138 research outputs found
The decline of educational hypergamy in the former Yugoslav republics
U radu se istražuju promene obrazovnog sastava braÄnih partnera u bivÅ”im jugoslovenskim republikama, posebno opadanje uÄeÅ”Äa brakova u kojima su muÅ”karci obrazovaniji od žena (hipergamni brakovi). U osnovi ovih promena jeste razvoj tercijarnog obrazovanja i nastanak polne asimetrije visokoobrazovanih lica, usled kojih je u prve dve decenije XX veka doÅ”lo do porasta uÄeÅ”Äa braÄnih zajednica u kojima su žene obrazovanije. Analiza se pretežno temelji na podacima Demografske (vitalne) statistike, a period koji je obuhvaÄen istraživanjem odnosi se na poslednjih pedeset godina. KoristeÄi se specifiÄnim pokazateljima (F i H indeksom) i korelacionom analizom, dobijeni su rezultati koji su pokazali da obrazovna hipergamija opada u svim istraživanim zemljama, kao i da je opadanje hipergamije povezano sa prevagom žena meÄu visokoobrazovanim licima. Rezultati daju nove uvide u tumaÄenje izbora partnera posredstvom socioekonomskih karakteristika, ali i ukazuju na kljuÄnu ulogu koju u procesu selekcije ima obrazovna kompozicija stanovniÅ”tva.This paper examines marital market preferences and opportunities, given the gender asymmetry of education and the limited supply of equally educated partners (especially in the higher education segment), and explores selection patterns resulting from new composite changes for at-risk populations entering into a marriage. Specifically, we are researching mixed marriages by education and changes in the patterns of that selection in the former Yugoslav republics. The assessment of the impact of changes in educational characteristics by gender on patterns of marital pairing and the reduction of the level of educational hypergamy was performed by analysing the movement of F-index and H-index values from 1970 to 2020 and their correlation coefficient. The results showed that educational hypergamy is declining in all countries and that the decline in hypergamy is positively associated with the predominance of women among highly educated individuals
Nadzorna ovlaÅ”Äenja poslodavaca u savremenim uslovima rada - analiza sluÄaja i moguÄi vidovi reÅ”avanja radnih sporova
U radu se razmatraju i analiziraju nadzorna ovlaÅ”Äenja poslodavaca u savremenim uslovima radne organizacije. SprovoÄenje nadzora integrisano je u svakodnevni život i rad ljudi, protoke kapitala i razvoj karijere. Informatizacija radne sfere doprinela je znaÄajnim promenama u odnosu poslodavca i zaposlenog. Stoga je predmet istraživanja fokusiran na prostorne, vremenske i etiÄke dimenzije poslodavÄevih ovlaÅ”Äenja, usled prihvatanja savremenih informacionih i komunikacionih tehnologija u proces rada. Zapravo, osnovni cilj istraživanja jeste prespitivanje i odreÄivanje poslodavÄevih nadzornih ovlaÅ”Äenja. U poslednjem delu rada, ukazuje se na moguÄe vidove reÅ”avanja sporova u kontekstu prekoraÄenja poslodavÄevÄevih ovlaÅ”Äenja, s posebnim osvrtom na domaÄu sudsku praksu
The change in educational assortative mating in Serbia and Slovenia, 1970ā2020
This paper explores marital matching patterns from the perspective of the partners' educational attainment, focusing on the link between gender asymmetry in education and educational hypergamy. In order to assess to what extent the tendency for women to marry men of higher educational status is related to the educational gender gap in Serbia and Slovenia, we calculate an index for women's educational advantage, and an index for the prevalence of educational hypergamy. Our results confirm the following: the growth of education is associated with an increase in female educational advantage; the relationship between female educational advantage and educational hypergamy is strongly negative; and there are no significant differences in assortative mating patterns between Serbia and Slovenia
Probability of failure of thermal power plant boiler tubing system due to corrosion
