrics such as teflon, nylon, and polymethyl methacrylate (PMMA) and for microwave-absorbing materials are reported     

Multimode solution for the reflection properties of an open-endedrectangular waveguide radiating into a dielectric half-space: theforward and inverse problems

Open-ended rectangular waveguides are extensively used in nondestructive dielectric material evaluation. The dielectric properties of an infinite-half space of a material are calculated from the measured reflection properties referenced to the waveguide aperture. This calculation relies on a theoretical and numerical derivation of the reflection coefficient likewise referenced to the waveguide aperture. Most of these derivations assume the dominant mode field distribution across the waveguide aperture. However, when dealing with low permittivity and low loss dielectric materials, there may exist significant errors when calculating the dielectric properties from the measured reflection coefficient. These errors have also shown to be more significant in the upper frequency portion of a waveguide band. More accurate results are obtained when higher order modes are considered in addition to the dominant waveguide mode. However, most studies incorporating higher-order modes have used various approximations when calculating the reflection properties and have not provided a full discussion on the influences of dielectric properties of the infinite-half space and the frequency of operation. This paper gives a rigorous and exact formulation in which the dominant mode and the evanescent higher-order modes are used as basis functions to obtain the solution for the reflection coefficient at the waveguide aperture. The analytic formulation uses Fourier analysis in addition to the forcing of the necessary boundary conditions at the waveguide aperture. The solution also readily accounts for the complex contributions of both TE and TM higher-order modes. Finally, the influences of the dielectric properties of the infinite-half space and the frequency of operation are investigated     

A free-space method for measurement of dielectric constants andloss tangents at microwave frequencies

For measurements of dielectric properties of planar slabs of ceramic and composite materials, the authors have developed a free-space measurement system in the frequency range of 14.5-17.5 GHz. The key components of the measurement system are a pair of spot-focusing horn lens antennas, a network analyzer, and a computer. Because of the far-field focusing ability of horn lens antennas, the free-space measurements can be made at microwave frequencies in a relatively compact and simple measurement setup. The time-domain gating feature of the network analyzer and the thru, reflect, and line calibration technique were used to eliminate the effects of undesirable multiple reflections. The dielectric constants and loss tangents were measured for standard materials such as fused quartz, Teflon, and polyvinylchloride (PVC). Dielectric properties for Teflon and PVC were also measured in a waveguide medium for purposes of comparison with the free-space method     

Permittivity determination of liquid materials using waveguide measurements for industrial applications

There is a need for a simple and relatively inexpensive microwave method for complex permittivity, ?, determination of liquid materials in industrial applications. Measurement cables that connect the measurement cell to the measuring instrument (usually a vector network analyser) may be either subject to severe phase instability particularly if they are long or metrology-grade cables are not available, or subject to thermal variation. In these instances, calibration techniques based on complex scattering parameter measurements may drastically suffer from extension of the reference plane to the boundaries of the measurement cell or sample end surfaces. In such situations, amplitude-only methods based on simplified calibration techniques such as response calibration are appropriate and very feasible. These methods have applications in industry such as thickness measurement and disbonding and delamination detection. The motivation of this study is to investigate a suitable method for ? determination of liquid materials using amplitude-only waveguide measurements. The analysis is restricted to lossy dielectric materials, which possess large enough attenuation such that the multiple reflections between their two end surfaces can be neglected. We derived an equation in terms of the loss tangent of these materials for selecting a suitable material thickness. We presented two approximations (frequency-independent and power-series representation) for ? determination of these materials. For validation of the method, we measured the ? of methanol and commercially available antifreeze solution by our method using a simple (frequency response) calibration and by another waveguide method using the thru-reflect-line calibration.     

New density-independent calibration function for microwave sensingof moisture content in particulate materials

Microwave techniques have been considered for a long time for moisture sensing in many food processing and agriculture-related industries. They are suitable for on-line real-time monitoring and control. However, with particulate materials, bulk density fluctuations cause significant errors in moisture content determination. To overcome this shortcoming, density-independent calibration functions are needed. In this paper, a new approach is presented in which both bulk density and moisture content are determined directly from measured microwave dielectric properties. A simple relationship between bulk density and the dielectric properties is identified, and a new density-independent function for moisture content prediction, exclusively dependent on the dielectric properties of the material under test (ϵ', ϵ"), is proposed. The validity and applicability of this function are demonstrated with an extensive data set obtained from measurements on a granular material (wheat), over wide ranges of frequency (11-18 GHz), temperature (-1°C-42°C), moisture content (10.6%-19.2%, wet basis), and bulk density (0.72-0.88 g/cm³). Explicit calibration equations for moisture prediction at different frequencies and temperatures are provided. Although data obtained by a transmission microwave measurement technique were used, this new approach remains valid in general for other techniques, provided that ϵ' and ϵ" are determined accurately     

可见光成像系统MRC测试技术的研究

通过分析国外现行的实现最小可分辨对比度(MRC)测量的方法,提出了一种采用两个重叠积分球对测试图案正反两面均匀照明,获取可调对比度的实现MRC测量的新方法,研制了相应的测量MRC仪器。采用新的方法测试图案的目标亮度和背景亮度可以单独调节,而且还能得到精确的控制。测试结果表明,该仪器测量亮度的误差在±0.3 cd/m2的范围内,MRC的扩展不确定度不超过3%,适合各种试验现场对可见光成像系统成像质量的在线评价。     

Resolving ambiguity in broadband waveguide permittivitymeasurements on moist materials

