Seminarske naloge

One of the biggest issues in mechanical engineering is to understand why there is so large scatter in measured fatigue life of seemingly same specimens, what contributes to fatigue life (loading, material), how to model it, open issues.

Dr. Samir El Shawish: Non-destructive techniques for damage characterization of structural components in NPPs

Introduce the ultrasound (and other) technique using physical principles, show the limitations (what is the smallest detectable crack, how does detection of cracks depend on the distance from the sensor), few examples with simple crack geometry and size.

Dr. Samir El Shawish: Ageing mechanisms in structural components of NPPs

Describe irradiation embrittlement, hardening and swelling, creep, thermal fatigue, SCC, IASCC, introduce basic principles behind each phenomenon and what are the engineering measures for its control, how we consider these mechanisms in simulations.

Dr. Samir El Shawish: Crystal plasticity theory

Describe basic principles behind crystal plasticity, define slip systems for FCC, BCC and HCP lattices, Schmid’s rule, criteria for slip through slip flow equation and hardening equation (without size effects), kinematic relations in small and finite strain approximation, discuss the validity (applicability) of the theory in various metals.

Dr. Samir El Shawish: Strain-gradient crystal plasticity theory

In comparison to conventional crystal plasticity, describe basic principles behind strain-gradient crystal plasticity, difficulties in FE implementation. Size effects.

Dr. Samir El Shawish: FFT-based method in computational crystal plasticity

The method should be introduced with corresponding equations and approximations, demonstrate applicability on a simple example, brief comparison to FEM

prof. dr. Iztok Tiselj: Kulinarična dinamika tekočin

V diplomski seminarski nalogi bo obravnavana vloga dinamike tekočin v kulinariki, pri čemer bo poudarek na fizikalnih pojavih, ki se pojavljajo pri pripravi hrane. Raziskali bomo osnovne pojme fluidne mehanike, kot so viskoznost, turbulentni in laminarni tok, ter njihovo aplikacijo v vsakdanji kulinarični praksi. Preučevali bomo, kako tekočine tečejo pri kuhanju, cvrtju, mešanju in emulgiranju ter kakšen vpliv imajo na teksturo in okus hrane. Obravnavane bodo tudi specifične kuharske tehnike, kot so penjenje, priprava omak in stabilizacija emulzij, s poudarkom na znanstveni razlagi procesov. S seminarsko nalogo želimo poudariti, kako znanje dinamike tekočin pripomore k boljšemu razumevanju in optimizaciji kuharskih tehnik ter kulinaričnih inovacij.

prof. dr. Iztok Tiselj: Metode za zajemanje medfazne površine v dvofaznih tokovih

Dvofazni tokovi so prisotni v številnih industrijskih procesih, kot so kemijska, jedrska, prehrambena in naftna industrija. Ključni izziv pri analizi dvofaznih tokov je natančno zajemanje in karakterizacija medfazne površine, saj ta vpliva na prenos toplote, maso in gibalne količine. V okviru seminarske naloge bi študent raziskal različne metode, ki se uporabljajo za zajemanje medfazne površine v dvofaznih tokovih, ter oceniti njihove prednosti in omejitve.

prof. dr. Iztok Tiselj: Metode POD, SPOD, DMD za diseminacijo koherentnih struktur v turbulentnih tokovih

V turbulentnih tokovih se pojavljajo kompleksne in dinamične koherentne strukture, ki igrajo ključno vlogo pri prenosu energije, mase in toplote. Razumevanje teh struktur je bistveno za izboljšanje napovedi in nadzora nad turbulentnimi procesi v različnih inženirskih in znanstvenih aplikacijah. Seminar se bo osredotočil na tri napredne metode za analizo in diseminacijo koherentnih struktur v turbulentnih tokovih: Proper Orthogonal Decomposition, POD), Spectral Proper Orthogonal Decomposition (SPOD) in Dynamic Mode Decomposition (DMD).

