Showing papers by "National Technical University of Athens published in 1980"
TL;DR: In this article, the distortion of the form of reflected caustic from the lateral faces of a dynamically loaded transparent and optically inert specimen containing a transverse crack running under constant velocity was studied on the basis of complex potential elasticity theory and the influence of this form on the value of dynamic stress intensity factor was given.
73 citations
TL;DR: The role of the boundary interphase on the adhesion efficiency between fiber and matrix in the case of polymers reinforced with unidirectional fibers was investigated in this article, where a theoretical model was introduced considering that the composite material consists of three phases; that is, the fiber, the matrix and the interphase, material which is the part of the polymer matrix lying at the close vicinity of the fiber surface.
Abstract: The role of the boundary interphase on the adhesion efficiency between fiber and matrix in the case of polymers reinforced with unidirectional fibers was investigated. A theoretical model was introduced considering that the composite material consists of three phases; that is, the fiber, the matrix and the interphase, material which is the part of the polymer matrix lying at the close vicinity of the fiber surface. The interphase material, having different physical properties from those of the bulk matrix, affects the overall behavior of the composite. Moreover, the quality of adhesion between the two main phases depends greatly on the nature of the interphase material. In this study we have considered that the interphase material is inhomogeneous in nature, with properties varying continuously from the fiber surface to the bulk matrix. The theory developed resulted in a criterion of the adhesion quality and in a prediction of the longitudinal modulus of elasticity of the fiber-composite.
70 citations
TL;DR: In this article, the central deflection of a ship's plate panel is expressed as a function of the plate slenderness ratio, the thickness of the stiffeners, the plate aspect ratio, and the weld throat thickness.
Abstract: Various efforts have been made in the past to express the central deflection of the plate panel as a function of the basic parameters, such as plate slenderness, welding thickness, etc., upon which it depends. Distortion tolerances of the plate could thus be specified as a function of the central deflection, which is easily established. The present paper analyzes more than 2000 measurements of plate panel central deflection in newly built ships collected over a number of years in a shipyard. The central deflection is expressed as a function of the plate slenderness ratio, the thickness of the stiffeners, the plate aspect ratio, and the weld throat thickness. A regression analysis is carried out to determine the coefficients of the functional relationship. From the regression analysis the influence of each parameter on the deflection is derived and new formulas for predicting maximum deflection in various cases are proposed. Finally, results of the analysis with various parameters included are compared with similar formulations previously given in the literature, and a new formula for specification of post-welding distortions is suggested.
46 citations
TL;DR: In this article, necessary and sufficient conditions for the stabilization of an /-input, m-output, time invariant, linear and minimal system while block decoupling it by state feedback and a constant non-singular input transformation are determined.
Abstract: Necessary and sufficient conditions arc determined for the stabilization of an /-input, m-output, time invariant, linear and minimal system while block decoupling it by state feedback and a constant non-singular input transformation. The fixed poles of the block decoupled system are related to a subset of the transmission zeros of tho uncompensated system nnd a method is presented for assigning arbitrarily all the non-fixed poles of the compensated system. Finally, the observability of the block decoupled system is examined.
46 citations
45 citations
TL;DR: In this paper, the direct quadrature method of numerical solution of Cauchy type singular integral equations encountered in plane elasticity crack problems is applied to the case where the loading distribution along the crack edges presents jump discontinuities.
39 citations
32 citations
TL;DR: A theoretical expression of the value of the glass transition temperature at any direction of orientation, defined by the angle ϑ subtended between this direction and the average axis of fibers, was introduced in this article.
Abstract: A theoretical expression of the value of the glass-transition temperatureTg at any direction of orientation, defined by the angle ϑ subtended between this direction and the average axis of fibers; was introduced. Dilatometric, as well as dynamic tests were executed for a type of unidirectional composite in order to compare the respective values ofTg derived from these tests. The experimental results were also compared with theoretical values derived from the theoretical expression. A satisfactory agreement between, theory and experiment was established.
31 citations
TL;DR: In this paper, the methods for direct numerical solution of Cauchy type singular integral equations of the first kind based on Gauss-Chebyshev or Lobatto-Chebyhev numerical integration and the reduction of such an integral equation to a system of linear equations are proved to converge under appropriate conditions.
Abstract: The methods for direct numerical solution of Cauchy type singular integral equations of the first kind based on Gauss-Chebyshev or Lobatto-Chebyshev numerical integration and the reduction of such an integral equation to a system of linear equations are proved to converge under appropriate conditions.
