Procena preostale čvrstoće metodom konačnih elemenata korozijom oštećenih čeličnih cevovoda u Hemijskoj industriji

Ovaj rad koristi napredne tehnike modeliranja korodiranih površina cevovoda, bazirane na Metodi konačnih elemenata (MKE), sa ciljem razvoja procedure za procenu preostale čvrstoće čeličnih cevovoda koji rade u uslovima sredine industrije hemijskih proizvoda. U radu je dat i prikaz mogućih oštećenja i posledica izazvanih korozijom čeličnih cevovoda u industriji hemijskih proizvoda, a razmotrena je i mogućnost preduzimanja mera da se takve pojave preventivno spreče

Low cycle fatigue of high strength low alloy steels

Imajući u vidu heterogenost svojstava zavarenog spoja, a naročito uske zone pod uticajem toplote, u ovoj disertaciji je eksperimentalno ispitivano i analizirano ponašanje niskolegiranog čelika povišene čvrstoće (HSLA) u uslovima niskocikličnog zamora, na glatkim okruglim epruvetama iz osnovnog metala i epruvetama dobijenim termičkom simulacijom zone pod uticajem toplote. Takođe je ispitivano ponašanje osnovnog metala i simulirane zone pod uticajem toplote u uslovima dejstva kavitacije. Čelik, Nionikral 70 (NN-70), odabran u ovoj disertaciji za istraživanje ponašanja pri zamornom opterećenju i dejstvu kavitacije, spada u grupu niskolegiranih čelika povišene čvrstoće koji se, između ostalog, primenjuju u brodogradnji kao i za izradu posuda pod pritiskom. Ciljevi istraživanja su da se, u odsustvu pouzdanijih teorija o zamaranju i kavitaciji, pogodnom metodom ispitivanja, tj. simuliranjem strukture materijala i dejstva opterećenja odnosno deformacija, u laboratorijskim uslovima na odgovarajućim uzorcima - epruvetama, dođe do rezultata koji su posredno ili neposredno primenljivi u proračunima i da se dobijeni rezultati eventualno iskoriste i za fundamentalna proučavanja tokova procesa zamaranja i kavitacije kako osnovnog metala, tako i zone uticaja toplote zavarenih spojeva.In this dissertation the experimentally investigated and analyzed behavior of a low-alloyed high strength steel exposed to the low cycle fatigue is done. Taking into account the welded joint properties heterogeneity, especially its narrow heat-affected zone, experiments were performed on two kinds of specimens: smooth round specimens made of parent material (PM) and specimens obtained by thermal simulation of the heat-affected zone (SHAZ). The behavior of PM and SHAZ affected by cavitation were also investigated. Steel designated as Nionicral 70 (NN-70), selected in this dissertation to investigate the behavior of fatigue and cavitations’ loads, belongs to a group of lowalloy steels of high strength, which are, among other things, applied in shipbuilding as well as for the production of pressure vessels. The research aims are that, in the absence of more reliable theories of fatigue and cavitation, a suitable test method, i.e. simulating the structure of the material and the load and/or deformation in laboratory conditions on the appropriate specimens, results are obtained which are directly or indirectly applicable in the calculations, and that the obtained results are possibly used for the fundamental study of the fatigue and cavitation processes of the parent material, as well as its heat-affected zone in welded joints

Presence of crack due the constant exploitation of turbine shaft

Designing, constructing and putting into operation a hydroelectric power plant includes complex tasks. Stresses of turbine and hydromechanical equipment of hydro power plants arise during the production of parts and assembly of equipment (residual stresses), in the process of performing functional requirements in exploitation and in the disturbed process of exploitation. After several decades of constant exploitation of the horizontal bulb turbine, which is an integral part of the hydroelectric power unit, empirically, the formation of a crack occurs in the turbine shaft due to the influence of corrosion, erosion and cavitation. Through experimental tests and calculations it has been determined that values of bending stresses of the turbine, which occur due to the action of fatigue and corrosion, as well as stress concentration, are bigger than 25 MPa for flanges exposed to water, and in other case bigger than 40 MPa for flanges exposed to `corrosive water` and can cause the occurrence of surface cracks on the transition radius between the cylindrical part of the shaft and the flange. It has been determined that stress values in the zone under the influence of bending stresses were bigger than allowable values, which led to the occurrence of many cracks due to fatigue corrosion. One of those cracks caused the failure of the shaft and of the whole turbine

