Showing papers by "National Technical University of Athens published in 1983"
126 citations
TL;DR: In this article, the T -criterion was applied in the case of the fracture of a ductile material and the predicted and measured quantities are in good agreement, especially in cases where existing other fracture criteria fail.
74 citations
TL;DR: The paper explores the effect that process design has on the synthesis of a control system and vice versa and attempts to organize all the above issues in a coherent system which can guide the control designer towards the synthesisation of effective control systems.
66 citations
TL;DR: In this paper, the existence of divergence instability for an elastically restrained column under a follower compressive force at its end is investigated and necessary and sufficient conditions for such instability are established.
65 citations
TL;DR: In this paper, the authors applied the Dugdale model to determine the plastic zones developed in the case of two collinear and unequal cracks within an homogeneous, Isotropic, elastic-perfectly plastic infinite plate.
40 citations
TL;DR: In this article, Second Fick's law of diffusion was used to predict the diffusion coefficient of the composite materials tested in all cases the matrix material was a diglycidyl ether of bisphenol-A polymer cured with 8 phr triethylene tetramine and filled with iron particles with an average diameter 150 μm at five distinct volume fractions.
Abstract: Water absorption in particulate composites at ambient temperature influences their thermomechanical properties. Second Fick's law of diffusion was used in this paper to predict the diffusion coefficient of the composite materials tested. In all cases the matrix material was a diglycidyl ether of bisphenol-A polymer cured with 8 phr triethylene tetramine and filled with iron particles with an average diameter 150 μm at five distinct volume fractionsv
f
=0, 0.05, 0.10, 0.16 and 0.20. The modification of the modulus of elasticity, ultimate stress, breaking strain and breaking energy due to moisture absorption was examined. Moreover, differential scanning calorimetry was used to study the influence of the time exposure into water and the filler concentration of the particulates on their glass transition temperature. Finally, the void occupancy in the composite was evaluated from free volume considerations.
32 citations
TL;DR: In this paper, the influence of moisture absorption on the extent of the boundary interphase in particulate composites is thoroughly studied, and an explanation of the observed relationship was advanced, based on a theoretical mechanism of absorption.
Abstract: The influence of moisture absorption on the extent of the boundary interphase in particulate composites is thoroughly studied. It was found that, during the process of moisture absorption there is a variation of the extent of the boundary interphase, closely related to the degradation of the mechanical behavior of the composite, as well as to the percentage amount of moisture absorbed. An explanation of the observed relationship was advanced, based on a theoretical mechanism of absorption. This study complements a previous one, where the observed degradation of the thermomechanical properties of particulate composites due to moisture absorption was shown to be intimately interrelated with the state and extent of the interphase between fillers and matrix.
32 citations
TL;DR: In this article, the problem of a thin sheet containing a slant crack and subjected to a biaxial load is examined by giving the expressions for stresses in a series form, which are equally valid near and far (r < 2a) from the crack tip.
Abstract: The problem of a thin sheet containing a slant crack and subjected to a biaxial load is examined by giving the expressions for stresses in a series form, which are equally valid near and far (r<2a) from the crack tip. The progressive importance of the higher-order terms in the series representation for stresses, which were added to the singular and constant ones was demonstrated by giving the isochromatic fringe patterns near and far the crack-tip area. Based on these expressions, a photoelastic determination of the exact values of the complex stress intensity factor was given without necessitating appropriate correction factors and diagrams. The values of SIFs thus determined are valid independently of the polar angle and distance of measurements from the crack tip.
30 citations
TL;DR: First the general problem of block matrix inversion and linear system solution is considered and a corresponding algorithm is developed, together with a block form of triangularization theorem, yielding most existing efficient algorithms.
29 citations
01 Jul 1983
TL;DR: In this article, the complex potentials describing the stress field due to a concentrated force, or a dislocation, applied at some point at the interior of either of the half planes bonded together, or being under some kind of contact between them, are calculated.
Abstract: The complex potentials Φ(z) and Ψ(z) describing the stress-field due to a concentrated force, or a dislocation, applied at some point at the interior of either of the half planes bonded together, or being under some kind of contact between them, are calculated. By using these potentials as Green's functions we have reduced the problem of crack and/or holes existing in either of these half-planes to an integral equation along the cracks or holes. The advantage of the method is that the integral equation along the interface between the two half-planes (y=0) is canceled out. Thus, the numerical evaluation of the state of stress is considerably reduced and simplified. The respective integral equations corresponding to some characteristic cases of bimaterial plates were derived. Finally, examples of cracked half-planes, either stressor displacement-free, or bonded with other half-planes, indicated the potentialities of the method.