29 citations
TL;DR: In this paper, the effects of transverse shear deformation and rotatory inertia of the mass of the column and of the positioning of the concentrated masses with or without their rotational inertia are analyzed.
26 citations
TL;DR: In this paper, a modification of the collocation method for the numerical solution of Cauchy type singular integral equations with generalized kernels is proposed, which assures the convergence of the method to the correct results since the error terms, usually neglected for the collocate points nearest to the end-points of the integration interval and generally tending to infinity, are now taken into consideration for the selection of collocation points.
TL;DR: In this article, a numerical technique is presented for the efficient solution of plane elasticity problems near corner points and the determination of the corresponding stress intensity factors under symmetry conditions so that only Mode I stress intensity factor exists.
TL;DR: In this article, the type of instability of a T-form frame with joint mass subjected to a compressive follower forceP applied at the joint is investigated on the basis of both the dynamic (variational) and static approach.
Abstract: In this paper the type of instability of a T form frame with joint massM subjected to a compressive follower forceP applied at the joint is investigated on the basis of both the dynamic (variational) and static approach. It is shown that the frame loses its stability through divergence and its critical load is independent of the joint stiffness when the support of the compressed bar is hinged; in contrast, if this support is fully fixed the frame, depending on the joint stiffness, may lose its stability either through divergence or through flutter. However, the region of flutter type instability is very limited which shows that the divergence instability is the rule and not the exception and accordingly the simple static(stability) approach is applicable. Moreover, it is found that the change of the buckling mechanism from flutter to divergence and vice versa is associated with a jump in the critical load.
TL;DR: In this paper, an investigation of the dependence of the crack-propagation velocity in complex bimaterial plates at different loading rates was undertaken and the results showed that for constant and given material characteristics of the bi-phase plate the crack propagation velocity in the first (notched) phase tends to increase with increasing strain rates.
TL;DR: It is concluded that energy models, though very poor forecasting devices, can be very useful to policy makers as tools for analysis; energy model developers must convince potential model users and for that purpose they can benefit immensely from the 35-year-long experience accumulated by their colleagues in Management Science.
Abstract: The Arab oil embargo in 1973 and the subsequent price rises and production restrictions have given birth to a distinct branch within Management Science: energy modelling. This paper gives a critical and selective review on energy modelling, an industry which though thriving in an era of general economic anxiety, is showing signs of arrogant immaturity. After giving a historical background, the paper classifies energy models into three groups: open loop demand or supply models; energy closed loop models; energy-economy closed loop models. For each group the problem area is analysed and some illustrative examples are described. In the last sections, an attempt is made to sum up the experience that has been gained with energy modelling: the basic deficiencies, the impact of this activity on policy formulation and its position within Management Science. It is concluded that energy models, though very poor forecasting devices, can be very useful to policy makers as tools for analysis; energy model developers must convince potential model users and for that purpose they can benefit immensely from the 35-year-long experience accumulated by their colleagues in Management Science.
TL;DR: In this article, the electromagnetic field is expressed in terms of circular cylindrical wave functions referred to both axes, in combination with related addition theorems, and exact closed-form expressions for the coefficients gnm in the resulting relation knm(d)=knm(0)[1+gnm(knmd)2+...] for the cutoff wavenumbers of the waveguide.
Abstract: The cutoff wavenumbers knm and the field of surface wave modes of a circular cylindrical conductor eccentrically coated by a dielectric are determined analytically. The electromagnetic field is expressed in terms of circular cylindrical wave functions referred to both axes, in combination with related addition theorems. When the solutions are specialized to small eccentricities kd, where d is the distance between the two axes, exact closed-form expressions are obtained for the coefficients gnm in the resulting relation knm(d)=knm(0)[1+gnm(knmd)2+...] for the cutoff wavenumbers of the waveguide. Similar expressions are obtained for the field. Numerical results for all types of modes are given. For certain values of the parameters, it is possible to enhance the operating bandwidth of the basic hybrid mode HE11 over the conventional concentric guide.
TL;DR: It is shown that a solution to the problem exists under very general conditions, and that one solution at least must have continuous pointwise error modulus.
Abstract: Segmented approximations with free knots for a given continuous function are discussed in a general form. This form includes any kind of continuous approximating segments, and a great variety of error criteria, such as anyL
s
-norm, or the maximum norm of the pointwise error. It is shown that a solution to the problem exists under very general conditions, and that one solution at least must have continuous pointwise error modulus. Finally, algorithms for the iterative reduction of the approximation error are proposed, which lead to good, but not necessarily best approximations.
