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1

Mechanical response of notched marble beams under bending versus acoustic emissions and electric activity

Kourkoulis S. K., Pasiou E. D., Dakanali I., Stavrakas I., Triantis D.

Journal of Theoretical and Applied Mechanics

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2018

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Vol. 56 nr 2

523--547

EN

An experimental protocol, including the combined application of both innovative and traditional sensing techniques, is described aiming to explore the mechanical response of marble and also to check the possibilities of detecting precursor phenomena designating upcoming catastrophic fracture. The protocol consisted of three-point bending tests with notched prismatic beams made of Dionysos marble, the material extensively used for restoration of the Acropolis of Athens monuments. The sensing system improvised included techniques relying on completely different physical foundations, which permit simultaneous detection and recording of the Pressure Stimulated Currents, Acoustic Emissions, three dimensional displacement fields and Notch Mouth Opening Displacements. Analysis of the results revealed interesting features of the mechanical response of Dionysos marble and indicated, also, that classical Continuum Fracture Mechanics fails to describe accurately the response of marble, at least in the presence of notches. In addition, strong correlations between the Pressure Stimulated Currents, the rate of acoustic hits and the rate of change of the opening of the pre-existing notch have been enlightened. Moreover, the onset of catastrophic crack propagation appears following distinguishable changes of the Pressure Stimulated Currents recorded. Therefore (and taking into account the very small size of the respective sensors as well as the simple complementary equipment needed), it is concluded that the specific technique could be considered as a simple and reliable tool for an alternative approach to the in-situ Structural Health Monitoring of classical stone monuments.

2

An experimentally validated model for the ilizarov fixator considering the loss of wire’s pretension

Kourkoulis S. K., Mitousoudis A.

Engineering Transactions

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2016

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Vol. 64, iss. 4

621--627

EN

The purpose of this study is to assess the biomechanical performance of the Ilizarov fixator and to attempt optimization of its configuration. Attention is focused to the quantification of the wires’ pretension loss. The study is implemented according to a combined experimental and numerical scheme: The data of a long series of experiments are used to calibrate and validate a numerical model. The model is then used for parametric analysis of the factors which, according to clinical experience, influence the efficiency of the device. It was concluded that the most critical factors are the pretension of the wires, the angle between them and the rings’ diameter; the role of the remaining factors is of rather minor importance.

3

An alternative approach for the interpretation of data from three-point bending of long bones

Kourkoulis S. K., Kouvaka A., Andriakopoulou C., Dontas I.

Engineering Transactions

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2016

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Vol. 64, iss. 4

401--407

EN

An alternative approach for elaborating the raw data obtained from three-point bending tests of long bones of Wistar rats is described. The aim of the study is to provide mechanical properties of the bone tissue which are independent of the bone geometric characteristics. The study is carried out experimentally ex vivo. It is indicated that ignoring the actual shape of the bone’s critical cross-section (i.e., the section where fracture occurs) and the inclination of the loading axis with respect to the principal centroidal axes of the critical section, may lead to erroneous results concerning the strength of the tissue.

4

A multi-layered ring under parabolic pressure

Markides Ch. F., Pasiou E. D., Kourkoulis S. K.

Engineering Transactions

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2016

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Vol. 64, iss. 4

433--440

EN

The stress- and displacement-fields developed in a circular ring consisting of a finite number of linearly elastic homogeneous and isotropic concentric layers are determined. The composite ring is subjected to a distribution of radial stresses (acting along two finite arcs of its periphery) varying according to a parabolic law. The problem is solved analytically adopting Savin’s approach for an infinite plate with a hole strengthened by rings. Taking advantage of the analytic solution, a numerical model is properly calibrated and validated by considering the case of a three-layered ring. It is concluded that the constructed model simulates reality in an excellent manner and therefore it can be safely used for a thorough parametric analysis of the numerous factors influencing the stress- and displacement-fields.

5

Stresses and Displacements in an Elliptically Perforated Circular Disc Under Radial Pressure

Markides Ch. F., Kourkoulis S. K.

Engineering Transactions

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2014

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Vol. 62, iss. 2

131--169

EN

The complex potentials governing the elastic equilibrium of a finite circular disc, elliptically perforated at its center, are obtained using Muskhelishvili’s formulation. The disc is subjected to non-uniform distribution of pressure along two symmetric finite arcs of its periphery. Given the complex potentials, the stress- and displacement-fields can be determined everywhere on the disc by introducing a novel flexible interpretation of the conformal mapping, suitably adjusted to the computational process. The results of this procedure are given for various strategic loci and are critically discussed. The length of the loaded arc is considered similar to that obtained from the solution of the intact disc-circular jaw elastic contact problem assuming that the disc is compressed between the steel jaws of the device suggested by the International Society for Rock Mechanics for the implementation of the Brazilian-disc test. Concerning the distribution of the externally induced pressure along the loaded arcs, it is proven that for the general asymmetric configuration (i.e. the axes of the elliptical hole are neither parallel nor normal to the loading axis) the induced asymmetric displacement field does not permit maintenance of equilibrium of the disc as a whole in case the disc is considered exclusively under a distribution of radial pressure: Additional tractions must be exerted along the loaded arcs, obviously in the form of frictional stresses. Besides, providing full-field, analytic expressions for stresses and displacements, the main advantage of the present solution is that, by properly choosing the ratio of the ellipse’s semi-axes, the solution of three additional configurations, of major importance in engineering praxis, are obtained: These of the intact disc, the circular ring and the cracked disc.

6

Revisiting the Reflected Caustics Method : the Accurate Shape of the “Initial Curve”

Markides Ch. F., Kourkoulis S. K.