Magistrske naloge

Dr. Mitja Uršič: Stability of the Auxiliary Feedwater System Regulation in Pressurized Water Reactor

The Auxiliary Feedwater (AFW) is a key safety system in pressurized water reactors (PWR), designed to prevent reactor overheating and potential core meltdown in the accidental event. AFW provides the feedwater to steam generators to provide a cooling source for the reactor system. In a two loop PWR, such as the Krško Nuclear Power Plant, the AFW system simultaneously supplies feedwater to both steam generators, with regulation managed through two independent valves on each steam generator line. The candidate will explore various mathematical solutions to model the behavior of this (weakly) coupled system, taking into account single-phase and two-phase feedwater flows, which could arise due to cavitation within the piping system.

Dr. Mitja Uršič: 3D modeliranje hlajenja razbitkov v eksperimentalni napravi FLOAT

V primeru težkih nesreč lahko pride do porušitve geometrije sredice jedrskega reaktorja. Za dolgoročno stabilizacijo razmer je v takem primeru pomembno, da je na novo nastala geometrija sredice hladljiva. Na tem področju se raziskave izvajajo računsko in eksperimentalno. Namen magistrske naloge je opraviti računalniške 3D simulacije eksperimentov FLOAT (Univerza v Stuttgartu, Nemčija) s programom MC3D (IRSN, Francija). Cilji magistrske naloge bo analizirati med eksperimentom opaženo 3D naravo popolavljanja testnega območja.

Dr. Mitja Uršič: Razvoj nadomestnega modela hlajenja razbitkov taline s pomočjo umetne inteligence

V primeru težkih nesreč lahko pride do porušitve geometrije sredice jedrskega reaktorja. Za dolgoročno stabilizacijo razmer je v takem primeru pomembno, da je na novo nastala geometrija sredice hladljiva. Na tem področju se raziskave izvajajo računsko in eksperimentalno. Namen magistrske naloge je razviti nadomestni model (ang. surrogate model) za napovedovanja ohlajanja razbitkov sredice. Cilj magistrske naloge je z nevronskimi mrežami učiti nadomestni model in sicer na podlagi serije 2D simulacij pogojev v eksperimentalni napravi FLOAT (Univerza v Stuttgartu, Nemčija) s programom MC3D (IRSN, Francija).

Dr. Mitja Uršič: Sklopljeno modeliranje hlajenja razbitkov taline v rekatorskih razmerah

V primeru težkih nesreč lahko pride do porušitve geometrije sredice jedrskega reakotrja. Nastala struktura je porozna. Namen naloge je omogočiti eksperimentalno opazovanje vzorcev toka hladila med poplavljanjem. Cilj magistrske naloge je vzpostavitev eksperimentalne postavitve za spremljanje poplavljanja v porozni snovi skupaj z izvedbo prvih meritev.

Dr. Mitja Uršič: Eksperiment poplavljanja porozne snovi

Območje plastnega uparjanja je zelo zanimivo za jedrsko industrijo. Med težkimi nesrečami namreč plastno uparjanje zavira hlajenje taline in povečuje možnost za nastanek parnih eksplozij. Namen magistrske naloge je prispevati k postavitvi eksperimenta, ki bo omogočil poskuse s prisilno konvekcijo pri nadzorovani temperaturi vroče površine in hladne kapljevine. Cilj naloge bo definiran glede na stanje postavitve v času opravljanja magistrske naloge (npr. metodologija za analizo morfologije plasti med kapljevino in paro).

Dr. Mitja Uršič: Plastno uparjanje med težko nesrečo

Dr. Mitja Uršič: Modeliranje razpada taline v razslojenih razmerah

V primeru težkih nesreč v jedrskih elektrarnah lahko pride do talitve sredice reaktorja. Ob interkaciji taline in hladila bi lahko prišlo do parne eksplozije, ki ogroža celovitost okoliških struktur. Pretekle raziskave so bile posvečene parnim eksplozijam bodisi ob prodiranju taline v hladilo bodisi ob razpadu taline v razslojenih razmerah. V realnosti je mogoče pričakovati preplet obeh konfiguracij. Namen magistrske naloge je simuirati parno eksplozijo ob prepletu obeh konfiguracijah. Cilj je določiti jakost parne eksplozije v votlini reaktorske posode.