Korozija predstavlja nepovratni proces degradacije materijala. Zbog nepovoljanog uticaja koji ima na radne osobine materijal, posebno materijala cevnog sistema kotla, ona predstavlja predstavlja neizostavni deo rada termoenergetskih postrojenja i glavni uzrok mnogih otkaza. Pored uobiÄajenih metoda za procenu preostalog radnog veka korisno je primeniti i metode za procenu pouzdanosti kotlovskih cevi termoenergetskih postrojenja koje su oÅ”teÄene korozijom. Korelacijom rezultata koji se dobijaju primenom ove dve razliÄite metode moguÄe je dobiti bolja saznanja o trenutnom stanju materijala cevi uz istovremeno izvoÄenje taÄnije procene njihovog ponaÅ”anja tokom dalje eksploatacije. Stoga je u ovom radu na primeru cevi ulazne zone paketa naknadnog pregrejaÄa prikazan integralni pristup procene preostalog radnog veka i pouzdanosti. Razmatranja ove vrste su veoma znaÄajna za pristupe u održavanju koji su zasnovani na riziku.Corrosion is irreversible process of material degradation. Due to detrimental effects exerted on the operating material characteristics, especially in the boiler tube system, it is of one the most important issues in the operation of thermal plants and is considered to be the root cause of many outages. Besides common remaining life assessment methods, it is useful to apply a method of reliability evaluation of the thermal power plant boiler tubes with corrosion damages. Correlation of results obtained by these two methods makes possible to obtain a better knowledge of current tubes material state as well as a more accurate assessment of their behavior during future exploitation. Therefore, an integrated approach of remaining life assessment and reliability evaluation, on the first stage reheater tubes system example, is presented in this paper. Considerations of this kind are very important for the risk-based maintenance programs
Probability of failure of thermal power plant boiler tubing system due to corrosion
Korozija predstavlja nepovratni proces degradacije materijala. Zbog nepovoljanog uticaja koji ima na radne osobine materijal, posebno materijala cevnog sistema kotla, ona predstavlja predstavlja neizostavni deo rada termoenergetskih postrojenja i glavni uzrok mnogih otkaza. Pored uobiÄajenih metoda za procenu preostalog radnog veka korisno je primeniti i metode za procenu pouzdanosti kotlovskih cevi termoenergetskih postrojenja koje su oÅ”teÄene korozijom. Korelacijom rezultata koji se dobijaju primenom ove dve razliÄite metode moguÄe je dobiti bolja saznanja o trenutnom stanju materijala cevi uz istovremeno izvoÄenje taÄnije procene njihovog ponaÅ”anja tokom dalje eksploatacije. Stoga je u ovom radu na primeru cevi ulazne zone paketa naknadnog pregrejaÄa prikazan integralni pristup procene preostalog radnog veka i pouzdanosti. Razmatranja ove vrste su veoma znaÄajna za pristupe u održavanju koji su zasnovani na riziku.Corrosion is irreversible process of material degradation. Due to detrimental effects exerted on the operating material characteristics, especially in the boiler tube system, it is of one the most important issues in the operation of thermal plants and is considered to be the root cause of many outages. Besides common remaining life assessment methods, it is useful to apply a method of reliability evaluation of the thermal power plant boiler tubes with corrosion damages. Correlation of results obtained by these two methods makes possible to obtain a better knowledge of current tubes material state as well as a more accurate assessment of their behavior during future exploitation. Therefore, an integrated approach of remaining life assessment and reliability evaluation, on the first stage reheater tubes system example, is presented in this paper. Considerations of this kind are very important for the risk-based maintenance programs
Hydrogen embrittlement of low carbon structural steel
Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20- St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials
