Прегледај по Аутор "Stefanović, Milentije"

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    ANALYSIS OF MICROSTRUCTURAL DAMAGE ON C45E STEEL IN THE PROCESS OF COLD UPSETTING OF A CYLINDER
    (Faculty of Technical Sciences, Institute for Production Engineering, 2016) Kraišnik, Milija; Vilotić, Dragiša; Šiđanin, Leposava; Šljivić, Milan; Stefanović, Milentije; Ilić, Jovan
    From the production point of view, the appearance of cracks of metal components is very negative, except in some metal forming processes when fracture is deemed necessary in order to obtain a product with the required characteristics. However, in general a plastic crack stops further forming processes and is thus a major factor limiting the increase of productivity, especially in cold metal forming. Previous studies have confirmed that the mechanism of nucleation, growth and coalescence of microcavities, which control the level of microstructure damage and lead to initiation and expansion of fracture, are influenced by various factors, with the generated stress state in the forming zone having the strongest influence. In this paper, these problems were investigated in the process of upsetting of cylindrical sample with flat plates. Based on the SEM examination of the microstructure of C45E steel with the aim to quantify microstructural damage and the FEM analysis of the stress state in the critical zone of forming, it can be concluded that there is a different impact of individual components of stress on the accumulation of critical level of damage.
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    APPLICATION POTENTIAL OF SOME CRITERIA OF DUCTILE CRACK IN BULK FORMING PROCESSES
    (University POLITEHNICA Timisoara, 2016) Kraišnik, Milija; Vilotić, Dragiša; Šiđanin, Leposava; Stefanović, Milentije; Anić, Jelica
    Ductile crack criteria are unavoidable tool for material formability analysis. Their basic task is foreseeing the location of crack occurrence and critical value of deformation level in the course of forming process. However, in order to provide successful application of certain criterion it is necessary to be familiar with material characteristics, specificities of forming processes and forming conditions. For all the above mentioned reasons this paper deals with the analysis of application potential of some widely used criteria of ductile crack in cold bulk forming processes according to representative literature resources. Results can contribute better understanding of phenomenon of material microstructure damage and adequate choice of ductile crack criterion in the analysis of formability in various processes and forming conditions.
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    MORPHOLOGY OF STEEL C45E MICROSTRUCTURE DURING UPSETTING
    (Faculty of Engineering Hunedoara, University POLITEHNICA Timisoara, 2015) Kraišnik, Milija; Vilotić, Dragiša; Šiđanin, Leposava; Petrović, Žarko; Šljivić, Milan; Stefanović, Milentije
    Entirely understanding of all factors and mechanisms which influence nucleation and accumulation of ultimate level of material failure is necessary for adequate choice of material and rational projecting of metal forming technological procedure. The first step in research refers to the morphology characterization of material micro constituents during processing. This paper contains qualitative-quantitative analysis of morphology changes of ferrite and colonies of pearlite in the steel C45E microstructure during upsetting of cylindrical specimen with flat dies. Relative microforming level of ferrite grains was determined based on direct measurements, while indirect way has been suggested for determination of pearlite colonies forming ability. The approach is based on monitoring of area fraction changes of pearlite colonies in relation to area fraction of ferrite basis during upsetting process. The results of SEM testing have shown that ferrite grains and pearlite colonies are formed unequally.
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    VARIOUS APPROACHES TO DEFINING THE CRITERIA OF DUCTILE CRACK IN COLD BULK FORMING PROCESSES
    (Faculty of Engineering Hunedoara, University POLITEHNICA Timisoara, 2015) Kraišnik, Milija; Vilotić, Dragiša; Šiđanin, Leposava; Stefanović, Milentije
    In the process of metal forming continuous development and accumulation of material microstructure damage takes place. Reaching the critical level of damage, macroscopic damages occur which is manifested by crack and destruction of specimen (i.e. the crack) in metal forming processes. Optimal and rational design of technological forming processes means recognizing the location the crack occurs and strain limiting value. The ductile crack criterion is used for the purpose of solving this very complex task of material formability. In this paper we provided overview of theoretical and experimental approaches used for the purpose of defining the criteria of ductile crack based on reference literary resources.