The dielectric properties of moist materials depend strongly on free water content. Moisture content may be conveniently sensed using microwaves, provided that calibration data is available. One common method utilizes network analyzer transmission measurements on waveguide cells filled with samples of known water content. Since this technique yields multiple solutions for the complex permittivity, a simple procedure has been developed to identify the correct solution. This has proved to be highly reliable and lends itself to measurement automation     

A novel numerical technique for dielectric measurement of generallylossy dielectrics

A method for determining the dielectric properties of infinite half-space of generally lossy dielectric materials is described. This method utilizes the measurement of the admittance of a rectangular waveguide radiating into such dielectrics. It is shown that the real part of the admittance is relatively insensitive to the variations of the imaginary part of the dielectric constant. A numerical procedure is initiated which provides a simple and fast-converging approach for calculating the dielectric properties. This numerical procedure lends itself to implementation by personal computers. The theoretical formulation for the expression of the admittance of an open-ended waveguide and the numerical procedure are discussed in detail. Results of several measurements of freespace and lossy dielectric samples (rubber with carbon black) to verify the theory and the numerical scheme are given. The results give good agreement with other measurement schemes. Comments on the accuracy of the results are also provided     

An improved open-ended waveguide measurement technique onparameters ϵγ and μγ ofhigh-loss materials

An improved technique of using rectangular waveguide aperture for simultaneous measurement of the electromagnetic parameters ϵ_γ and μ_γ of materials is developed in this paper. Both multilayer and single-layer medium sheet samples can be tested. Samples are sandwiched between a flange of an open-ended waveguide and a shorting plate. The parameters are obtained by using an optimization technique by fitting the theoretical values of the reflection coefficients Γ(ϵ_γ,μ_γ) to the measured values with ϵ_γ,μ_γ as the argument. The related details, test theories, waveguide design, sample preparation, and error analysis are also discussed in this paper. The experimental results are validated by the measurements performed using the reflection-transmission method using an automatic network analyzer and the published data from manufactures. By virtue of its open-ended waveguide configuration, this technique is well suited for sheet or coating materials, and it might be applied for industrial on-the-worksite testing or biomedical analysis     

热电材料塞贝克系数的不确定度评定

热电材料是一类可以将热能和电能直接相互转化的新型功能材料.塞贝克系数作为评价热电材料性能的重要参数,其准确测量尤为关键.基于准确测量方法,建立了塞贝克系数的溯源路径,研究了塞贝克系数测量仪器的溯源性,验证了测量方法的准确性,以P型碲化铋Bi2Te3块体热电材料作为测量对象进行了测量不确定度评定,其相对扩展不确定度为0....     

基于共面波导传输线的宽频带介电常数提取方法

提出了一种基于共面波导传输线的宽频带低损耗材料介电常数提取方法。设计5根共面波导传输线作为标准覆盖待测样品介电常数频率范围,采用在片矢量网络分析仪测得各传输线未经修正的S参数,经过多线TRL校准算法提取传播常数;采用quasi-TEM模型计算传输线的单位长度电阻R和单位长度电感L;最终求得0.1~50GHz GaAs材料的介电常数。相比于传统的腔体法只能点频测试,且需设计待测样品腔体结构,该测试方法支持宽频带介电常数的连续测试,且测试效率更高;测试结果与谐振腔法偏差±2.5%,满足工程实践需要。     

A technique for dielectric measurement of cylindrical objects in arectangular waveguide

In this paper, the inverse scattering problem of a homogeneous dielectric post in a rectangular waveguide is considered. A novel inversion algorithm, based on the method of moments and eigen analysis, for computation of the dielectric constant of the post (ϵ) from the measured voltage reflection coefficient is introduced. In this method the integral equation for the polarization current induced in the dielectric post is cast into a matrix equation, and then the contribution of ϵ to the resulting reflection coefficient is expressed explicitly using the eigen analysis. It is shown that the dielectric constant can be obtained from the solution of a complex polynomial function which in turn can be obtained numerically using the conjugate gradient method. Practical aspects of dielectric measurement using this technique are discussed. The HP-8510 network analyzer is used to measure the reflection coefficient of dielectric posts in an X-band waveguide sample holder. Metallic and known dielectric posts are used to determine the accuracy of the dielectric measurement technique     

Modified Infinite Sample Method for Routine Permittivity Measurements at Microwave Frequencies

A modified infinite sample method for routine permittivity measurements of granular or powder as well as liquid materials is described. The method permits continuous monitoring of the temperature, moisture content, and density of the test sample. Analytical and graphical solutions, which lead to the material permittivity from experimental data and measurement uncertainty, are presented. The method is particularly valuable when a large number of samples must be tested as a function of temperature, moisture content, and density, or when a continuous monitoring of the permittivity is required during irradiation by microwave power.     

Precision Determination of the Dielectric Properties of Nonmagnetic High-Loss Microwave Materials

New tests by the author to determine the, dielectric properties of high-loss nonmagnetic microwave materials used for waveguide absorbers and anechoic chamber linings reveal two significant results: dielectric parameters may be calculated from simple formulas without solving transcendental equations, and accuracy is limited only by the homogeneity of the sample and the precision of its fit in the waveguide. The technique used is based upon prior work by other researchers but is refined for materials with loss tangents exceeding 0.1. These previously have received little attention in the literature. Results of tests in the S-band yield loss tangents between 0.4 and 0.8, and dielectric constants from 1.5 to 2.0 for samples completely filling the cross section of WR284 waveguide. The author reviews other methods, analyzes error sources, and describes his laboratory work.