Hydrogen embrittlement of low carbon structural steel
Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20- St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials
Life estimation of first stage high pressure gas turbine blades
Posle pojave prevremenih lomova koji su se javili kod lopatica prvog stepena turbine visokog pritiska odreÄenog tipa turboreaktora u jednoj lokalnoj avio kompaniji, usledila su istraživanja u cilju odreÄivanja njihovog sigurnog (pouzdanog) radnog veka. Lopatice prvog stepena turbine visokog pritiska su tokom rada izložene simultanom delovanju pritiska gasa iz komore za sagorevanje, centrifugalne sile u sluÄaju rotorskih lopatica, velikim temperaturnim promenama, kao i agresivnoj radnoj sredini. Kombinacija svih navedenih Äinilaca izaziva vrlo kompleksno naponsko stanje lopatica kao i moguÄnost pojave viÅ”estrukih mehanizama oÅ”teÄivanja: zamora izazvanog fluktuacijama u mehaniÄkom naponu, termo-mehaniÄkog zamora usled temperaturnih promena i korozije naponski optereÄenih delova. Da bi se odredio radni vek u navedenim uslovima, neophodno je proceniti napone kojima su lopatice izložene uzimajuÄi u obzir nekoliko promenljivih koje se u radu tretiraju deterministiÄki. Razmatran je prenos toplote izmeÄu sagorelih gasova i metala lopatica turbine. IzraÄunat je ukupni napon na dve vrste lopatica imajuÄi u vidu termiÄke efekte i mehaniÄko optereÄenje. Naponski ciklus je zatim izraÄunat za razliÄite faze rada turboreaktora uz varijacije termiÄkih i mehaniÄkih osobina. Procena sigurnog radnog veka je izvrÅ”ena primenom dva pristupa: inicijalnog modela i pristupa sa tolerancijom oÅ”teÄenja uzimajuÄi u obzir mehaniÄki rast oÅ”teÄenja i rast korozionog pita. ProraÄun je primenjen i na statorske i na rotorske lopatice turbine visokog pritiska turboreaktora koje su izraÄene od Äelika NI 738. NaÄeno je da su ovo komponente visokog rizika, tako da je procenjen i rizik sa aspekta potencijalnih posledica od loma. Dobijeni rezultati su razmatrani u cilju reÅ”avanja problema i donoÅ”enja sigurne odluke sa aspekta procedure održavanja ali i konstrukcijskog reÅ”enja.Based on very early occurring ruptures found in the first stage high pressure turbine blades of a turbo reactor in a local aviation company, this study has the aim to determine their safe life. The first stage blades are subjected to simultaneous action of gas pressure coming from the combustion chamber, centrifugal forces in the case of the rotor blades and to important temperatures transients, which progress in a very aggressive environment due to hot gases. These combined parameters cause a high state of stress involving several complex mechanisms of damage, such as: fatigue caused by mechanical stress fluctuations, thermo-mechanical fatigue caused by temperature variations and corrosion caused on the stressed elements. Life cycle determination asks for stress evaluation of blades regarding several variables which are approached deterministically in the study. Heat exchange between combustion gases and metal blades is considered. The total stress on two kinds of blades is calculated by the addition of the thermal effect and the mechanical loading. The stress cycle is then calculated for different steps of the engine function during the operation by considering the variation of the thermal and the mechanical properties of the system. Safe life determination is done by two different approaches: the safe life approach by the initiation model and the damage tolerance approach considering the defect growth mechanics and considering the pitting corrosion effect. The calculation is applied for stator and rotor blades of an aero engine high pressure turbine made of NI 738. Since these parts are high risk components from the point of view of potential failure consequences, the risk is assessed as well. The results obtained are studied to determine the solution to the problem, and to propose a safe decision to be taken about the design or maintenance procedures.
