摘要:

Non-destructive evaluation of the permittivity of materials at microwave frequencies requires a set of calibration data. The accuracy of this data is important in the inversion of measurement values to dielectric constants and loss tangents. In this paper, a mathematical method for the interpretation of measurements taken by the open microstrip and open slot resonators is presented. The method is efficient computationally and its high accuracy has been supported experimentally for a rectangular microslrip patch resonator and a slot resonator. Example results including variations of the resonant frequency, return-loss and input impedance with the permittivity are presented and a technique for the inversion of the return-loss (or the input impedance)is demonstrated for low-loss dielectrics. The inversion of measurement results on a perspex taken by a slot resonator show that the dielectric constant obtained is very close to (within 0.5% of) that specified by the manufacturer.

ISSN:1058-9759    

DOI:10.1080/10589759708953029

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The general aim of nondestructive testing is the detection of critical flaws without affecting the quality of intact components. Ultrasonic inspection is a very popular method; however, it requires direct contact with the object. Of particular interest are those methods which do not require any mechanical contact with the inspected component. This advantage is highly appreciated for X-rays. Basically the same should be true for other electromagnetic waves that interact with materials, e.g. for microwaves. Microwaves are transmitted through many nonmetals and hence can be used to reveal imperfections and to image them in a raster-like fashion. The purpose of this paper is to give some examples how microwaves can be applied to characterise materials or flaws. We will show the characterisation of fields in the near zone of waveguides by using lockin-thermography to improve resolution. Moreover, we present a low cost sensor technique to obtain phase- and magnitude images which provide further information.

ISSN:1058-9759    

DOI:10.1080/10589759708953030

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

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).

ISSN:1058-9759    

DOI:10.1080/10589759708953031

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Nondestructive surface crack detection is an important issue in many industrial, manufacturing and structural environments. Although there exist several standard nondestructive techniques, each possesses their respective advantages and disadvantages. Recent research activities in using open-ended microwave sensors has proved to be a viable practical approach. Microwave techniques offer many advantages including the fact that they may be used in a noncontact fashion while detecting exposed, filled and covered surface cracks. This paper explores the noncontact characteristics of employing an open-ended rectangular waveguide sensor at K-band. The results show the possibility of detecting thin and long machined slots/ cracks on aluminum plates at standoff distances of several millimeters. In addition, the results show that these cracks/ slots are readily detected when exposed, filled with rust and covered with several layers of common paint. A discussion of the findings is also presented.

ISSN:1058-9759    

DOI:10.1080/10589759708953032

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor