Materials and Technologies

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Authors: Liliana Sandu, Nicolae Faur, Cristina Bortun, Sorin Porojan
Abstract: Several studies evaluated the removable partial dentures by the finite element analysis, but none of them evaluated thermal stresses. The purpose of the study was to explore the influence of thermal oral changes induced by hot/cold liquids and food on the circumferential cast clasps of removable partial dentures. A 3-dimensional finite element method was used to explore the temperature distribution, thermal stress and the influence of thermal changes on stresses and displacements of circumferential clasps during functions. Thermal variations induce stresses in dental clasps, high temperatures having a more aggressive effect than lower one. Cold liquids and food induce high stresses in the retentive clasp arms while hot ones in the occlusal rests of the clasps and for the back action clasp also in the minor connector. The study suggests the importance of consFigureidering thermal variations for stress analyses of the cast clasps.
229
Authors: Alina Prună, V. Brânzoi, F. Brânzoi
Abstract: The influence of electrolyte additions on the corrosion of zinc in aqueous solutions of KOH has been determined using electrochemical and nonelectrochemical techniques. These included anodic and cathodic polarization resistance and potentiodynamic method. The inhibitors studied included ZnO and tetra-alkylammonium bromides in different concentrations. From the data provided corrosion currents were calculated. The effectiveness of the inhibitors was compared and it was found that combinations of zinc oxide with tetra-alkylammonium salts were the most effective. Surface analysis obtained with scanning electron microscopy (SEM) revealed morphology characteristics developed at the zinc surface.
233
Authors: L. Velciu, T. Meleg, M. Mihalache
Abstract: The aim of this study is to discuss the morphology (the shape and orientation) of the hydride platelets in the pressure tube material (Zr-2.5%Nb alloy) in function of the modified microstructure by heat treatment.The different microstructures were obtained by annealing and hydrides by hydrogenation (using the electrolytic deposition and the diffusion of the hydrides in the block of samples by a thermal treatment of ageing). The microstructures and hydride morphology were characterised by microscopic analysis, hydrogen content measurements, hydride orientation factor, microhardness Vickers measurements.Based on the results obtained, the increased recrystallization degree involves changes in the hydride distribution. This fact is reflected in the increase of the orientation factor of hydrides and in micro-hardness diminution.
237
Authors: Doru Romulus Pascu, A. Bobic, G. Băltean, Horia Mateiu
Abstract: The paper presents the experimental results regarding the influence of the multiple thermal cycles on the structural and mechanical characteristics of the weldable C-Mn steel used in energetic. Macro- and microscopic examinations, and mechanical testing made on the intersection welds show that the structural and mechanical characteristics are within the reference range of CMn (type S355JR) steel. So, the hardness of the pearlitic – ferritic structures in the welded joints characteristic zones is less than 216 HB; the tensile strength of the welded joints is more than 430 [N /mm2], imposed value for the C-Mn steel. Also, the weld toughness is between 38 [J] and 52 [J] (at 20 °C), more than 27 [J], value what must be assured for the investigated steel. So, the tendency to brittle fracture of the welded joints is diminished. The presence of some imperfections within the intersection welds imposes a carefully monitoring of all these welded joints by adequate nondestructive examinations.
241
Authors: Maria Magdalena Şovar, Diane Samelor, Alain Gleizes, P. Alphonse, S. Perisanu, C. Vahlas
Abstract: Alumina thin films were processed by MOCVD from aluminium tri-iso-propoxide, with N2 as a carrier gas, occasional addition of water in the gas phase, deposition temperature in the range 350-700°C, total pressure 0.67 kPa (2 kPa when water was used). The films do not diffract Xray when prepared below 700°C. At 700°C, they start to crystallize as γ-alumina. EDS, EPMA, ERDA, RBS, FTIR and TGA revealed that films prepared in the range 350-415°C, without water in the gas phase, have an overall composition Al2O3-x(OH)2x, with x tending to 0 with increasing temperature. Al2O3 is obtained above 415°C. When water is added in the gas phase, the film composition is Al2O3, even below 415°C. Coatings deposited in these conditions show promising protection properties.