Methodology for determining the region of stabilization of low-cycle fatigue

In present paper, the behaviour of high-strength low-alloyed (HSLA) steel under conditions of low-cycle fatigue (LCF) has been experimentally tested and analyzed. Based on the experimental results obtained in the programme EXCEL, characteristic regions of low-cycle fatigue of steel NIONIKRAL 70 (NN-70) have been determined, the most important being the region of stable behaviour of materials, so-called "the region of stabilization". From this region, on the basis of pre-defined requirements, characteristic stabilized hysteresis have been isolated for each strain level, based on which the Indicators of low-cycle fatigue of steel NN-70 have been identified

Metodologija određivanja pokazatelja ponašanja HSLA čelika pri delovanju niskocikličnog zamora

U radu je analizirano eksperimentalno istraživanje ponašanja niskolegiranog čelika povišene čvrstoće (HSLA) u uslovima niskocikličnog zamora (LCF), sa kontrolisanom i potpuno reverzibilnom deformacijom (/2 = const, R = min/max = -1). Ispitivanja niskocikličnog zamora obavljena su na seriji glatkih epruveta od čelika Nionikral 70 (NN-70), sa poluamplitudom kontrolisane deformacije, /2=0.35, 0.45, 0.50, 0.60, 0.70 i 0.80. Za izbor stabilizovanih histereza i obradu rezultata ispitivanja niskocikličnim zamorom, korišćene su preporuke standarda ISO 12106:2003 (E) i ASTM E 606-04, kao i dve nove metode: Metoda sredine stabilizacije (MSS) i Metoda praga NDT (MpNDT), što u ovom radu predstavlja četiri rezultata za poređenje. Za svaki nivo deformacije izabrane su karakteristične stabilizovane histereze, određene prema preporuci standarda ISO 12106:2003 (E) i ASTM E 606-04, MSS i MpNDT, sa kojih su očitani potrebni podaci za dalju obradu rezultata i konačno, konstruisanje karakterističnih krivih niskocikličnog zamora. Konstruisane krive su međusobno poređene, a ocenjen je uticaj izbora karakterističnih stabilizovanih histereza na karakterizaciju ponašanja čelika NN-70. Rezultati eksperimentalnih ispitivanja su nam dali važne podatke o razumevanju zamornog ponašanja čelika NN-70. U radu je izvršeno i poređenje rezultata statičkog i dinamičkog, odnosno monotonog i zamornog ponašanja materijala, što predstavlja praktični doprinos oceni ponašanja čelika NN-70 u uslovima delovanja niskocikličnog zamora

Analiza opravdanosti modeliranja sučeono zavarenih spojeva kod proračuna posuda pod pritiskom

U radu je posle proračuna metodom konačnih elemenata (MKE) analizirana raspodela napona na modelu dela oplate rezervoara pod pritiskom bez zavarenih spojeva (ZS), sa obimnim ZS, podužnim ZS i ukrsnim mestom ZS. Takođe, u radu je na primeru rezervoara za tečni ugljendioksid, na kome su pored limova oplate i danaca sa potrebnim otvorima i elementima kao što su sedla sa osloncima i uške modelirani obimni i podužni sučeoni ZS, urađen proračun MKE i uporedna analiza uticaja ZS na raspodelu napona

Finite element method pressure vessel calculation and analysis of the effects of the butt welded joints on a carrying capacity of a structure

In present paper, after calculation using the finite element method (FEM), stress distribution on the model of a part of the pressurized boiler shell without welded joints (WJ), with transversal WJ, longitudinal WJ and crosspoint location of WJ Is analyzed. Also, FEM calculation and comparative analysis of the effects of WJ on the stress distribution have been conducted using the tank for liquid carbon dioxide as an example, on which. In addition to the shell sheet-metal and heads with necessary openings and elements such as saddles with supports and hangers, transversa! and longitudinal butt WJ, have been modelled