Dr. Mitja Uršič: Jakost parne eksplozije v reaktorskih razmerah

a semi-mechanistic boiling model embedded within the multiphase mixture framework will be used to simulate heat transfer on the heated wall during the subcooled boiling. The model is based on the heat transfer augmentation due to boiling effects, where the heat transfer is expressed as a weighted sum of nucleate boiling and forced convective heat fluxes. Its formulation is very useful for simulations of realistic nuclear systems, but requires thorough validation. The candidate wil use and test the semi-mechanistic model applied in ANSYS Fluent code and verify its applicability at different operating conditions.

Safe long-term operation of nuclear power plants depends strongly on the reliability and integrity of safety relevant components. Understanding the ageing processes of metals that may limit the long-term safe operation is therefore of immense importance. The InterGranular Stress-Corrosion Cracking (IGSCC) is one of the most significant ageing degradation mechanisms in structural metallic alloys which corresponds to the initiation and propagation of cracks along the grain boundaries. To understand and accurately predict the IGSCC initiation, the knowledge on grain boundary stresses is required. The candidate will perform a series of finite element simulations with commercial code Abaqus to obtain the probability density distributions of intergranular normal stresses present in a polycrystalline metal when exposed to different external loading conditions. Using the statistical analysis of the results, the candidate will try to identify the most influencing parameters that contribute to the largest grain boundary stresses relevant for IGSCC initiation.

In irradiated steels, dislocation channel (also clear channel) refers to the localization of plastic deformation within slip bands that appear to be free of radiation defects. A dislocation channel is a microstructural object, only few tens of nm wide and with a length comparable with the grain size, which is created when loading an irradiated material. Its complex interaction with defects on the nanoscale affects the behaviour of the metal at the macroscopic scale (loss of ductility, reduced uniform elongation). Because of their interaction with grain boundaries, dislocation channels are thought to have a crucial role in the initiation of intergranular cracks (Irradiation-Assisted Stress Corrosion Cracking - IASCC). The candidate will perform finite element simulations with commercial code Abaqus to analyse the effects of dislocation channels on grain boundary stresses. Dislocation channels will be imposed topologically to the aggregate model and appropriate crystal plasticity laws will be assigned to different model regions in order to reproduce the response of an aggregate on a macroscopic scale. The influence of the dislocation channel width and spacing on intergranular stresses will be investigated and a possible correlation with the initiation of IASCC will be examined.

In irradiated steels, dislocation channel (also clear channel) refers to the localization of plastic deformation within slip bands that appear to be free of radiation defects. A dislocation channel is a microstructural object, only few tens of nm wide and with a length comparable with the grain size, which is created when loading an irradiated material. Its complex interaction with defects on the nanoscale affects the behaviour of the metal at the macroscopic scale (loss of ductility, reduced uniform elongation). Because of their interaction with grain boundaries, dislocation channels are thought to have a crucial role in the initiation of intergranular cracks (Irradiation-Assisted Stress Corrosion Cracking - IASCC). The candidate will model slip transmission on grain boundaries by modifying the hardening behavior as a promising mesoscopic way to account for the observed dependence of cracking on slip discontinuity without having to describe explicitly the dislocation channels.

The reactor pressure vessel (RPV) is an indispensable component in a nuclear power plant which structural integrity must be assured under all possible events. A limiting event for the long-term operation (LTO) of the RPV is a pressurized thermal shock (PTS). A PTS event typically follows a loss-of-coolant accident (LOCA), where the subsequent injection of cold water into the hot RPV may induce high thermal stresses in the RPV wall. Under these circumstances, it is important to ensure that existing crack-like flaws in the RPV will not propagate under brittle fracture. This is even more crucial as nuclear power plants enter LTO beyond the designed operational lifetime. A PTS analysis involves thermal-hydraulic (TH) simulations of a selected transient event, followed by a thermo-mechanical (TM) analysis of the RPV to evaluate the temperature and stress distributions through the vessel's wall. These are employed in subsequent fracture-mechanics (FM) analyses to obtain stress-intensity factors (SIFs) of postulated cracks, which are finally compared with the fracture toughness of the RPV material to determine the likelihood of brittle fracture. For given TH data from a selected transient, extended finite element method (XFEM) with the submodeling technique in ABAQUS computer code will be used to predict initiation and arrest of cracks in the RPV under PTS transient loads. The results will be compared with semi-analytical tools for crack growth predictions.