Life estimation of first stage high pressure gas turbine blades
Posle pojave prevremenih lomova koji su se javili kod lopatica prvog stepena turbine visokog pritiska odreÄenog tipa turboreaktora u jednoj lokalnoj avio kompaniji, usledila su istraživanja u cilju odreÄivanja njihovog sigurnog (pouzdanog) radnog veka. Lopatice prvog stepena turbine visokog pritiska su tokom rada izložene simultanom delovanju pritiska gasa iz komore za sagorevanje, centrifugalne sile u sluÄaju rotorskih lopatica, velikim temperaturnim promenama, kao i agresivnoj radnoj sredini. Kombinacija svih navedenih Äinilaca izaziva vrlo kompleksno naponsko stanje lopatica kao i moguÄnost pojave viÅ”estrukih mehanizama oÅ”teÄivanja: zamora izazvanog fluktuacijama u mehaniÄkom naponu, termo-mehaniÄkog zamora usled temperaturnih promena i korozije naponski optereÄenih delova. Da bi se odredio radni vek u navedenim uslovima, neophodno je proceniti napone kojima su lopatice izložene uzimajuÄi u obzir nekoliko promenljivih koje se u radu tretiraju deterministiÄki. Razmatran je prenos toplote izmeÄu sagorelih gasova i metala lopatica turbine. IzraÄunat je ukupni napon na dve vrste lopatica imajuÄi u vidu termiÄke efekte i mehaniÄko optereÄenje. Naponski ciklus je zatim izraÄunat za razliÄite faze rada turboreaktora uz varijacije termiÄkih i mehaniÄkih osobina. Procena sigurnog radnog veka je izvrÅ”ena primenom dva pristupa: inicijalnog modela i pristupa sa tolerancijom oÅ”teÄenja uzimajuÄi u obzir mehaniÄki rast oÅ”teÄenja i rast korozionog pita. ProraÄun je primenjen i na statorske i na rotorske lopatice turbine visokog pritiska turboreaktora koje su izraÄene od Äelika NI 738. NaÄeno je da su ovo komponente visokog rizika, tako da je procenjen i rizik sa aspekta potencijalnih posledica od loma. Dobijeni rezultati su razmatrani u cilju reÅ”avanja problema i donoÅ”enja sigurne odluke sa aspekta procedure održavanja ali i konstrukcijskog reÅ”enja.Based on very early occurring ruptures found in the first stage high pressure turbine blades of a turbo reactor in a local aviation company, this study has the aim to determine their safe life. The first stage blades are subjected to simultaneous action of gas pressure coming from the combustion chamber, centrifugal forces in the case of the rotor blades and to important temperatures transients, which progress in a very aggressive environment due to hot gases. These combined parameters cause a high state of stress involving several complex mechanisms of damage, such as: fatigue caused by mechanical stress fluctuations, thermo-mechanical fatigue caused by temperature variations and corrosion caused on the stressed elements. Life cycle determination asks for stress evaluation of blades regarding several variables which are approached deterministically in the study. Heat exchange between combustion gases and metal blades is considered. The total stress on two kinds of blades is calculated by the addition of the thermal effect and the mechanical loading. The stress cycle is then calculated for different steps of the engine function during the operation by considering the variation of the thermal and the mechanical properties of the system. Safe life determination is done by two different approaches: the safe life approach by the initiation model and the damage tolerance approach considering the defect growth mechanics and considering the pitting corrosion effect. The calculation is applied for stator and rotor blades of an aero engine high pressure turbine made of NI 738. Since these parts are high risk components from the point of view of potential failure consequences, the risk is assessed as well. The results obtained are studied to determine the solution to the problem, and to propose a safe decision to be taken about the design or maintenance procedures.
Prediction and prevention of boiler tubing systems erosion in thermal plant
The problem of boiler tube erosion is a very complex problem and present in practically all fossil fuel power plants burning low heating value coals irrespective of their particular design and operating characteristics. Predicting the rate of material degradation processes due to operating conditions is the essential feature for assessing the remaining lifetime. This clearly includes the knowledge of the type and kinetics of erosion phenomenon. In order to follow the erosion process and provide a reliable prediction of erosion rate and thus tube time to failure, models taking into account all relevant effects have to be developed. In this paper the is presented results of testing carried out on the evaporator hopper tubes of 350MW unit exposed to erosion and were used to assess their remaining life
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