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Mamalis A.G.,Project Center for Nanotechnology and Advanced Engineering | Theodorakopoulos I.D.,National Technical University of Athens | Vortselas A.K.,National Technical University of Athens
Technische Mechanik | Year: 2012

High quality, ex-situ powder-in-tube (PIT) MgB 2 superconductors are fabricated using the explosive compaction technique. During the treatment, the precursor materials are densified under high strain-rates using PETN as the explosive medium. It has been found that the product quality depends on the porosity of the compact, which affects the critical current density of the superconductor by introducing changes in the interparticle bonding of the material, as well as the peak shockwave pressure which has an effect on the maximum tensile stress imposed to the specimen. This determines the crack formation in the consolidated powder and the uniformity of the product's final shape. The explosive compaction process has been modeled using the LS-DYNA explicit finite element code where the compacted MgB 2 powder is treated as a porous soil-like material with a customised yield surface. The results of the numerical simulation include the compact porosity, the pressure, the temperature and strain rate profiles as well as the dimensions of the final product, which are used as input data in order to assess the efficiency of the explosive compaction process. The process is non-parametrically optimised for the above mentioned quality factors, and the optimal dimensions of the explosive charge and container tube are determined. Source


Mamalis A.G.,Project Center for Nanotechnology and Advanced Engineering | Theodorakopoulos I.D.,National Technical University of Athens | Vortselas A.K.,National Technical University of Athens
Materials Science Forum | Year: 2011

High Tc MgB2 superconductors were fabricated using the ex-situ and in-situ powder in tube (PIT) technique. During treatment, the precursor materials were shock consolidated under high strain-rates using PETN as the explosive medium. After compaction, the superconducting properties of the steel-sheathed MgB2 samples were examined by means of magnetization measurements using a SQUID magnetometer. Bean's critical state model was applied to investigate the critical current density characteristics of the samples at fields up to 5T. The superconducting transition temperature of the specimens was determined by examining the material temperature dependence of magnetisation in zero field cooled (ZFC) and field cooled (FC) states. An assessment of the explosive compaction technique was carried out by simulating the procedure using the LS-DYNA explicit finite element code. The sample is modelled as a porous soil-like material with a customised yield surface. The numerically estimated final product dimensions, porosity and hardness are compared to the experimental results; furthermore, the numerically obtained pressure, temperature and strain rate profiles are used to assess the efficiency of the compaction process for different explosive quantities and powder compositions. © (2011) Trans Tech Publications. Source


Kundrak J.,University of Miskolc | Mamalis A.G.,Project Center for Nanotechnology and Advanced Engineering | Gyani K.,University of Miskolc | Bana V.,University of Miskolc
International Journal of Advanced Manufacturing Technology | Year: 2011

In high-speed cutting of hardened steels, the surface layer is strongly affected by thermal effects and mechanical forces. Due to this, the surface layer of the machined material changes noticeably. Microhardness, one parameter of the surface integrity, is the most important. This paper deals with an investigation of microhardness. Measuring results are presented, and reasons for the sometimes significant changes in microhardness are analysed. It is proved on which part of the cutting edge the material removal will not take place but the cutting edge deforms the material plastically and how this part of the cutting edge can be reduced. With the measurement of the cutting force, the hypothesis is proved that the values of the cutting force components related to each other are different compared to the traditional turning. The passive force is 1.88-2.25 times higher than the main cutting force. Hence, the force taking place on the flank face of the cutting tool is very high and the friction power significantly influences the cutting temperature. The friction taking place on the flank face of the cutting tool generates 2.8-3.9 times higher heat than the cutting force. Due to the changes that occur in the surface layer, the hardness of this layer is higher with 100-150 HV in depth of 0.1 mm than the original hardness or the hardness prescribed in the technical drawings. This phenomenon can be observed not only in internal hard turning but also machining of external and conical surfaces. Microhardness is compared after hard turning and after grinding. According to the measurements, the ground surface has not become a harder surface layer but softer. As an average result of many measurements, it has been proven that the original hardness (700 HV) after case hardening will increase to 800-850 HV after hard turning and will decrease to 500- 550 HV after grinding. Microhardness changes are analysed considering the typical chip removal characteristics of hard turning. In this article, the focus is on how changes in microhardness influence the functional behaviour of the components and may affect their lifetimes. In this article, it has been proven that independently from the surface of the machined gear (bore, conical or face surface) the changes in the surface layer regarding microhardness do not differ. Source


Mamalis A.G.,Project Center for Nanotechnology and Advanced Engineering | Vortselas A.K.,National Technical University of Athens | Kouzilos G.,National Technical University of Athens
Journal of Applied Polymer Science | Year: 2012

In the study described in this article, we aimed to apply multiparametric optimization to the processing conditions in a spider die used for the extrusion of polymers, in this case, high-density polyethylene tubes. Product quality is affected by the homogeneity of the flow speed and the temperature at the die exit. Inhomogeneity causes shape distortions and material weak spots (weld lines), and its major cause in spider dies is the discontinuity and distortion of the flow caused by spider legs, the ties by which the inner mandrel of the die is secured to its external casing. The Nelder-Mead nonlinear optimization technique was applied to the numerical model to pinpoint the processing conditions, namely, the inlet pressure, inlet temperature of the melt, temperature of the die walls, and temperature of the spider legs. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 Copyright © 2012 Wiley Periodicals, Inc. Source


Mamalis A.G.,Project Center for Nanotechnology and Advanced Engineering | Spentzas K.N.,National Technical University of Athens | Papapostolou D.P.,National Technical University of Athens | Pantelelis N.,National Technical University of Athens
Thin-Walled Structures | Year: 2013

In the present paper, the explicit finite element code LS-DYNA3D was used to investigate the influence of selected material properties in the crash energy absorption characteristics of composite sandwich panels subjected to in-plane compressive loading. The first step in this investigation was to simulate as accurate as possible representative tests corresponding to the collapse modes that occurred in a series of static edgewise compression tests performed in the National Technical University of Athens (NTUA) using various types of composite sandwich panels. These sandwich panels were candidate materials for use in the new type of composite front-end bumper of a transportation vehicle. Subsequent to the precise reproduction of the collapse modes, a step-by-step approach was followed in order to examine the influence of selected faceplate and foam core material properties on the crash energy absorption characteristics of the in-plane loaded sandwich panels. More specifically, several series of finite element models were created, by altering the value of only one material parameter per time of the initial FEA models used for the simulation of the sandwich panels collapse modes. The results from the processing of these series of FEA models were used to create figures that graphically depict the influence of the material properties on the energy characteristics. More over the simulation results were analysed in order to express by means of simple linear or polynomial functions the dependence of the crash energy absorption characteristics on the sandwich material parameters. The findings of this series of investigations were recorded aiming to be used as reference to the design of composite sandwich panels in various crashworthiness applications. © 2012 Elsevier Ltd. Source

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