27 citations
TL;DR: In this article, a closed-form solution for the stress field in an infinite plate which contains a slant crack was presented, and its accuracy was independent of the normalized to the crack-length distance from the crack tips.
TL;DR: In this paper, the optical method of reflected caustics was employed to investigate the variation of the coefficient of friction in the roll gap during strip rolling, and an almost linear decrease of the friction coefficient was observed from a maximum value, at the entry plane to zero at the neutral plane and then a parabolic, tending to linear, increase of it, towards the exit plane.
TL;DR: In this article, a method for the derivation of an internally balanced minimal-state-space description of a SISO system, given its transfer function, was proposed, which requires only once the computation of the eigenvalues-eigenvectors of a symmetric matrix.
Abstract: A method is proposed for the derivation of an internally balanced minimal-state-space description of a SISO system, given its transfer function. The method requires only once the computation of the eigenvalues-eigenvectors of a symmetric matrix. Solution of the Lyapunov matrix equation is not needed.
21 Mar 1983
TL;DR: This work characterize complementary views and show that finding a minimum complement of a given view is NP-complete and defines and study a new kind of dependencies the explicit functional dependencies, which intuitively state that some part of the database information can be computed from the rest.
Abstract: We study the problem of translating updates of database views. View updates are disambiguated by requning that a specified view complement (i.e. a second view which contnms all the information omitted from the given view) remains constant during the translation. We study some of the computational problems related to the application of this general methodology in the context of relational databases. We restrict our attention to projective views of databases which consist of a single relation and satisfy functional dependencies. We first characterize complementary views and show that finding a minimum complement of a given view is NP-complete. We then study in detail the problem of translating the insertion of a tuple into a view and extend our results to the cases of deletion and replacement of a tuple. Finally we define and study a new kind of dependencies the explicit functional dependencies, which intuitively state that some part of the database information can be computed from the rest.
TL;DR: In this paper sufficient conditions are derived to ensure the convergence of the Elliott and Hunter types of quadrature rules for the evaluation of weighted Cauchy principal-value integrals of the form.
Abstract: In this paper sufficient conditions are derived to ensure the convergence of the Elliott and Hunter types of quadrature rules for the evaluation of weighted Cauchy principal-value integrals of the form:
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The simultaneous convergence in the interval (−1, 1) of both quadratures was established for a class of Holder-continuous functionsf(f∈Hμ) Corrections of some previous statements on the subject of convergence of such quadratures are also included
TL;DR: This paper is a review of recent theoretical work on the problems which arise when many users access the same database.
TL;DR: In this paper, the optical method of caustics and the T-criterion of fracture were applied to in vestigate and explain the process of interface disruption, which takes place when a moving transverse crack reaches and coalesces with a weak longitudinal interface of a bi-phase plane specimen.
Abstract: The optical method of caustics and the T-criterion of fracture were applied to in vestigate and explain the process of interface disruption, which takes place when a moving transverse crack reaches and coalesces with a weak longitudinal interface of a bi-phase plane specimen. A quantitative criterion was then established, based on the T-criterion, which relates the mechanical properties of the interface with the mechanical properties of subsequent phase of penetration of the crack for the critical condition of crack propagation either along the interface, or along the second phase.
TL;DR: In this article, a simple asymptotic elastic theory and a verification by the method of reflected caustics were used to determine the boundary of the core region of brittle fracture.
TL;DR: In this paper, the authors used caustics with high-speed photography to study the dynamic behavior of a transverse crack in bimaterial plates, with slant interfaces, and found that the influence of the interface region to the crack propagation velocities was more intense when the crack propagates from a more brittle to a more ductile phase, than in the opposite case.
TL;DR: In this article, the optimal load flow problem is formulated as a non-linear constrained optimization problem, recognizing system losses; operating limits on the generation units and line security limits, and solved by an iterative scheme.
Abstract: This paper presents a methodology for solving the Optimal Load Flow problem. The problem is formulated as a non-linear constrained optimization problem, recognizing system losses; operating limits on the generation units and line security limits. The optimization problem is solved by an iterative scheme. At each iteration the Optimal Load Flow problem is decomposed into a real subproblem and a rqactive subproblem. The two subproblems are transformed to quadratic programming problems by utilizing Z-mattix techniques and the Generalized Generation Distribution Factors.