TL;DR: In this article, the Radau-Chebyshev method of numerical integration and solution of singular integral equations is modified, through a variable transformation, so as to become applicable to the numerical solution of single integral equations along semi-infinite intervals, and the direct determination of stress intensity factors at the crack tips.
TL;DR: In this article, a new experimental technique was developed for the production of crosslinked polystyrene (PS) particles in a suspension crosslinking reaction, and the suspending medium being used was silicone oil saturated by 1,2-dichloroethane (DCE), without any suspension stabilizer.
Abstract: A new experimental technique was developed for the production of crosslinked polystyrene (PS) particles in a suspension crosslinking reaction. The suspending medium being used was silicone oil saturated by 1,2-dichloroethane (DCE),without any suspension stabilizer. Antimony pentachloride, 1,4-dimethyl-2,5-dichloromethyl benzene and DCE were used as a catalyst, crosslinking agent, and polystyrene solvent, respectively. The products were colorless transparent spherical particles of a very high purity and they could be produced in short reaction times.
TL;DR: In this paper, the thermomechanical behavior of polymeric matrices reinforced with long unidirectional fibres in their glass transition region was investigated based on rigorous theories for the mechanical moduli and the coefficients of thermal expansion for viscoelastic composites.
Abstract: The thermomechanical behaviour of polymeric matrices reinforced with long unidirectional fibres in their glass transition region was investigated. Based on rigorous theories for the mechanical moduli and the coefficients of thermal expansion for viscoelastic composites, it was proved analytically that the introduction of reinforcing fillers in polymeric matrices causes glass transition temperature to increase to an extent proportional to the reinforcing effect. This is confirmed by numerous experimental results available in the literature, except for highly-filled composites, where imperfect adhesion may counterbalance the reinforcing effect. Dilatometric as well as two different dynamic mechanical tests confirmed the latter. The variation of glass transition temperature as function of the various aspects of mechanical reinforcement was investigated.
TL;DR: The numerical solution of one-dimensional singular integral equations with singularities in their kernels can be achieved in several cases by reducing such an equation to a system of linear equations through use of appropriate numerical integration rules and collocation points as mentioned in this paper.
TL;DR: In this article, the communication process between two hierarchical systems each possessing two levels is modelled as finite state parametrized Markov chains, and the control parameters upon which the transition probabilities at the levols W, W′depend are: (1)collective properties of the dynamical deliberations on levels Q, Q′ which measure (a) the percentage of occupancy of a state selected a priorias homeo-static and (b) the cross-correlation(s) between the state sequences of the levy (Q, Q), or (Q′, W)
Abstract: The communication process between two hierarchical systems each possessing two levels is dealt with. The dynamics at all levels are modelled- as finite state parametrized Markov chains. For the lower levels Q, Q′the transitions are parametrized on control variables stemming from an underlying game, which simulates collectively all hierarchical levels below Q, and Q′respectively. The higher levels W, Ware modelled by semi-Markov chains with holding times following a geometrical distribution. The control parameters upon which the transition probabilities at the levols W, W′depend are : (1)collective properties of the dynamical deliberations on levels Q, Q′which measure (a) the percentage of occupancy of a state selected a priorias homeo-static and (b) the cross-correlation(s) between the state sequences of the levols (Q, Q′) or (Q′, W) respectively; and (2)the holding time statistics. The feedforward control is exercised from the higher levels Wand W′towards the lower levels Q, Q′correspondingly in...
TL;DR: Five typical groups of triangular elements are introduced and their properties and recurring formulae referring mainly to nodal and triangular numbering are given in detail, used later for developing the computer programme in FORTRAN.
TL;DR: In this paper, an analytical procedure for the nonlinear elastic analysis of a cantilever circular beam subjected to a concentrated co-planar force at its free end is presented.
Abstract: In this investigation an analytical procedure for the non-linear elastic analysis of a cantilever circular beam subjected to a concentrated co-planar force at its free end is presented. According to this method the non-linear differential equations, describing the equilibrium of the deformed beam, were decoupled and a closed-form solution under the form of elliptic integrals was obtained.
In dieser Arbeit wird eine analytische Methode fur die nichtlineare elastische Analyse eines freitragenden kreisformigen Kragbalkens, an dessen freiem Ende eine konzentrierte komplanare Kraft wirkt, entwickelt. Nach dieser Methode werden die nichtlinearen Differentialgleichungen, welche das Gleichgewicht des deformierten Kragbalkens beschreiben, entkoppelt, und es wird eine geschlossene Losung mit Hilfe elliptischer Integrale angegeben.