Papers by Author: Ionel Chirica

Abstract: In this paper the influence cell honeycomb geometry on the mechanical behaviour of a composite sandwich plate is analyzed. Three cell geometries (circular, hexagonal and square) are static analysed so that to select the best type of honeycomb that will be used in the manufacturing the sandwich plate core. The main aim is to develop approach models of equivalent orthotropic materials to replace the real model of honeycomb core with their properties so that to quickly calculate the sandwich plate made out of composite when is used a finite element analysis code. Geometry and material properties of the honeycomb are delivered by the material provider. Comparative analysis, by using Finite element analysis is performed for all geometries, in the same boundary conditions. Since in the impact loading of the composite sandwich plate the core is mainly loaded to compression, comparative study of the three cell geometries honeycomb was performed for this type of compressive loading. Since the cell is the basic element of the honeycomb core, the calculus is performed for one unit volume of sandwich, concerning also the part of skins.

Abstract: This paper presents the FEM approach of fatigue life estimation of a layered composite plate having a central elliptical cut-out subjected to time variation loads. Some properties and material data, such as data used for the S-N curve / Wöhler curve of the material, represented in cyclic stress versus number of cycles to failure coordinates, which is required for starting the fatigue analysis, were taken from the OPTIMAT database.The plate analysis was parameterized and modeled with the Shell281 element type. This type of element can be used in a triangular form more efficiently than other types of shell elements available in version 15 of ASYS APDL.Initially, the static analysis was performed to identify the node with the maximum principal stress (hot spots). This node was used for performing the fatigue analysis. The fatigue parameter to estimate fatigue life is cumulative fatigue usage factor. This factor is obtained by adding together all individual usage factors calculated for each variable load that is applied to the analyzed plate. The fatigue usage factor is used to estimate the percentage of life that is consumed after applying the fatigue loads. The value of the usage factor is in the range from 0 to 1.

Abstract: . In this paper a parametric static and torsion analysis on a catamaran ship structure made out of composites materials is performed. The best solution for material selection is based on specific criteria for certain analysis. For dynamic analysis, the main criteria is to obtain the structure natural frequencies in a range, far from the resonance frequencies produced by the ship propulsion installation, wave loads etc. For static analysis the comparative criteria is to obtain the lowest weight of structure for the same strength. Also, buckling item is other criteria for comparative analysis. The comparative analysis is performed between rule based scantling structure and a FEM based structure for a composite material twin hull deck, in order to reduce the total hull weight. Material used for ship deck structure of the analyzed ship is e-glass polyester.

Abstract: Magnetorheological elastomers (MREs) are a kind of smart materials, which change the mechanical properties (viscoelastic characteristics) under the magnetic field action. In the paper the determination of damping properties (reactive force) of specimens made out of magnetorheological elastomers is presented. The specimens made out of MREs have been fabricated as a composite with matrix made out of silicone rubber with certain contents of magnetisable particles (carbonyl iron powder). The cylindrical specimens have been tested in compression loading, controlled by an electro-mechanic system. The MRE characteristics of the specimens have been determined in the presence of a magnetic field produced with an electromagnet (coil device). The reactive force occurring in the MRE specimen has been determined on the basis of the measured data during loading. The variation curves of the reactive force versus magnetic field induction are drawn. As a conclusion, the rigidity of the MRE specimen is increasing since the magnitude of the magnetic field induction is increasing.

Abstract: Parametric study to evaluate the influence of the sides ratio on the axial buckling and postbuckling behaviour of the layered composite plates used in ship deck structure, is presented. In the deck structure, the plates (outlined by two pairs of longitudinal and transversal stiffeners) have obviously quadrilateral shape (from triangle to quadratic shapes). The presence of the in-plane loading, occurred from the ship hull general bending, may cause buckling of layered panels. The parametric calculus was performed for various values of ratio between the minimum and maximum value of the sides. The numerical analysis on the trapezoidal plates is presented, including the graphical results for all values of loading and side ratios.

Abstract: The paper addresses the study of the damping characteristics estimation and behaviour of the magnetorheological elastomers (MREs) in the absence of magnetic field. This type of material actively changes the size, internal structure and viscoelastic characteristics under the external influences. These particular composite materials whose characteristics can vary in the presence of a magnetic fields are known as smart materials. The feature which causes the variation of properties in magnetic fields is explained by the existence of polarized particles which change the material form by energy absorbing. Damping is a special characteristic that influences the vibratory of the mechanical system. As an effect of this property is the reducing of the vibration amplitudes by dissipating the energy stored during the vibratory moving. The main characteristic that is based on the determination of the damping coefficient is the energy loss, which is the subject of the present paper. Before to start the characteristics determination in the presence of the magnetic field, it is necessary to study these characteristics in the absence of magnetic field. The MRE specimens have been manufactured and tested under the light conditions (non magnetic field). A special experimental test rig was built to investigate the response of the MRE specimens under the charging force. The experimental results show that the loss energy of the MRE specimen can be determined from the charging-discharging curves versus displacement. The results of the MRE specimen are presented in this paper: MRE with feromagnetic particles not exposed in magnetic field during fabrication.

Abstract: In the paper, a methodology to evaluate the influence of the central circular delamination on the buckling and postbuckling behaviour of the sandwich plates under shear and compression loading, used in ship hull structure, is presented. The parametric calculus was done for various values of loading ratios. The numerical tests on the delaminated plates are presented. Sandwich composite plates containing central delaminations are analyzed with respect to combined boundary shear and axial compression buckling. Finally, the graphical results for all values of loading ratio are presented.

Abstract: The paper presents selected results of a study concerning the protective capacity of ship hull plates, made out of layered composite plates. A scenario to evaluate the behaviour of the ship structure plate under blast loading is presented. The nonlinear analysis on a 3-D FEM model by using composite elements was performed. The methodology for the blast pressure charging and the mechanism of the blast wave in free air are given. The space pressure variation is determined by using Friedlander exponential decay equation. According to the methods used in this paper an individual pressure-time history to each element based on its distance from the blast is assigned. The dynamic response of the composite plate is shown.

Abstract: In this paper, results obtained on buckling behavior of ship deck plates made of composite materials are presented. The fabrication technologies used in ship structures made of composite materials are mostly based on the hand made. So, the probability to occur material imperfections is too high. The influence of the imperfections on the buckling characteristics of the panels is analyzed. The results obtained so by numerical and by experimental analysis of the composite plates with initial transversal imperfections are compared with the results obtained for the plates made of steel, having the same type of imperfections.

Abstract: The paper describes the results from mechanical testing and numerical analyses for the adhesive joints used in ship structures. The work undertaken in the numerical and experimental tests of different connections (steel-glass fiber polyester resin; steel - Carbon fibre epoxy) is summarized. The numerical nonlinear calculus is done due to the nonlinear properties of the adhesive and also due to the nonlinear behaviour of the adhesive joint.