The turbulent mixing of fluids at different temperatures in T-junctions may be the cause of fatigue crack growth due to the varying thermal stresses in the pipes caused by the temperature fluctuations. The analysis of such long-transient and interdisciplinary phenomenon requires rather complex and computationally expensive simulations. Using the results of a computational fluid dynamics simulation of a fluid mixing case already available, the extended finite element method (XFEM) in ABAQUS computer code will be used to analyze the growth of semi-elliptical inner-surface cracks in a pipe surrounding the mixing fluids. The two-dimensional and time-dependent growth will be predicted with remodeling/remeshing technique implemented by the applicant. The results will be compared with semi-analytical tools for crack growth predictions.

The applicant will review the different damage models and techniques available in the Abaqus code. Then, the applicant will design and execute different case studies to evaluate their performance.

The safe and long-term operation of engineering structures largely relies on the ability to predict their fatigue life under loads that are complex in nature. Examples of such complex loads are seismic, the loads acting on vehicle parts as well as a great variety of fluid turbulence-related loads acting on aircraft structures and on safety-related components of power plants. Typically known as spectrum loads, these complex loads are characterized by variable amplitude and multi-frequency content. The fatigue analyses are then needed to predict the lifetime using the loads and experimental fatigue resistance data. An important contributing factor to predicted lifetimes are the number and amplitude of the cycles estimated from the stress history that drives the fatigue damage. The applicant will review, develop their own computer implementation and verify the different cycle-counting methods in the literature. The methods’ influence on predicted fatigue lives will be evaluated using different complex stress histories.

InterGranular Stress Corrosion Cracking (IGSCC) represents one of the most critical ageing mechanisms affecting austenitic stainless steels in Nuclear Power Plants. As a structural material, austenitic stainless steel is extensively utilized in NPPs due to its high corrosion resistance and mechanical strength. However, under certain environmental conditions, particularly those involving high temperatures and corrosive media, these steels become susceptible to IGSCC. The IGSCC corresponds to the initiation and propagation of microcracks along the Grain Boundaries. A comprehensive understanding of IGSCC processes and their smart integration over different scales is crucial for accurately predicting the onset and progression of IGSCC in real-world components. In this work, the candidate will generate a mesoscopic finite element model that accounts for the interplay between microstructure, mechanical behaviour, and various associated mechanisms (such as oxidation), at the scale of several grains in a polycrystalline aggregate. The goal is to create a transient crystal plasticity finite element model that would faithfully simulate IGSCC-relevant processes in the material in front of the crack tip where the mechanical stresses and concentrations of oxygen and hydrogen (or chlorides, sulfates) is largest and thus the local microstructure is most important.

InterGranular Stress Corrosion Cracking (IGSCC) represents one of the most critical ageing mechanisms affecting austenitic stainless steels in Nuclear Power Plants. As a structural material, austenitic stainless steel is extensively utilized in NPPs due to its high corrosion resistance and mechanical strength. However, under certain environmental conditions, particularly those involving high temperatures and corrosive media, these steels become susceptible to IGSCC. The IGSCC corresponds to the initiation and propagation of microcracks along the Grain Boundaries (GBs). A comprehensive understanding of IGSCC processes and their smart integration over different scales is crucial for accurately predicting the onset and progression of IGSCC in real-world components. In this work, the candidate will generate a probabilistic mesoscopic model where appropriate (time-independent) GB normal stress distributions and time-dependent GB strength distributions will be considered due to uneven GB oxidation or defect precipitation kinetics observed in IGSCC. The goal is to estimate a fraction and direction in space of overloaded GBs, and the corresponding time needed to achieve this stage for a given external loading conditions.

It is important to ensure that existing crack-like flaws in components will not propagate under loads that present in in-service conditions. Fracture mechanics is a powerful tool that allows performing structural integrity evaluations of cracked components under mechanical and thermal loads. While analytical formulations of stress-intensity factors (SIF) exist for simple geometries, the finite element method (FEM) and extended FEM (XFEM) are needed to evaluate SIF for complex geometries and loadings. The development of complex meshes needed in FEM can be avoided with XFEM. However, due to the rather recent implementation of XFEM in computer codes, it is necessary to gain experience in its use and confidence in its capability to assess the structural integrity of components. The goal of the proposed work is to verify the XFEM solutions against those provided with common FEM meshes as well as with analytical formulations.