TL;DR: In this paper, the required joint stiffness as well as the lateral bracing stiffness of unbraced rectangular frames subjected to vertical loading are discussed in conjunction with their loadcarrying capacity.
Abstract: The required joint stiffness as well as the lateral bracing stiffness of unbraced rectangular frames subjected to vertical loading are discussed in conjunction with their loadcarrying capacity. The sway-buckling mode of such frames (implying a considerable decrease in their load-carrying capacity) can be prevented by chosing a suitable combination of values of the above parameters which satisfy a nonlinear interaction equation at each level of the external loading. By providing a lateral support of very small stiffness to an initially unbraced frame, its load-carrying capacity can be increased up to the level of the corresponding capacity of the same frame with no sidesway.
TL;DR: In this paper, the authors studied the way in which Santorin earth acts during its hydration with portland cements and specifically, the rates of its action and its optimum content, the amount of Ca(OH) 2 derived during the hydration has been quantitatively determined, by means of thermogravimetry.
TL;DR: In this article, the qualitative influence of the solid support sorptive properties on the reflected wavelengths from cholesteric liquid crystals has been reported for the first time, where it was visually observed that the reflected light band shifted to longer wavelengths as the sorptive abilities of solid support increased.
Abstract: This work is a continuation of that published in Part I [Mol. Cryst. Liq. Cryst., 61, 31 (1980)] under the same title, where the qualitative influence of the solid support sorptive properties on the reflected wavelengths from cholesteric liquid crystals has been for the first time reported. It was visually observed that the reflected light band shifted to longer wavelengths as the sorptive abilities of the solid support increased. In the present work, quantitative measurements (visible diffuse reflectance spectroscopy) at several temperatures on bare metal (Zn, Fe, Cu, Al) surfaces mechanically treated, on chemically and electrolytically prepared ZnO and Fe3O4, on electrolytically prepared γ1- and γ2–Al2O3, on copper surfaces electroplated with copper and on electiolytically prepared γ1-Al2O3 under several current densities, covered with the same mixture of cholesterics as the one used in the previous work, were carried out. Thus, the results of the visual observations of the previous work were c...
TL;DR: In this paper, the authors investigated the interactions between energy and economy in the group of Developing and Industrializing Countries (DIC's) and pointed out that energy models used in the DIC's must capture the particular features of energy policy in these countries, such as rapid economic development fueled by expensive, depleting and often imported energy resources; dependence on foreign resources such as energy, capital, technology, etc.; management of indigenous resources, social structural changes, rapid population growth, urbanization and industrialization.
TL;DR: In this article, the authors analyzed the influence of short-circuit duration on the dynamic stress in certain cases of busbar structures and gave the maximum dynamic stress for the entire range of relevant mechanical fundamental frequencies.
Abstract: The short-circuit peak current and the electromagnetic short-circuit peak force appear at about 10 ms after the short-circuit inception. However, the maximum value of dynamic stress due to the electromagnetic forces may occur much later and depends on the fundamental frequency of the structure. A decrease in dynamic stress can be obtained even if the short-circuit duration is much longer than 10 ms. In the presentwork, the dynamic stress on stiff conductors and insulators during the short-circuit, as well as, the subsequent free oscillation after fault clearing, are analytically determined and the influence of short-circuit duration on the dynamic stress in certain cases of busbar structures is investigated. Furthermore, the shortest short-circuit durationwhichresults in maximum dynamic stress is given for the entire range of the relevant mechanical fundamental frequencies.
01 May 1983
TL;DR: In this paper, a new technique for the solution of singular integral equations is proposed, where the unknown function may have a particular singular behaviour, different from the one defined by the dominant part of the singular integral equation.
Abstract: A new technique for the solution of singular integral equations is proposed, where the unknown function may have a particular singular behaviour, different from the one defined by the dominant part of the singular integral equation. In this case the integral equation may be discretized by two different quadratures defined in such a way that the collocation points of the one correspond to the integration points of the other. In this manner the system is reduced to a n × n system of discrete equations and the method preserves, for the same number of equations, the same polynomial accuracy. The main advantage of the method is that it can proceed without using special collocation points. This new technique was tested in a series of typical examples and yielded results which are in good agreement with already existing solutions.