Showing papers by "National Technical University of Athens published in 1979"
TL;DR: In this paper, the fundamental problem of printed antennas is addressed, and the Green's function to the problem is obtained in dyadic form by solving the problem of a Hertzian dipole printed on a grounded substrate.
Abstract: The fundamental problem of printed antennas is addressed. The printed or microstrip dipole is considered, and its radiation characteristics are investigated. The Green's function to the problem is obtained in dyadic form by solving the problem of a Hertzian dipole printed on a grounded substrate. Input impedance computations are presented, and the numerical solution for the Sommerfeld integrals is discussed.
160 citations
TL;DR: In this paper, the scattering from eccentrically stratified spheres is considered and the method of separation of variables is used in conjunction with translational addition theorems for spherical vector waves.
Abstract: In this paper the scattering from eccentrically stratified spheres is considered. For the case of a spherical inhomogeneity embedded inside a dielectric sphere, the method of separation of variables is used in conjunction with translational addition theorems for spherical vector waves. Analytical results are obtained when the difference in the dielectric constants of the two spheres is small by employing a special perturbation technique. Scattering properties such as distortion of the scattering patterns, variation of total and backscattering cross sections, and depolarization for randomly oriented scatterers are investigated. Methods of detection and identification of inhomogeneities are discussed.
72 citations
TL;DR: In this paper, the authors considered the plane elasticity problem of an arbitrary system of curvilinear cracks in an isotropic elastic half-plane bonded to another halfplane consisting of a different elastic material and reduced to a complex Cauchy type singular integral equation along the cracks.
Abstract: The plane elasticity problem of an arbitrary system of curvilinear cracks in an isotropic elastic half-plane bonded to another half-plane consisting of a different isotropic elastic material is formulated by using the complex variable technique and reduced to a complex Cauchy type singular integral equation along the cracks. The special cases of a half-plane with a stress-free boundary and of a periodic array of curvilinear cracks are also treated. The numerical techniques available for the solution of complex Cauchy type singular integral equations are presented and a discussion on them is made. Finally, three applications to some special cases of straight or circular-arc-shaped cracks are made.
39 citations
TL;DR: In this article, the Gauss-Chebyshev method was applied to two crack problems in plane isotropic elasticity and the numerical results obtained illustrate the powerfulness of the method.
Abstract: As is well-known, an efficient numerical technique for the solution of Cauchy-type singular integral equations along an open interval consists in approximating the integrals by using appropriate numerical integration rules and appropriately selected collocation points. Without any alterations in this technique, it is proposed that the estimation of the unknown function of the integral equation is further achieved by using the Hermite interpolation formula instead of the Lagrange interpolation formula. Alternatively, the unknown function can be estimated from the error term of the numerical integration rule used for Cauchy-type integrals. Both these techniques permit a significant increase in the accuracy of the numerical results obtained with an insignificant increase in the additional computations required and no change in the system of linear equations solved. Finally, the Gauss-Chebyshev method is considered in its original and modified form and applied to two crack problems in plane isotropic elasticity. The numerical results obtained illustrate the powerfulness of the method.
32 citations
TL;DR: In this article, a numerical method is proposed for the approximate solution of a Cauchy-type singular integral equation (or an uncoupled system of such equations) of the first or the second kind and with a generalized kernel, in the sense that the kernel has also a Fredholm part presenting strong singularities when both its variables tend to the same end point of the integration interval.
31 citations
TL;DR: In this paper, the problem of a V-notched isotropic elastic halfplane under generalized plane stress or plane strain conditions can be reduced by using the complex variable technique, to a complex Cauchy type singular integral equation along one of the Vnotch edges.
Abstract: The problem of a V-notched isotropic elastic half-plane under generalized plane stress or plane strain conditions can be reduced, by using the complex variable technique, to a complex Cauchy type singular integral equation along one of the V-notch edges. This equation can be numerically solved by use of the Gauss-Jacobi method and reduction to a system of linear equations. The values of the stress intensity factorKI at the V-notch tip were evaluated for some notch angles in the case of pure tension and the results obtained are in accordance with the available results in the case of a V-notched finite isotropic plane elastic medium. The difficulties faced in evaluatingKI are investigated and a discussion on them is made. The method is also applicable even when the V-notch edges are curvilinear and their loading arbitrary.
31 citations
TL;DR: In this article, a modification of the usual way of determining mode I stress intensity factors at crack tips by using the experimental method of caustics is proposed, where both principal diameters of an experimentally obtained caustic should be taken into account so that the influence of the part of the stress field behaving like r 1/2 near the crack tip is minimized.
Abstract: A modification of the usual way of determining mode I stress intensity factors at crack tips by using the experimental method of caustics is proposed. In accordance with the proposed modification, both principal diameters of an experimentally obtained caustic should be taken into account so that the influence of the part of the stress field behaving like r1/2 near the crack tip is minimized (r being the polar distance from the crack tip). This modification makes the method of caustics much superior to the photoelastic and interferometric methods for the evaluation of mode I stress intensity factors, not only from the experimental but also from the theoretical point of view. Some interesting geometrical properties of the caustics formed at crack tips are also deduced in this paper.
