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Showing papers by "National Technical University of Athens published in 1978"


Journal ArticleDOI
TL;DR: In this article, the authors evaluated the thermomechanical behavior of a particle composite under the assumption that a boundary interphase between the matrix and the filler particles develops, upon which the overall behavior of the composite depends.
Abstract: The thermomechanical behaviour of a particle composite was evaluated under the assumption that a boundary interphase between the matrix and the filler particles develops, upon the thermomechanical properties of which the overall behaviour of the composite depends. This interphase exists in reality and it contains both areas of adsorption interaction in polymer surface layers onto filler particles as well as areas of mechanical imperfections. Under the assumption that the interphase is homogeneous and isotropic exhibiting perfect adhesion with both main phases a theory was developed providing quantitative means of assessing the adhesion efficiency between the phases and its effect on the thermomechanical behaviour of the composite. Experimental results are in good agreement with theoretical predictions.

108 citations


Journal ArticleDOI
TL;DR: In this paper, the buckling of multistory, multibay frames is investigated and a method of analysis is presented for estimating directly critical conditions, for both bifurcational and limit point type of instability.
Abstract: The buckling of multistory, multibay frames is investigated. A method of analysis is presented for estimating directly critical conditions, for both bifurcational and limit point type of instability. This method is based on a nonlinear formulation, and on the simultaneous solution of the equilibrium and buckling equations, and it is demonstrated through two simple frame problems.

49 citations


Journal ArticleDOI
TL;DR: The application of the dynamical theory to the problem of a ship moving with constant forward speed on a free surface has been extended to include the exciting forces in oblique regular waves.
Abstract: The application of the dynamical theory to the problem of a ship moving with constant forward speed on a free surface has been extended to include the exciting forces in oblique regular waves. As a result, it has become possible to derive a new formulation for the equations of motion, for a ship moving with five degrees of freedom. The application of the same theory has yielded formulas for the calculation of the mean added resistance and drift force in oblique regular waves and the calculation of all mean forces and moments for the forced oscillations of a ship in calm water.

41 citations


Journal ArticleDOI
TL;DR: In this article, the Lobatto-Jacobi method of numerical solution of Cauchy type singular integral equations and determination of stress intensity factors, based on the use of the corresponding numerical integration rule, was modified so as to become applicable to the evaluation of a special class of generalized stress intensity factor associated with a real singularity.
Abstract: The Lobatto-Jacobi method of numerical solution of Cauchy type singular integral equations and determination of stress intensity factors, based on the use of the corresponding numerical integration rule, was modified so as to become applicable to the evaluation of a special class of generalized stress intensity factors associated with a real singularity. The same technique can also be used for the evaluation of generalized stress intensity factors associated with a pair of complex conjugate singularities. An application of the method to a plane elasticity problem is also made. Finally, the modification of the method valid for a pair of complex conjugate singularities is illustrated in detail in a numerical example.

41 citations


Journal ArticleDOI
TL;DR: A series of cold-setting epoxy polymers, plasticized with different amounts of plasticizer, ranging between 0 and 90% by weight of the amount of the epoxy prepolymer, were studied for their mechanical, optical, and fracture behavior properties as discussed by the authors.
Abstract: A series of cold-setting epoxy polymers, plasticized with different amounts of plasticizer, ranging between 0 and 90% by weight of the amount of the epoxy prepolymer, were studied for their mechanical, optical, and fracture behavior properties. Quantities defining the mechanical properties were considered: the elastic modulus E, Poisson's ratio v, and fracture tensile stress σf. These were accurately measured with electric strain gauges in specimens tested in a 5-ton Instron tester. The optical behavior was characterized by the stress optical coefficients of the materials in both principal directions, α and β, as well as by the coefficient of optical anisotropy, ζ. The values of these quantities were measured by a Fizeau interferometric method. Finally, the optical method of caustics was applied to cracked epoxy polymer specimens to provide a new experimental technique for determining the stress optical properties of these polymers in terms of their mechanical properties. This method was used to check the previous results found by established experimental methods.