Engineering Transactions

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2013

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Vol. 61, iss. 4

265–-287

EN

The shape of the “initial curve”, i.e. the locus of material points, which if properly illuminated provide (under specific conditions) the “caustic curve”, is explored. Adopting the method of complex potentials improved formulae for the shape of the “initial curve” are obtained. Application of these formulae for two typical problems, i.e. the mode-I crack and the infinite plate with a finite circular hole under uniaxial tension, indicates that the “initial curve” is in fact not a circular locus. It is either an open curve or a closed contour, respectively, the actual shape of which depends also on the in-plane displacement field.

7

Smooth elastic contact of cylinders by caustics: the contact length in the Brazilian disc test

Kourkoulis S. K., Markides Ch. F., Bakalis G.

Archives of Mechanics

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2013

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Vol. 65, nr 4

313--338

EN

The smooth contact of two elastic cylinders compressed against each other along a common generatrix is studied analytically and experimentally. The main objective is the quantification of the length of the contact arc. For the analytic study, the complex potentials method is employed while experimentally the contact arc’s length is determined by extending the reflected caustics technique. A series of experiments are then carried out using the device suggested by the International Society for Rock Mechanics for the standardized implementation of the Brazilian test and the typical set-up of the reflected caustics method. The experimental results are compared to the analytically determined ones. The agreement is satisfactory especially for low load levels, ensuring validity of the linear elasticity assumption.

8

Bending of fragmented architraves restored with bolted titanium bars: a numerical analysis

Kourkoulis S. K., Ganniari-Papageorgiou E.

Engineering Transactions

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2008

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Vol. 56, iss. 2

159-180

EN

The mechanical behaviour of restored structural members of ancient monuments is studied in the present paper with the aid of the Finite Element Method. The study is motivated by the needs of the conservation project in progress on the Parthenon Tempie of the Acropolis of Athens; the results however could be valuable for various stone monuments under conserva-tion. Centrally fractured prismatic marble architraves (epistyles) of rectangular cross-section restored with either threaded or smooth titanium bars are modelled. The architraves are rest-ing on marble blocks simulating the capitals (abacuses) of the columns of the tempie. They are subjected to bending under uniformly distributed loading along their span, following the results of earlier studies, concerning the influence of the loading modę on the overall behav-iour of restored structural members. The method used for determination of the reinforcement reąuired is the one introduced recently by the scientists working for the restoration of the Acropolis monuments. All the loads that could be applied on the member after it is replaced in its initial positiori in the monument were taken into account, including the own weight of the member, the weights of the members that will rest on it after the restoration, as well as possible dynamie (earthąuake) loads. Emphasis is laid on the influence of the threads of the bolted bars in comparison to the results for the unbolted ones, in an effort to ąuantify the maximum anchoring length reąuired in order to minimize the intervention on the authentic Stones. The distribution of the stress and strain fields all over the architrave-abacus-reinforcing bar system is investigated and conclusions are drawn concerning the extreme stresses and the points where they are developed.

9

The process zone around the tip of cracks in metal matrix composites

Kourkoulis S. K.

Archives of Mechanics

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2003

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Vol. 55, nr 5-6

407-430

EN

Some characteristics of the process zone developed around the tips of cracks as well as the fractography of the fracture surfaces constitute the subject of the present paper. The study was carried out using Scanning Electron Microscopy for the determination of the Crack Tip Opening Displacement, while numerical analysis and experimental results were combined for the determination of the J-integral. A modern particulate Metal Matrix Composite and its matrix alloy were used for the fabrication of relatively thin tensile specimens of two different types, i.e. single- and double-edge-notched. Dependence of the above quantities on the thickness of the specimen is detected: both the critical Crack Tip Opening Displacement and the critical J-integral are found to increase in an almost linear manner with thickness. For comparison, intact prismatic specimens were considered, in order to study the dependence of ductility and fracture on the thickness of the specimen. It is observed that they depend also on the specimen thickness in a similar manner. Variation of these two properties with the direction of the specimen with respect to the rolling axis is also detected, indicating the importance of the plastically induced anisotropy due to the manufacturing process. Concerning the mechanisms leading to failure, it is concluded that void coalescence is active although void nucleation due to the presence of particles appears to be unavoidable.

10

The influence of cracks on the mechanical behaviour of particulate MMCs: an experimental study

Kourkoulis S. K.

Archives of Mechanics

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2001

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Vol. 53, nr 4-5

439-456

EN

The deterioration of the mechanical properties of particulate Metal Matrix Composites (MMCs) caused by macroscopic cracks is studied, experimentally, in comparison to the behaviour of the respective matrix alloys. Two Al-based MMCs, reinforced with fine SiC particles, extensively used in aerospace engineering, are employed. The study is focused on the influence of cracks on the fracture strength of MMCs and on the variation of the critical Crack Opening Displacement (COD) versus the inclination of the crack with respect to the loading direction. The role of the plastic anisotropy induced during manufacturing is also investigated.

11

The displacement discontinuity technique in fracture mechanics: the subsurface crack problem

Kourkoulis S. K., Exadaktylos G. E.

Archives of Mechanics

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2001

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Vol. 53, nr 4-5

421-437

EN

The displacement discontinuity technique is employed for the determination of the critical conditions causing propagation of a preexisting finite fracture that lies close to the free boundary of a semi-infinite medium. The technique is applied by using a suitably adapted commercial indirect boundary element code, permitting the determination of the displacement, strain and stress fields in multiply-connectedelastic bodies. The critical conditions are then determined by employing a suitable failure criterion. The results of the numerical analysis are compared with the respective ones obtained from a series of experiments with pre-cracked specimens simulating the semi-infinite medium. The agreement is proved to be satisfactory.