Scattering of an induced surface electromagnetic field by fatiguecracks in ferromagnetic metals

In crack detection and sizing by the alternating current field measurement technique, U-shaped wires or coils excited by a high-frequency AC current source can be used to induce the surface field in the workpiece. The authors present a modeling technique for the interaction of a fatigue crack in a ferromagnetic metal with the surface field resulting from an inducer with two U-shaped wires. This work is an extension of a previous modeling technique to have developed for infinitely long (one-dimensional) cracks. In the present technique, the boundary of the fatigue crack is approximated by a circular arc, leading to a formulation for an efficient computation of the field-flaw interaction. Various numerical and experimental results supporting the modeling and illustrating the behavior of the magnetic field and electric potential at the metal surface around circular-arc cracks are presented     

The influence of skin depth on crack measurement by the ac field technique

A general solution is given to describe the ac field around surface-breaking cracks for arbitrary values of the ratio of skin depth δ to crack depthd. This solution allows accurate interpretation of crack depth from voltage readings taken by a Crack Microgauge instrument in cases where δ andd are comparable. It is shown to agree with asymptotic formulae obtained for the cases where δ/d is very small and very large. Provided that the probe length Δ is large compared with δ, the calculations show that a very good approximation to the crack depth may be obtained, irrespective of the ratio δ/d, by using the thin-skin asymptotic formula $$d = \frac{1}{2}\Delta (V_2 - V_1 )/V_1 $$ whereV ₁ andV ₂ are probe voltages registered at positions just off and across the crack, respectively. The problem is solved for a crack of infinite aspect ratio which has no opening, but it can be applied with confidence at the centerline of cracks of fairly large aspect ratio.     

Characteristics of a coplanar waveguide HEMT mount on GaAssubstrate

In a special integrated coplanar waveguide HEMT mount, designed for a wide band (1-60 GHz) on-wafer measurement of the characteristics of HEMTs on GaAs, significant power loss as high as 30% of the input power over a range of frequencies is observed. This power loss is mainly attributed to the radiation through two via holes connecting the coplanar waveguide ground planes to the backside metallization in the mount. Based on this assumption, an approximate theoretical model is developed to substantiate the experimental observations     

Dispersion in shielded coupled coplanar waveguides

A full-wave solution is developed to investigate the dispersion in shielded coupled coplanar waveguides. This study shows that the quasi-TEM data available in the literature for the coupled coplanar waveguide cannot accurately predict the performance of this structure at high frequencies.     

Accurate determination of modes in dielectric-loaded cylindricalcavities using a one-dimensional finite element method

A unified approach is presented for calculating the resonant frequencies of all the modes in cylindrical cavities axisymmetrically loaded with dielectrics. In this method, the radial variations of the field components in the resonator are expressed in terms of first-degree finite-element polynomials, whereas the axial variations of the field components are approximated by trigonometric functions. To calculate the resonant frequencies, an H-vector variational formulation is employed and minimized with respect to the coefficients of the expanded field components. Spurious solutions which are inherent in the finite-element technique are effectively eliminated by means of a penalty term included in the variational expression, imposing a divergence-free magnetic field constraint. To show the capability of the method, resonant frequencies of several cylindrical cavities, including those loaded with dielectric rods and dielectric rings, were calculated. A mode chart is presented which can be used for designing certain multimode dielectric-loaded cavity filters. In contrast to other rigorous techniques reported in the literature, the present method is highly efficient when dielectrics are fully extended along the cavity length     

Analysis of bilateral finline couplers

Although characteristics of bilateral finline as a transmission line have been studied in detail, the behaviour of this structure as a coupler has not received much attention so far. The letter represents a study of phase constants and characteristic impedances of the even and odd modes of bilateral finlines as couplers, leading to the exposition of some useful design information.     

Surface electromagnetic fields around surface flaws in metals

In an earlier paper⁽³⁾ it was shown mathematically and confirmed by experiment that when a uniform surface alternating conduction current is incident on a spherical cap indentation at the surface of a metal there is no perturbation to the current flow on the plane surface outside the cap. It is shown here that this result is true of all axisymmetric surface cavities which are interrogated by any form of surface Laplacian current flow. The result applies, for example, to surface cavities in the form of right cylindrical stopped holes or to conical pits. The paper describes experiments confirming the result in those two cases for uniform incident currents. It also describes the case of an annular cylindrical hole where the theory predicts that a uniform surface current of lower strength than the incident uniform current is established on the central part of the surface plane. This gives rise to a linear variation in surface probe readings in a center line traverse which is also confirmed by experiment. These results are in marked contrast to the perturbations to the surface currents produced by surface cracks, which have been described in earlier work⁽²⁾ by the use of unfolding algorithms.     

Probe characterization in ac field measurements of surface crack depth

In the ac field method of crack depth measurement by the Crack Microgauge, the area of the loop formed in the probe gives rise to an induced voltage, which can introduce errors into the depth measurement. In this paper, a method for measuring the probe area is given, and the quality of the probe is thereby characterized. The underlying theory was given previously, and it is applied here to the probe characterization problem. The probe area is determined by two voltage measurements taken on an artificial rectangular flaw machined in an arbitrary metal. By measurements on several such specimens with the same probe, it is confirmed that the area so obtained is a characteristic of the probe and is independent of the specimen material. Thereafter, measurements on various rectangular flaws with probes of different characteristic area were taken, and very good agreement between predicted and real depths was achieved. Both theory and experiments show that probe characterization is of particular importance when this method is used to measure surface crack depths in metals of low permeability such as aluminum.     

Short-circuited feed terminations on beam steering square loop antennas

Theoretical and experimental investigations of a beam steerable square loop antenna (SLA), including a four-port practical feeding mechanism, is presented. Exciting one of the feeds and short-circuiting the other three results in a linearly polarised beam tilted up from the ground plane and directed away from the active feed. Thus, by exciting one feed at a time, the antenna steers the beam in four different quadrants. A qualitative explanation of the radiation characteristics is also presented using the current distribution. The SLA is designed to operate at 4.5 GHz and has a gain of 6.7 dBi.     

Computation of resonant frequencies of dielectric loadedrectangular cavity using TLM method

The electromagnetic analysis of a dielectric loaded rectangular cavity using the 3D transmission line matrix method is presented. It is found that in order to excite all the modes within a frequency range, the field polarisation and location of the excitation brick in the TLM analysis must be properly selected. For the resonator of concern, the results of computation of the first 11 resonant frequencies show excellent agreement with the experimental results