27 citations


Journal ArticleDOI
TL;DR: Then-dimensional generalisation of this theorem, due to C. M. Miranda, is used for the construction of an error estimation procedure for approximate solutions of nonlinear systems of equations.
Abstract: A well known theorem by Bolzano states that, whenever a continuous real function changes sign in some interval, it must have a zero in this interval. Then-dimensional generalisation of this theorem, due to C. Miranda, is used for the construction of an error estimation procedure for approximate solutions of nonlinear systems of equations.

23 citations


Journal ArticleDOI
TL;DR: In this paper, the authors considered the electromagnetic scattering from finite-length dielectric fibers with a diameter much smaller than the wavelength and for a perpendicular incidence case is considered, where the induced fields are assumed to be uniform on the cross section of the fiber.
Abstract: The electromagnetic scattering from finite-length dielectric fibers with a diameter much smaller than the wavelength and for a perpendicular incidence case is considered. The induced fields are assumed to be uniform on the cross section of the fiber. This yields a one-dimensional integral equation for the inner field which is solved by employing Galerkin’s method. Numerical results for the scattering amplitude are obtained for specific cases. In addition, it is shown that the energy finiteness criterion is satisfied.

20 citations


Journal ArticleDOI
TL;DR: The comrade matrix as mentioned in this paper is a generalization of the companion matrix, and arises when a polynomial is expressed in terms of a basis set of orthogonal polynomials.
Abstract: The comrade matrix is a generalization of the companion matrix, and arises when a polynomial is expressed in terms of a basis set of orthogonal polynomials. The work begun in a previous paper is here continued for multivariable systems, and a number of generalizations of standard results are described. Topics covered include controllability, canonical forms, polynomial and state-space realizations and linear feedback. The flexibility offered by an arbitrary choice of basis promises to be useful for applications.

20 citations



Journal ArticleDOI
TL;DR: In this paper, a new simple method for the photoelastic determination of Mode I stress intensity factors from isochromatics is proposed, which takes into account the fact that a considerable part of the error committed in the photoELastic determination at crack tips, based on experimentally obtained isochromaatic fringe patterns, is due to ignoring the non-singular parts of the stress field near the crack tips for the evaluation of these factors.

13 citations



Journal ArticleDOI
01 Nov 1978
TL;DR: A variational method for investigating the nonlinear lateral stability is applied to a rectangular, geometrically perfect, two-bar frame which is loaded eccentrically to the center line of one of the bars as mentioned in this paper.
Abstract: A variational method for investigating the nonlinear lateral stability is applied to a rectangular, geometrically perfect, two-bar frame which is loaded eccentrically to the center line of one of the bars. The frame loses its stability through a limit point. The individual, and primarily the coupling effects on the nonlinear sway-buckling load are fully assessed for a variety of parameters such as transverse shear deformation, cross-sectional shape, loading eccentricity, slenderness ratio and moment of inertia ratio of the two bars. The conclusions presented herein give a better insight into the actual response of the frame.

Journal ArticleDOI
TL;DR: The criterion for the selection of the intrasystem control algorithm(s) (code) is based on the maximization of the ‘Joint Figure of Merit’ related to the long-term average of an appropriately weighted sum of conflicting terms: Probabilities of homeostasis versus adequate matching with the partner.
Abstract: The interaction between two self-organizing 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) (code) is based on the maximization of the ‘Joint Figure of Merit’ related to the long-term average of an appropriately weighted sum of conflicting terms: Probabilities of homeostasis versus adequate matching with the partner.