245
Authors: Alice Dinu, M. Radulescu, D. Ionescu, Silviu Florea
Abstract: The main cause of failure of CANDU-type fuel bundles during their operation is the Stress Corrosion Cracking (SCC). This mechanism appears in presence of the simultaneous action of a corrosive agent and of mechanical stress. The presence of precipitated zirconium hydride platelets can affect the propagation of the SCC cracks. For this reason, we studied the influence of content of hydrogen on the initiation and the propagation of SCC crack. To do this study, some Crings samples with different content of hydrogen (100-500ppm) were exposed in a corrosive environment containing iodine (10-4g/cm2) at 320°C. The stress state, the concentration of the corrosive environment and the temperature were maintained constant. For comparison, in the same conditions some as-received samples were exposed. To relieve the morphology of the precipitates of the zirconium hydride the samples were been investigated using metallographic method, before and after thermal treatments. While before the experiment a circumferential orientation of the hydrides precipitates was observed, after the thermal treatments a reorientation of hydrides in a radial direction was been remarked. Investigation by SEM method revealed the propagation modes of cracks in the case of the as-received samples and it was put in evidence the existence of the microcracks on the inner surface of tested samples.
249
Authors: Gabriel Andrei, Constatin Gheorghies, Laurenţia Andrei, Iulian Gabriel Birsan
Abstract: The main purpose of this work is to reveal the structural changes occured after the impact test on ball bearing steel samples, relative to their ferrite-pearlite phase. The XRD analysis has been used to investigate the level of first and second order internal stresses, the dimensions of mosaic blocks as well as the dislocation density in crystal lattice. The influence of the impact velocity, material hardness and surface roughness on fine structural parameters, mentioned above, is also analysed. On the basis of structural changes it is possible to control the material response during the impact loading.
253
Authors: Brandusa Ghiban
Abstract: Orthopaedic implants represent mechanical devices which may be used for different purposes in human skeleton. such as either repairing of soft tissues (ligaments) or hard tissues (fractures. osteotomy. partial or total replacements). The aim of present paper is to put in evidence some macro and micro structural features revealed at state of surface after 5 months of implant in a female femoral fracture. Investigated surfaces were analyzed by optical and scanning electron microscopy and by X-Rays diffraction.
257
Authors: Brandusa Ghiban
Abstract: In the present paper there are presented results concerning comparative corrosion resistance of some austenitic stainless steels, with or without silicon content. The silicon content is varying in the range of 1-5 %, in a matrix of alloyed austenite, containing 20% Cr and 15-18%Ni and with very low carbon contents (lower than 0,03% C). There are investigated different types of corrosion resistance: intergranular corrosion in nitrogen media, stress corrosion resistance in chloride media, and transpassive behaviour in sulphuric media. The testing media contained nitrogen, as it follows: Huey test (65% HNO3, at boiling temperature, 244 hours maintain), and 5N HNO3 +1g/l Cr6+ (144 h, at boiling temperature). The media containing chloride is 45% MgCl2 at boiling temperature, with a period of 1000 hours. The transpassive behaviour is tested in 10% HSO4. The investigations were made by optic and electronic microscope, and the corroded surfaces were examined by scanning electron microscopy.
261
Authors: I. Vida-Simiti, Nicolae Jumate, M. Guzun, V. Ajder, J. Bobanova
Abstract: The paper reports on a study regarding the structure of composite layers obtained by electrochemical deposition. The depositions were achieved in a bath formed of a mixture of aqueous solutions of iron salts (iron chloride), cobalt (cobalt sulphate) and solid particles of silicon carbide (SiC) in suspension. Following the electrochemical deposition on composite structures are formed as a thin layer with a metallic matrix (FeCo alloy), reinforced with hard particles of SiC. The structure of the composite layer is uniform and very fine, with crystalline granules under 500 nm. The electrochemically deposited FeCo alloy representing the metallic matrix of the composite layer has a high micro-hardness (864 HV), superior to the same alloy obtained by casting.
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