Alien Registration- Niskanen, Greta I (Paris, Oxford County)

https://digitalmaine.com/alien_docs/21132/thumbnail.jp

Mean-field crystal plasticity compared to full-field crystal plasticity in fatigue modelling

Abstract. Understanding fatigue as a phenomenon is crucial, as most machine components fail due to fatigue fracture. The fatigue behavior of a component is highly dependent on the microstructure of the material. Crystal plasticity models are efficient computational tools for investigating the micromechanical behavior of the material and therefore obtaining a better understanding of fatigue. This work compares two different types of crystal plasticity models regarding fatigue modeling, a crystal plasticity finite element model and a self-consistent crystal plasticity model. The microstructure of the material can be modeled explicitly with crystal plasticity finite element model. The self-consistent model does not use an explicit microstructure, instead the grains experience the effects of neighboring grains through homogenization. The main goal of the work is to determine if the self-consistent model is able to produce comparable results to the computationally more demanding finite element model in fatigue loading. The models are calibrated to fatigue and tensile test data of a martensitic steel with a new proposed fitting method separately addressing the plasticity and damage behavior. Two different types of microstructural morphologies are used to compare the models with only plasticity in fatigue loading, a prior austenite like microstructure and a martensitic microstructure, which both have their own grain orientation distributions. The damage behavior is compared with martensitic microstructures with embedded inclusions. With only plasticity active, the models produce quite similar results with a prior austenite structure. With the more intricate geometry of a martensitic structure, larger differences in plasticity and stress distribution are noticed. The difference in the model formulations causes obvious differences once the damage is introduced. While the finite element model can properly model the crack propagation, the self-consistent model can only estimate the damage in the microstructure.Homogenisaatiokideplastisuusmallin ja elementtimenetelmäkideplastisuusmallin vertailu väsymisessä. Tiivistelmä. Väsymisen ymmärtäminen ilmiönä on tärkeää, sillä suurin osa koneenosien vaurioitumisesta johtuu väsymismurtumista. Väsymiskäyttäytyminen riippuu hyvin paljon käytetyn materiaalin mikrorakenteesta. Kideplastisuusmallit ovat tehokkaita laskennallisia työkaluja materiaalin mikromekaanisen käyttäytymisen ja täten väsymisen tutkimisessa. Tässä työssä vertaillaan kahta erilaista kideplastisuusmallia, kideplastisuuselementtimallia ja -homogenisaatiomallia. Elementtimalli mahdollistaa todellisuuden mukaisen mikrorakenteen mallintamisen, kun taas homogenisaatiomallilla materiaalin rakeet kokevat ympäröivän mikrorakenteen vuorovaikutukset homogenisaation kautta. Työn pääasiallinen tavoite on selvittää, kykeneekö homogenisaatiomalli tuottamaan vertailukelpoisia tuloksia laskennallisesti raskaamman elementtimallin kanssa väsymiskuormituksessa. Työssä esitellään uusi menetelmä mallien kalibrointiin, jossa plastinen käyttäytyminen ja vauriokäyttäytyminen on erotettu toisistaan. Mallit kalibroidaan martensiittisen teräksen koetuloksiin. Malleja vertaillaan ilman vauriota jäännösausteniittisella ja martensiittisella mikrorakenteella, joilla on kummallakin oma raeorietaatiojakaumansa. Lisäksi malleja vertaillaan vaurion kanssa martensiittisella rakenteella sulkeuman kanssa. Ilman vauriota mallit tuottavat kohtalaisen samankaltaisia tuloksia austeniittisella rakenteella, mutta martensiittisella rakenteella, jossa mikrorakenteen geometrian rooli on merkittävämpi, havaitaan suurempia eroja plastisuus- ja jännitysjakaumissa. Kun vaurio aktivoidaan, havaitaan mallien välillä selkeä formulaation aiheuttama ero. Elementtimalli kykenee mallintamaan särönkasvun materiaalissa realistisesti, kun taas homogenisaatiomalli kykenee vain arvioimaan vaurion määrää rakenteessa