Journal ArticleDOI
TL;DR: In this article, the authors untersuchten Eigenspannungen in Epoxydharzen nach Hartung wahrend eines willkurlichen Polymerisationsprozesses.
Abstract: Es wurden die in Epoxydharzen nach Hartung wahrend eines willkurlichen Polymerisationsprozesses entwickelten Eigenspannungen untersucht. Die Spannungsanalyse erfolgt mit den ublichen Methoden der Spannungsoptik. Interessante Ergebnisse zeigen sich fur die Entwicklung von Eigenspannungen. Ergebnisse, die von Wert fur eine Verbesserung des Giesprozesses sein durften.

Journal ArticleDOI
TL;DR: In this paper, a variational approach based on a quasi-static approach is proposed to compute the characteristics of microwave circuits of finite conductor thickness on anisotropic and inhomogeneous substrates, where the charge distribution on each side of the conductor can be represented by a spatially dependent set of exponential functions.
Abstract: The reproducibility of microwave integrated circuits can be improved by employing substrates which are anisotropic. A method is presented which facilitates the computation of the characteristics of distributed integrated microwave circuits of finite conductor thickness on anisotropic and inhomogeneous substrates. The method is variational and it relies on a quasi-static approach which assumes that the charge distribution on each side of the conductor can be represented by a spatially dependent set of exponential functions.

Journal ArticleDOI
TL;DR: It is proved in the paper that the optimal (cost minimizing) decision system in an infinite horizon is of the control limit rule type, rapidly obtained by policy improvement algorithms.




Journal ArticleDOI
TL;DR: In this paper, a class of plasticized epoxy polymers were subjected to indentation tests over a wide range of temperature and plasticizer percentage, and the materials exhibited creep behavior according to the temperature and the amount of plasticizer.
Abstract: A class of plasticized epoxy polymers were subjected to indentation tests over a wide range of temperature and plasticizer percentage. Constant or continuously varying temperature tests were carried out under constant load, and the materials exhibited creep behavior according to the temperature and the amount of plasticizer. Thermal expansion properties were also studied, and glass transition temperature as well as thermal expansion coefficients a1 and a2 were determined for each individual material. Theoretical predictions were found valid under the present conditions.

Journal ArticleDOI
TL;DR: In this paper, a system of two-dimensional singular integral equations for the solution of the three-dimensional problem of a plane crack of arbitrary shape is investigated, and the main idea for this solution is to apply the boundary properties of a double-layer potential for the determination of certain limiting values.
Abstract: In the present report, the formulation of a system of two-dimensional singular integral equations for the solution of the three-dimensional problem of a plane crack of arbitrary shape is investigated. The main idea for this solution is to apply the boundary properties of a doublelayer potential [2,3] (which can be considered as the generalization of the Plemelj formulas for one-dimensional singular integrals) for the determination of certain limiting values. Similar equations are given in [1,4], where the Betti-Rayleigh theorem is used at first for the formulation of the problem in terms of surface quantitites. For the estimation of the l~miting values involved in [4], it seems that the dislocation function was expanded in some kind of special series around the singular point, which is cut out by a disk of radius e centered on it. As e ÷ O, the limit of the integral is calculated. The same results are obtained by using a simpler method which is suggested below.

Journal ArticleDOI
TL;DR: The L1 approximation of strictly convex functions by means of first degree splines with a fixed number of knots with main theoretical results a system of equations for the knots, which solves the problem, and an estimate of the approximation error.
Abstract: The L1 approximation of strictly convex functions by means of first degree splines with a fixed number of knots is studied The main theoretical results are a system of equations for the knots, which solves the problem, and an estimate of the approximation error The error estimation allows the determination of bounds for the number of knots needed so that the L1 approximation error does not exceed a given number Finally, an algorithm is used, by means of which a solution to the system can be obtained

Journal ArticleDOI
TL;DR: In this paper, the effect of particle shape and imperfect bonding between matrix and filler particles of a composite system with constituents of equal shear moduli was investigated, and it was found that the particle shape does not affect the effective moduli of the composite.