23 citations
22 citations
TL;DR: In this paper, the authors used the optical method of pseudocaustics for the experimental solution of plane elasticity, smooth contact problems for finite or infinite media in contact of arbitrary shape.
Abstract: T he optical method of pseudocaustics can be used for the experimental solution of plane elasticity, smooth contact problems for finite or infinite media in contact of arbitrary shape. This technique constitutes an alternative to the various numerical and experimental techniques for the approximate solution of plane elasticity contact problems. The success in the application of the method of pseudocaustics to plane elasticity contact problems is due to the possibility inherent in this method of the direct determination of the derivative Φ'( z ) of the complex potential Φ( z ) of N.I. Muskhelishvili along the boundaries of the media in contact. Then, the Muskhelishvili complex potentials, Φ( z ) and Ψ( z ), completely characterizing the state of stress and strain in a plane elastic medium, can easily be determined at any point of the media in contact after simple algebraic calculations. Two applications of the proposed method to contact problems of practical interest are also made.
21 citations
TL;DR: In this paper, it was shown that the Steffensen's or the secant method converges without reduction of their order of convergence, if they are applied onh(x) instead off(x).
Abstract: Ifs is a zero of the functionf(x) with multiplicityn, and |f (n+1)(x)| is bounded in a neighborhood ofs, thens is a simple zero of the function $$h\left( x \right): = f^2 \left( x \right)/\left( {f\left( {x + f\left( x \right)} \right) - f\left( x \right)} \right)$$ and |h?(x)| is bounded in some neighborhood ofs. Iteration methods like Newton's Steffensen's or the secant method converge therefore tos without reduction of their order of convergence, if they are applied onh(x) instead off(x).
19 citations
TL;DR: In this paper, the Gauss-type quadrature method with a logarithmic weight function was extended to the evaluation of Cauchy-type integrals and to the solution of integral equations by reduction of the latter to a linear system of algebraic equations.
Abstract: The Gauss-type quadrature methods with a logarithmic weight function can be extended to the evaluation of Cauchy-type integrals and to the solution of Cauchy-type integral equations by reduction of the latter to a linear system of algebraic equations. This system is obtained by applying the integral equation at properly selected collocation points. The poles of the integrand lying in the integration interval were treated as lying outside this interval. The efficiency of the method, both in evaluating integrals and solving integral equations, is exhibited by a numerical example. Finally, an application of the method to a crack problem of plane elasticity is made.
TL;DR: In this paper, critical conditions for three simple two-bar frames, loaded eccentrically and suddenly by a constant load of infinite duratation, are presented for the case of infinite delay.
Abstract: This note presents critical conditions for three simple two-bar frames, loaded eccentrically and suddenly by a constant load of infinite duratation.
TL;DR: In this article, a series of simultaneous tensile creep measurements of the longitudinal extension and lateral contraction in standard thin tension specimens yielded individual curves for the variation of the lateral contraction ratio (Poisson's ratio) at different loading and temperature steps throughout the viscoelastic spectrum.
TL;DR: In this paper, the problem of interaction between a curvilinear crack and a misfitting inclusion of arbitrary shape inside an infinite isotropic elastic medium of the same material as the inclusion was solved by using the complex potential technique and reducing the problem to a complex Cauchy type singular integral equation along the crack only (or the system of cracks).
Abstract: The problem of interaction between a curvilinear crack (or a system of such cracks) and a misfitting inclusion of arbitrary shape (or a system of such inclusions) inside an infinite isotropic elastic medium of the same material as the inclusion was solved by using the complex potential technique and reducing the problem to a complex Cauchy type singular integral equation along the crack only (or the system of cracks).
TL;DR: The criterion for the selection of the intrasystem control algorithm(s) is based on the maximisation of the 'Joint Figure of Merit' related to the long-term average of an appropriately weighted sum of conflicting terms: Probabilities of homeostases versus adequate matching with the partner.
Abstract: The interaction between two self-organising systems, each possessing two hierarchical levels, is dealt with. The communication process is pursued as a bidirectional information transfer. The criterion for the selection of the intrasystem control algorithm (s) is based on the maximisation of the ‘Joint Figure of Merit’ related to the long-term average of an appropriately weighted sum of conflicting terms: Probabilities of homeostases versus adequate matching with the partner.
TL;DR: In this paper, a method for the investigation of certain two-dimensional singular integrals, frequently accounted in the three-dimensional theory of elasticity, was presented for the case of typical cases of 3D elasticity.
TL;DR: In this article, the second fundamental problem of plane isotropic elasticity for a cracked infinite medium, that is the problem in which the displacement derivatives are given along the crack edges, as well as the closely related rigid line inclusion problem in plane elasticity are treated by using the method of complex potentials.