The goal of this work is to verify the performance of the FENICS finite element (FE) code against the ABAQUS code for the same type of computational problems in solid mechanics. The problems will include typical linear elastic and elasto-plastic material constitutive models, mesh convergence of different element types and parallelization performance in computer clusters.

V tej nalogi bi študent izvajal LES (Large Eddy Simulation) simulacije toka skozi kvadraten kanal pri različnih resolucijah in gostotah mrež. Poudarek bi bil na analizi sekundarnih tokov, kjer bi se študent osredotočil na raziskovanje vzrokov za njihov nastanek in na vprašanje, ali je velikost teh tokov odvisna od mreže in LES modela, ki se uporablja. Tema vključuje tako numerične simulacije kot analizo rezultatov glede na različne mrežne konfiguracije. Cilj naloge je razumeti dinamiko sekundarnih tokov in izboljšati natančnost LES simulacij.

Tema se osredotoča na raziskovanje toplotne konvekcije v sloju tekočine, ki je ogrevan od spodaj in ohlajen od zgoraj. Študent bi s pomočjo numeričnih simulacij preučeval začetke naravne konvekcije ter analiziral razvoj konvekcijskih celic. Poleg tega bi raziskoval prehod med laminarno in turbulentno konvekcijo ter vpliv parametrov, kot sta temperaturni gradient in geometrija plasti.

Tema se osredotoča na proučevanje toka tekočine skozi porozne medije, kot so peščena tla ali porozni filtri. Študent bi s pomočjo numeričnih metod in eksperimentalnih podatkov preučeval vpliv velikosti in oblike por na pretok ter analiziral, kako Darcyjev zakon opisuje tok tekočine skozi kompleksne strukture. Tok skozi porozne medije ima pomembne aplikacije v hidrologiji, filtracijskih sistemih ter naftni industriji, kjer je razumevanje toka skozi porozne materiale ključno.

V magistrski nalog bo študent razvijal model za simulacijo združevanja ali razpada mehurjev plinav kapljevini z uporabo enofluidne formulacije metode volumnov tekočine (VOF) v 3D računalniških simulacijah. Cilj je izboljšati natančnost in zanesljivost modeliranja mehurjev v dvofaznih tokovih, kar je ključno za optimizacijo procesov v različnih industrijah. Študent bo analiziral obstoječe metode za zajemanje in sledenje medfaznih površin, identificiral njihove omejitve ter razvijal nove pristopke za učinkovitejše združevanje ali razpad mehurjev. Rezultate simulacij bo primerjal z eksperimentalnimi podatki, ki so nastali v laboratoriju THELMA, R4 - primer Taylorjevega mehurja.

3D Lagrangian particle tracking (3D-LPT) is the most advanced state-of-the-art method for non-intrusive measurement of the velocity in fluids. The method combines recordings from multiple high-speed cameras and applies shake-the-box algorithm for tracking of individual seeding particles in fluid flow. The candidate will utilize the 3D-LPT method to measure the velocity field in turbulent pipe flow.

Pri simulacijah stisljivih tokov v katerih hitrost tekočine dosega in presega hitrost zvoka in se v toku pojavljajo udarni valovi, se v sistemu Navier-Stokesovih enačb (kontinuitetna, gibalna in energijska enačba) običajno zanemari viskozni člen v gibalni enačbi in člen toplotne prevodnosti v energijski enačbi. Rezultat takšne aproksimacije so tri parcialne diferencialne enačbe prvega reda - Eulerjeve enačbe. Eulerjeve enačbe so hiperboličnega tipa, kar se v veliki meri izkorišča pri konstrukciji numeričnih shem za njihovo reševanje. Študentka naj sestavi svoj računalniški program, ki temelji na "high-resolution shock-capturing" numeričnih shemah in z njim simulira prehod udarnega vala skozi zožitev dvodimenzionalne cevi. 