Journal ArticleDOI
TL;DR: In this article, it was shown that all the elements in an arbitrary regular Routh array can be obtained from minors of a single suitably defined observability (or controllability) matrix.
Abstract: It is shown that all the elements in an arbitrary regular Routh array can be obtained from minors of a single suitably defined observability (or controllability) matrix. This provides an alternative to the Bezoutian matrices of a previous paper. Similar expressions for the minors of Bezoutian-type matrices associated with generalized polynomials are also derived. This enables the Routh array for generalized polynomials to be obtained directly without conversion of the polynomials to power form.

Journal ArticleDOI
TL;DR: In this article, a solution is presented to the buckling problem of a continuous beam resting on a tensionless foundation model, which was proposed by Winkler and by Reissner.
Abstract: In this paper, a solution is presented to the buckling problem of a continuous beam resting on a tensionless foundation model that was proposed by Winkler and by Reissner.

Journal ArticleDOI
01 Jan 1978
TL;DR: In this paper, a general computational method together with the relevant computer program which can be easily used to perform reliability analysis on electric power installation configurations is presented. But the program is very conducive to analysis work of reliability sensitivity as a function of the failure rates of the configuration components.
Abstract: The paper presents a general computational method together with the relevant computer program which can be easily used to perform reliability analysis on electric power installation configurations. The mathematical reliability model of the program is based on the concept of frequency of failures during the useful life period. In other words, it starts from basic configuration component failure-rate data and uses the tie-set approach to carry out the reliability analysis of the system under study. For purposes of illustration the computer program has been applied to a practical h.v./m.v. substation configuration. The program is very conducive to analysis work of reliability sensitivity as a function of the failure rates of the configuration components.


Proceedings ArticleDOI
01 Sep 1978
TL;DR: In this paper, the cutoff number knm and the field of three types of two-conductor metallic and two types of Goubau waveguides are determined analytically.
Abstract: The cutoff wavenumbers knm and the field of three types of two-conductor metallic and two types of Goubau waveguides are determined analytically. The types are: Eccentric circular conductors of radii Rl, R2 and distance d between their axes, elliptic inner with circular outer and circular inner with elliptic outer conductor, while in the case of Goubau lines, dielectrically coated eccentric circular as well as concentric elliptic conductor are considered. For small eccentricities kd in the case of eccentric circular and h= ka/2 in the case of circular-elliptic(where a is the inter-focal distance of the elliptic conductor) exact, closed-form expressions are obtained for the coefficients gnm in the resulting relations knm(d or h)= knm(0)[1+gnm?2+ ...], where ?= knmd or h, for the cutoff wavenumbers of the corresponding waveguides. Similar expressions are obtained for the field. Numerical results for all types of modes and comparisons are also included.

Journal ArticleDOI
TL;DR: In this article, it was shown that the influence of roll on the equations of motion of sway and yaw is small and can be neglected for all practical purposes, and the rolling motion can be treated separately as a one degree of freedom with the inclusion of the "linear" influence of yaw.
Abstract: On the basis of an order of magnitude analysis and numerical calculations it is shown that: The influence of roll on the equations of motion of sway and yaw is small and can be neglected for all practical purposes. The rolling motion can be treated separately as a one degree of freedom with the inclusion of the "linear" influence of sway and yaw. This development permits the easier treatment of the troublesome roll motion problem as a non-linear system both in regular waves and for non-linear statistics in a random seaway, with or without an anti-rolling device.

Journal ArticleDOI
TL;DR: In this article, the authors evaluate the radial and horizontal displacements of the surface of the Earth and the gravity perturbation due to the loading effect of the M2 oceanic tide for 13 European laser-ranging sites Most of them are situated in coastal regions
Abstract: Summary The expectation for high accuracies in laser ranging in the years to come and the increasing geodynamical implications in satellite geodesy require the investigation of the effect of time-dependent geodynamical phenomena on the laser-ranging sites In this paper we evaluate the radial and horizontal displacements of the surface of the Earth and the gravity perturbation due to the loading effect of the M2 oceanic tide for 13 European laser-ranging sites Most of them are situated in coastal regions