Abstract: The second fundamental problem of plane isotropic elasticity for a cracked infinite medium, that is the problem in which the displacement derivatives are given along the crack edges, as well as the closely related rigid line inclusion problem in plane elasticity are treated by using the method of complex potentials. Without restrictive assumptions on geometry and boundary conditions, these problems are reduced to a complex Cauchy type singular integral equation along the crack or the inclusion, the numerical solution of which can easily be obtained by using the Lobatto-Chebyshev method.
TL;DR: A modification of the Lobatto-Chebyshev method for the numerical solution of Cauchy type singular integral equations appearing in plane or antiplane elasticity crack problems and the determination of stress intensity factors at crack tips is presented in this article.
Abstract: A modification of the Lobatto-Chebyshev method for the numerical solution of Cauchy type singular integral equations appearing in plane or antiplane elasticity crack problems and the determination of stress intensity factors at crack tips is presented. This modification, based on a variable transformation, permits the selection of abscissas and collocation points used to be modified so as to obtain rapid convergence of the numerical results for the stress intensity factors to their correct values. The proposed technique is seen to be particularly effective for crack problems with crack tips near boundaries, interfaces or other geometrical discontinuities. Two applications of the method, to a periodic array of cracks in plane elasticity and to an antiplane shear crack near a boundary, show the effectiveness of the method.
TL;DR: In this article, a modification of the numerical techniques of solution of Cauchy-type singular integral equations and determination of stress intensity factors at crack tips in plane elastic media is proposed.
Abstract: A modification of the numerical techniques of solution of Cauchy-type singular integral equations and determination of stress intensity factors at crack tips in plane elastic media is proposed. This technique presents some advantages under appropriate geometry conditions.
TL;DR: In this paper, the dynamic mechanical properties of a class of plasticized epoxies were investigated over a wide temperature range by means of a Rheovibron Viscoelastometer.
Abstract: The dynamic mechanical properties of a class of plasticized epoxies were investigated over a wide temperature range by means of a Rheovibron Viscoelastometer. Master curves for storage and loss moduli were produced by means of the frequency-temperature superposition principle, as well as for relaxation modulus by a conversion formula. The effect of plasticizer percentage on the said parameters, as well as the loss peaks, corresponding toγ andα relaxations was investigated.
TL;DR: In this paper, an improvement of the one-parameter extrapolation method of photoelastic determination of complex (mixed-mode) stress intensity factors at straight or curvilinear crack tips in a plane isotropic elastic medium due to Smith et al. [12, 13] was achieved by measuring the absolute value of such a factor on the isochromatic fringes along properly selected polar directions.
TL;DR: In this paper, a detailed analysis of the strain field in the whole specimen at definite loading steps before and during the neck initiation and propagation is undertaken by the mechanical interference (moire) method.
TL;DR: In this article, reflected caustics were applied to the study of singularities in the stress field appearing at the crack tip of symmetric plates containing a symmetric crack and submitted to bending loads.
TL;DR: In this paper, based on the principle of virtual work, the formulation of the flexibility matrix and the static analysis of a skew-curved beam in the most general case of loading and response are presented.
01 Jun 1979
TL;DR: In this article, the radiation properties of microstrip dipole antennas were investigated using the variational principle and the stationary phase method, and the radiation field was determined by employing the stationary-phase method.
Abstract: The radiation properties of microstrip dipole antennas are investigated. Input and mutual impedance calculations are performed by employing the variational principle. Also, the radiation field is determined by employing the stationary phase method.
TL;DR: In this paper, the authors systematize known results related to the problem of plane infinite wedge and clarify obscurities and misunderstandings occurring during the various forms of solutions of this classical problem.
Abstract: The objective of this paper is to systematize known results related to the problem of plane infinite wedge and to clarify obscurities and misunderstandings occurring during the various forms of solutions of this classical problem. For this purpose the problem was formulated as a well posed one by imposing the appropriate regularity conditions. Then, the existence and rate of convergence of the involved complex inversion integral was established, as well as the differentiability of the solution. Thus, an asymptotic expansion of the solution was given and a particular application was made for the special case of a half plane. Moreover, another numerical method which is based on Laguerre polynomials, was introduced and as an application the stress distribution at the interior of the wedge for certain values of the wedge angle 2α were calculated and checked for the case of a uniformly loaded wedge.
TL;DR: In this paper, the singular integral is considered as an iterated integral and these line integrals are calculated by using the modified quadrature formulas for integrals possessing real or complex poles, i.e. for singular integrals or regular ones.
Abstract: Some cubature formulas for the evaluation of surface singular integrals are presented. The singular integral is considered as an iterated integral and these line integrals are calculated by using the modified quadrature formulas for integrals possessing real or complex poles, i.e. for singular integrals or regular ones. The error is obtained as the sum of two contour integrals. An estimate of the error for the Gauss-Jacobi and the Gauss-Chebyshev case is obtained.