The candidate will perform boiling flow simulations of the experiments in an annular test section, located in THELMA laboratory of Reactor Engineering Department JSI. Two-fluid approach within OpenFOAM or ANSYS Fluent code will be used for simulations. The task will require setting up the geometry of the test section, mesh generation, input model development, performing simulations on the computer cluster and analysis of the results.

A semi-mechanistic boiling model embedded within the multiphase mixture framework will be used to simulate heat transfer on the heated wall during the subcooled boiling. The model is based on the heat transfer augmentation due to boiling effects, where the heat transfer is expressed as a weighted sum of nucleate boiling and forced convective heat fluxes. Its formulation is very useful for simulations of realistic nuclear systems, but requires thorough validation. The candidate wil use and test the semi-mechanistic model applied in ANSYS Fluent code and verify its applicability at different operating conditions.

Heat pipe is a highly energy-efficient heat transfer device based on the processes of evaporation and condensation. Due to their passive operation and efficient heat transfer, heat pipes have recently gained increasing interest in the technology of small modular reactors and fusion divertor cooling applications. In this study, the candidate will use Computational Fluid Dynamics (CFD) simulations to design the classroom heat pipe experiment. Theoretical calculations and CFD analyses will be used to optimise the heat pipe design in terms of appropriate geometrical characteristics and operating conditions.

The candidate will perform CFD simulations of individual bubbles in boiling flow using interface tracking methods available in OpenFOAM. The focus will be either on the dynamics of bubble formation at a nucleation site on the heated wall, or to investigate coalescence or breakup of bubbles in the flow.

Infrared (IR) camera measurements will be used to determine the average temperature of the heated foil in contact with the boiling flow in a squared channel. After performing IR camera calibration, the candidate will conduct measurements of the foil temperature at different opearting conditions of the test section .

Two-phase flow patterns of the subcooled boiling flow in the top-heated squared channel will be analysed using the High-Speed (HS) camera recordings. Two HS cameras, located at the bottom and at the side of the horizontal transparent channel will be used to determine the bubble size distribution and integral void fraction during flow boiling. Energy flow balance will be obtained using the calorimetry measurements at the inlet and outlet of the test section.

Boiling flow is investigated in an annular test section, where the inner copper tube is heated, while the outer tube is transparent and allows visual observation of the boling flow of primary fluid (refrigerant R245fa that boils at room temerature). The inner tube is heated by a secondary fluid (water), therefore the supplied heat flux can be controlled by the temperature or the mas flux of the heating water. The candidate will explore the boiling flow regimes in the vertical layout of the test section. The effect of heat flux on he two-phase flow pattern during boiling flow will be investigated. The heat flux profile along the heated wall tube will be determined from thermocouple measurements inside the copper pipe. Using a high-speed visualization and image processing techniques, the candidate will determine the bubble size distribution, shape and the integral distribution of the vapor volume fraction along the tube and evaluate the experimental uncertainties.

Flow boiling is an extremely effective cooling mechanism that is used in many power generation systems, fusion reactors among others. The candidate will investigate flow boiling in a square channel with one-sided heat load. Using a high-speed infrared camera with micro-optics, the candidate will find the nucleation sites on a thin metal foil and measure the local temperatures at these spots. Later, the nucleation site densities at different heat loads and coolant flow rates will be determined.

Toplotno neizolirane slepe cevi (cevi z zaprtim koncem), termohidravlično povezane s primarnim tokom vroče tekočine, izkazujejo kompleksno dinamiko toka, ki je posledica medsebojnega vpliva vdirajočega vrtinčnega toka na odprtem koncu ter stratificiranega toka v bližini zaprtega konca cevi. Te inherentne nestabilnosti v toku lahko v praksi generirajo ciklična temperaturna nihanja v steni cevi, ki potencialno vodijo do termičnega utrujanja materialov in nastanka razpok. Takšne neizolirane cevi najdemo v nekaterih tlačnovodnih reaktorjih z dvema hladilnima zankama, kjer so cevi varnostnega vbrizgavanja priključene neposredno na reaktorsko tlačno posodo. Cilj predlaganega dela je analizirati dinamiko toka v slepi cevi cevovoda varnostnega vbrizgavanja poenostavljene geometrije tlačnovodnega reaktorja.

