Papers by Keyword: Plastic Deformation

Abstract: An atomic model is proposed which describes the formation of dislocations and deformation twins through a direct plus reverse martensitic transformation localized in two or several slip planes. The reasons and mechanisms for the formation of partial dislocations and mechanical twinning in nanocrystals under plastic deformation are discussed.

Abstract: On the properties of exploitation of the parts, a major influence has the quality parameters of working surface. Under the term "quality superficial layer" it is understood the integrity of such indices as: geometric precision, undulation, form precision, work surface microgeometry, physical and mechanical properties of the superficial layer. In most cases, the influence of these indicators on the exploitation characteristics of the surfaces are examined separately, but of their reciprocal interaction is evident. Roughness and precision machining of the surfaces have a significant influence on the characteristics of the machine. In most cases, the roughness and precision machining is indicated depending on the operating conditions of the surface. Varying with processing parameters, we can manage forming the surface roughness and surfaces precision. In this paper will present how influencing processing parameters on surface roughness and precision surface.

Abstract: The examination of the main machining methods applied in manufacturing processes from machine building and based on material removal from workpiece highlights essentially the existence of distinct processes able to generate material. An analysis of certain machining methods able to develop processes of material removal from workpieces was initiated by taking into consideration the principle machining schema and the capacity of generating machined surfaces. One concluded that within distinct machining processes, various phenomena are applied in order to obtain material removal from workpiece.

Abstract: When loaded over their yield strength, materials suffer significant plastic deformations. In such zones, the degree of plastic deformation depends on the characteristics of the material and also on the nature of the loading. As known, the degree of plastic deformation may be correlated with the value of hardness, determined in the plastically-deformed zone. The present paper discusses the three types of testings applied, which led to the fracture of some specimens and, consequently, to plastic deformations in the immediate vicinity of the fractured surfaces. The testings, performed on Compact Tension Specimens, aimed at determining the following characteristics developed by the Mechanics of fracture: the crack growth rate, da/dN, fracture toughness for materials with brittle behavior, K_Ic, and fracture toughness for the materials with a preponderantly ductile character, J_Ic. Along the surfaces fractured during these testings, plastic deformations occur, differentiated exactly by the load differences. Under such conditions, testings were made upon the 1C45 (1.053) steel, with the characteristics established by the manufacturer, as well as upon the same steel, yet subjected to an annealing thermal treatment, and the variations of Vickers hardness were recorded both along the fractured surfaces and from one specimen to another.

Abstract: In this paper we present the experimental results obtained after determining hardness on samples previously subjected to fatigue. Firstly, 6 identical samples have been subjected to stress fatigue in the elastic range a number of 10⁵, 5∙10⁵, 10⁶, 2∙10⁶, 3∙10⁶ and 4∙10⁶ cycles. For all samples we used the same form of stress cycle, respectively, sinusoidal, and the same values of maximum, minimum force and the amplitude of the cycle (50 kN, 30 kN and 10 kN). It is noted that the maximum value of the load was less than that at which samples fall within the plastic deformation. Therefore, the original loading of samples was performed in the elastic range. For each of the six samples determinations of levels of hardness were performed, with the value of indentation load of 10 kgf. Indentations were made on the samples, along the length of the calibration, at a distance of 10 mm one within the other. As it will be seen below, in the areas with the highest hardness were performed several indentations, in order to determine the highest hardness area, which, in our view, also presents the largest degree of plastic deformation.

Abstract: Nowadays the surface fine finishing processes ensure, according to the quality of the initialsurface, the initial quality of the cut metal, the operating mode used for processing, the value of roughness with values between Ra = ( 0.4 .. . 0.02) μm. This leads to the reduction of the size of the layer deformed in the previous processing, and to the decrease of the micro-hollows of the resulted roughness serving as lubricant stores contributing to a decrease in the abrasion resistance of the contact surface of the cylinder. From this context it results that bearing area or rather the work area tends towards 100%. This study will present a method of plastic deforming of the inner cylindrical surface by rotary-percussion. I intend to apply this plastic deformation between the honing operation for the scalping of the heat engines cylinders and the finishing honing. The micro-hollows created by percussion will provide oil deposits to eliminate the dry friction, and their density on the cylinder surface will contribute to reducing the load-bearing work surface. The load-bearing work surface obtained with this process will be the total surface minus the surface of the concavities executed through rotary percussion. This procedure applied to the final machining operation by honing the cylindrical surfaces aims at replacing the finishing honing with lapping thus eliminating the running operation at the commissioning of the heat engine. The process has several technological and economic advantages, such as: the malleability of the metal which the cylinder is built of can be higher, the wear of the piston’s surface will decrease the effect of polishing between the piston and the cylinder , the engine will be able to work at higher speeds because the effect of lubricating the contact surfaces will be enhanced; the elimination of the running operation will decrease the cost and time of the installation parameters specified by the manufacturer, the micro-hollows are not compatible with each other so that the oil in the concavities is not blown by the burning gases before the piston passes by them.

Abstract: Manufacture of fuel tablets from UO₂ powder includes many processes, among which shrinkage without application of external loading till now requires explanations. Particles of UO₂ are brittle up to ~ 700°C. By cold pressing due to the porous structure of powder billets separate particles are pressing one into another with accompanying rise of residual elastic stress. Under following heating and sintering plasticization of UO₂ occurs, its yield stress decreases and conditions for creep and resulting shrinkage of tablets takes place.

Abstract: Owing to its selectivity, diffraction is a powerful tool for analysing the mechanical behaviour of polycrystalline materials at the mesoscale, i.e. phase and grain scale. In situ synchrotron diffraction (transmission mode) during tensile tests and modified self-consistent elastoplastic model were used to study elastic and plastic phenomena occurring in polycrystalline specimens during deformation. The evolution of stress for grains which belong to different phases of duplex stainless steel and pearlitic steel was analyzed.

Abstract: The The basic hypothesis of this article focuses on the study of accompanying phenomenas in the cutting zone of the stainless steels, with regard to the elimination of occurrence of poor-quality holes when drilling into new austenitic ELC (Extra Low Carbon) stainless steels. The problem of drilling holes with diameter D=1 to 9 mm resides in the fact that 20 to 28 % of these holes do not comply with prescribed requested requirements. The result of the deformation is very often the unforeseen destruction of the cutting tools; therefore their operational tool life is reduced. This article presents the results of experiments focusing on the study of the plastic deformation of the chips, at drilling with diameter d=5 mm when drilling into a new austenitic ELC stainless steels. This study also includes an analysis of accompanying phenomena in the cutting zone by measuring some selected parameters. The results of the experiments were compared with Cr18Ni8 steel and then verified when drilling holes into specific products.

Abstract: The basic hypothesis of this article focuses on the changes in the plastic deformation under the machined surface during drilling. For experiments applied a new Cast Iron GTW 35-04, Stainless steels DIN 1.4301 and ISO C45 carbon steel. The problem of drilling holes with diameter D=2 to 15 mm resides in the fact that 20 to 30% of these holes do not comply with prescribed requested requirements. This article presents the results of experiments focusing on the study of the damage process in helical drills with diameter d=6.0 mm. This study also includes an analysis of accompanying phenomena in the cutting zone by measuring some selected parameters.