Dr. Ivo Kljenak: Gravity-driven injection from accumulators

Velike mehanske napetosti med kristalnimi zrni lahko povzročijo nastanek medkristalnih razpok v kovinskem polikristalnem skupku. Velikosti teh napetosti niso odvisne le od zunanje obremenitve, pač pa tudi od snovnih lastnosti, oblik ter orientacij kristalnih zrn. Za posamezen primer kovinskega skupka je možno z računsko zahtevnimi simulacijami dokaj natančno napovedati velikosti medkristalnih napetosti, v splošnem pa je tak pristop nepraktičen. Trenutne raziskave potekajo v smeri iskanja bližnjice, s katero bi lahko enostavno a še vedno dovolj natančno napovedali obnašanje medkristalnih napetosti v poljubnem kovinskem skupku. Na ta način bi lahko hitro ocenili, kakšna je verjetnost za nastanek medkristalnih razpok v danem materialu pri dani obremenitvi.

Študent(ka) bo s pomočjo numeričnih simulacij na različnih polikristalnih modelih (program ABAQUS) in statistične analize rezultatov poskušal(a) identificirati skupne značilnosti teh modelov. Na podlagi teh spoznanj bo lahko poiskal(a) ključne parametre, ki prispevajo k medkristalnim napetostim v splošnem polikristalu. S pomočjo eksperimentalnih podatkov (sodelovanje s CEA, Francija) bo lahko tudi preveril(a), ali so lokacije nastanka razpok res korelirane z največjimi medkristalnimi napetostmi.

Notranje strukture lahkovodnih reaktorjev so povečini narejene iz nerjavnega avstenitnega jekla, ki je znano po dobrih mehanskih lastnostih. Pod vplivom nevtronskega sevanja se lastnosti teh jekel poslabšajo, še zlasti opazimo zmanjšanje lomne žilavosti in povečanje občutljivosti na sevalno napetostno korozijsko pokanje (ang. Irradiation-Assisted Stress Corrosion Cracking, IASCC). Znano je, da IASCC povzroča poškodbe vijakov pregradnih plošč v posodah tlačnovodnih reaktorjev. Zaradi kompleksnih fizikalnih procesov v ozadju tega pojava trenutno še ne znamo podati zanesljivih napovedi za nastanek IASCC.

Študent(ka) bo s pomočjo numeričnih simulacij na različnih polikristalnih modelih (program ABAQUS) pomagal(a) izboljšati trenutno razumevanje ključnih mehanizmov za nastanek IASCC v avstenitnih nerjavnih jeklih pod pogoji, ki so značilni za tlačnovodne reaktorje. Delo se bo v grobem delilo na kalibracijo že obstoječega mikromehanskega modela kristalne plastičnosti, ki opisuje nelinearen mehanski odziv kristalnih zrn v obsevanem avstenitnem nerjavnem jeklu (sodelovanje s CEA, Francija) ter na modeliranje lokaliziranih plastičnih pasov, ki naj bi igrali ključno vlogo pri nastanku medkristalnih razpok zaradi IASCC.

Safe long-term operation of nuclear power plants depends strongly on the reliability and integrity of safety relevant components. Understanding the ageing processes of metals that may limit the long-term safe operation is therefore of immense importance. The InterGranular Stress-Corrosion Cracking (IGSCC) is one of the most significant ageing degradation mechanisms in structural metallic alloys which corresponds to the initiation and propagation of cracks along the grain boundaries. To understand and accurately predict the IGSCC initiation, the knowledge on grain boundary stresses is required.

The candidate will perform a series of finite element simulations with commercial code Abaqus to obtain the probability density distributions of intergranular normal stresses present in a polycrystalline metal when exposed to different external loading conditions. Using the statistical analysis of the results, the candidate will try to identify the most influencing parameters that contribute to the largest grain boundary stresses relevant for IGSCC initiation.

Dr. Samir El Shawish: Explicit modelling of dislocation channels in neutron-irradiated stainless steel

In irradiated steels, dislocation channel (also clear channel) refers to the localization of plastic deformation within slip bands that appear to be free of radiation defects. A dislocation channel is a microstructural object, only few tens of nm wide and with a length comparable with the grain size, which is created when loading an irradiated material. Its complex interaction with defects on the nanoscale affects the behaviour of the metal at the macroscopic scale (loss of ductility, reduced uniform elongation). Because of their interaction with grain boundaries, dislocation channels are thought to have a crucial role in the initiation of intergranular cracks (Irradiation-Assisted Stress Corrosion Cracking - IASCC).

The candidate will perform finite element simulations with commercial code Abaqus to analyse the effects of dislocation channels on grain boundary stresses. Dislocation channels will be imposed topologically to the aggregate model and appropriate crystal plasticity laws will be assigned to different model regions in order to reproduce the response of an aggregate on a macroscopic scale. The influence of the dislocation channel width and spacing on intergranular stresses will be investigated and a possible correlation with the initiation of IASCC will be examined.

Dr. Samir El Shawish: Modelling slip transmission at grain boundaries of irradiated stainless steel

In irradiated steels, dislocation channel (also clear channel) refers to the localization of plastic deformation within slip bands that appear to be free of radiation defects. A dislocation channel is a microstructural object, only few tens of nm wide and with a length comparable with the grain size, which is created when loading an irradiated material. Its complex interaction with defects on the nanoscale affects the behaviour of the metal at the macroscopic scale (loss of ductility, reduced uniform elongation). Because of their interaction with grain boundaries, dislocation channels are thought to have a crucial role in the initiation of intergranular cracks (Irradiation-Assisted Stress Corrosion Cracking - IASCC).

The candidate will model slip transmission on grain boundaries by modifying the hardening behavior as a promising mesoscopic way to account for the observed dependence of cracking on slip discontinuity without having to describe explicitly the dislocation channels

Dr. Samir El Shawish: Deterministic mesoscale modelling of IGSCC in austenitic stainless steels

InterGranular Stress Corrosion Cracking (IGSCC) represents one of the most critical ageing mechanisms affecting austenitic stainless steels in Nuclear Power Plants. As a structural material, austenitic stainless steel is extensively utilized in NPPs due to its high corrosion resistance and mechanical strength. However, under certain environmental conditions, particularly those involving high temperatures and corrosive media, these steels become susceptible to IGSCC. The IGSCC corresponds to the initiation and propagation of microcracks along the Grain Boundaries. A comprehensive understanding of IGSCC processes and their smart integration over different scales is crucial for accurately predicting the onset and progression of IGSCC in real-world components.

In this work, the candidate will generate a mesoscopic finite element model that accounts for the interplay between microstructure, mechanical behaviour, and various associated mechanisms (such as oxidation), at the scale of several grains in a polycrystalline aggregate. The goal is to create a transient crystal plasticity finite element model that would faithfully simulate IGSCC-relevant processes in the material in front of the crack tip where the mechanical stresses and concentrations of oxygen and hydrogen (or chlorides, sulfates) is largest and thus the local microstructure is most important.

Dr. Samir El Shawish: Probabilistic mesoscale modelling of IGSCC in austenitic stainless steels

InterGranular Stress Corrosion Cracking (IGSCC) represents one of the most critical ageing mechanisms affecting austenitic stainless steels in Nuclear Power Plants. As a structural material, austenitic stainless steel is extensively utilized in NPPs due to its high corrosion resistance and mechanical strength. However, under certain environmental conditions, particularly those involving high temperatures and corrosive media, these steels become susceptible to IGSCC. The IGSCC corresponds to the initiation and propagation of microcracks along the Grain Boundaries (GBs). A comprehensive understanding of IGSCC processes and their smart integration over different scales is crucial for accurately predicting the onset and progression of IGSCC in real-world components.

In this work, the candidate will generate a probabilistic mesoscopic model where appropriate (time-independent) GB normal stress distributions and time-dependent GB strength distributions will be considered due to uneven GB oxidation or defect precipitation kinetics observed in IGSCC. The goal is to estimate a fraction and direction in space of overloaded GBs, and the corresponding time needed to achieve this stage for a given external loading conditions.