Virtuaalinen väsymistestaaminen

Tiivistelmä. Väsymisen tutkiminen ilmiönä on tärkeää, sillä se on usein rakenteiden kestoiän määrittävä tekijä. Ilmiön ennustamiseksi on kehitetty useita erilaisia malleja. Näiden mallien tarkkuudet vaihtelevat ja kaikki mallit eivät toimi kaikissa tilanteissa, minkä vuoksi yksikään malli ei ole vielä saanut yleistä hyväksyntää. Työssä perehdytään virtuaalisen väsymistestaamisen historiaan, tulevaisuuden näkymiin, sekä tarkastellaan erilaisten kappaleiden käyttäytymistä yhden tällaisen mallin avulla. Lisäksi työssä käydään pintapuolisesti läpi materiaalien väsymistä ilmiönä. Tarkasteltava malli on kiteiden plastisuuteen perustuva elementtimalli. Kideplastisuusmallin tarjoama lähestymistapa mahdollistaa väsymisen tarkastelemisen mikromekaanisella tasolla. Jaksollisen kuormituksen vaikutuksen plastisuuteen ja mikrorakenteellisten ominaisuuksien ymmärtäminen on oleellista, jotta voidaan muodostaa kattava käsitys väsymisestä ilmiönä. Yksinkertaisuuden vuoksi mallista on jätetty materiaalin vaurioitumisen vaikutus pois ja se on muutettu toimimaan pienten siirtymien teorian mukaisesti. Kideplastisuusmalli otettiin käyttöön Zset-ohjelmistossa, jossa sillä suoritettiin elementtimenetelmäsimulaatioita. Tulosten käsittelyssä käytettiin Pythonia. Mallilla suoritetaan puhtaan materiaalin ja sulkeuman sisältävien kappaleiden vetokokeita sen toiminnan arvioimiseksi. Sulkeuman sisältävää materiaalia tarkastellaan myös väsyttävässä jaksollisessa kuormituksessa. Tarkasteltaville kappaleille on annettu martensiittisen suurlujuusteräksen arvot. Sulkeumat ovat pyöreitä ja täydellisesti kiinnittyneitä materiaalin matriisiin. Sulkeuman tarkastelussa on käytetty jokaisessa tilanteessa alumiinioksidille tyypillistä suurta kimmokerrointa, lukuun ottamatta osiota, jossa tarkastellaan sulkeuman kimmomoduulin vaikutusta materiaalin käyttäytymiseen. Kideplastisuusmallin havaitaan tuottavan realistisia tuloksia plastisen käyttäytymisen perusteella. Myös kideorientaation vaikutus on selvästi havaittavissa koetilanteissa.Virtual fatigue testing. Abstract. Research of fatigue as a phenomenon is important as it is often the defining factor of a component’s life. Many models have been developed to predict material fatigue, yet all of these models have inaccuracies and do not work in every situation. Therefore there is no universal acceptance for any model as the best for modeling fatigue. This work focuses on the history of fatigue modeling, future of virtual fatigue testing and fatigue as a phenomenon. A model based on crystal plasticity and its functioning is inspected in greater detail. The inspected model uses the finite element method for simulations. The crystal plasticity approach enables investigating fatigue as a micromechanical phenomenon. Understanding the effects of cyclic loading on plasticity and microstructural properties is crucial for complete understanding of material fatigue. For simplicity, material damage is not considered in this work. The crystal plasticity model is also transformed for the theory of small deformations. The model was used for FEM simulations in Zset-software and Python was used to process the results. The model was used to simulate a tensile test for clean material and for material with a round inclusion. Clean material was also simulated in cyclic loading. The values of a high strength martensitic steel were used for the material. The inclusions used were bonded perfectly to the material matrix. They were given a Young’s modulus typical of aluminum oxide inclusion in all the simulations except for the study of material behavior’s dependence on inclusion stiffness. The crystal plasticity model produces realistic results based on the plastic behavior. It also simulates accurately the effect of crystal orientation

Shot-noise-driven escape in hysteretic Josephson junctions

We have measured the influence of shot noise on hysteretic Josephson junctions initially in macroscopic quantum tunnelling (MQT) regime. Escape threshold current into the resistive state decreases monotonically with increasing average current through the scattering conductor, which is another tunnel junction. Escape is predominantly determined by excitation due to the wide-band shot noise. This process is equivalent to thermal activation (TA) over the barrier at temperatures up to about four times above the critical temperature of the superconductor. The presented TA model is in excellent agreement with the experimental results

On decidability and complexity of low-dimensional robot games

A robot game, also known as a Z-VAS game, is a two-player vector addition game played on the integer lattice Zn, where one of the players, Adam, aims to avoid the origin while the other player, Eve, aims to reach the origin. The problem is to decide whether or not Eve has a winning strategy. In this paper we prove undecidability of the two-dimensional robot game closing the gap between undecidable and decidable cases. We also prove that deciding the winner in a robot game with states in dimension one is EXPSPACE-complete and study a subclass of robot games where deciding the winner is in EXPTIME

Detailed comparison of the pp -> \pi^+pn and pp -> \pi^+d reactions at 951 MeV

The positively charged pions produced in proton-proton collisions at a beam momentum of 1640 MeV/c were measured in the forward direction with a high resolution magnetic spectrograph. The missing mass distribution shows the bound state (deuteron) clearly separated from the $pn$ continuum. Despite the very good resolution, there is no evidence for any significant production of the $pn$ system in the spin-singlet state. However, the $\sigma(pp\to \pi^+pn)/\sigma(pp\to \pi^+d)$ cross section ratio is about twice as large as that predicted from $S$-wave final-state-interaction theory and it is suggested that this is due to $D$-state effects in the $pn$ system.Comment: 8 pages, 3 figure

Reachability problems in low-dimensional nondeterministic polynomial maps over integers

We study reachability problems for various nondeterministic polynomial maps in Zn. We prove that the reachability problem for very simple three-dimensional affine maps (with independent variables) is undecidable and is PSPACE-hard for both two-dimensional affine maps and one-dimensional quadratic maps. Then we show that the complexity of the reachability problem for maps without functions of the form ±x+a0 is lower. In this case the reachability problem is PSPACE for any dimension and if the dimension is not fixed, then the problem is PSPACE-complete. Finally we extend the model by considering maps as language acceptors and prove that the universality problem is undecidable for two-dimensional affine maps

Weighted Automata on Infinite Words in the Context of Attacker-Defender Games

The paper is devoted to several infinite-state Attacker–Defender games with reachability objectives. We prove the undecidability of checking for the existence of a winning strategy in several low-dimensional mathematical games including vector reachability games, word games and braid games. To prove these results, we consider a model of weighted automata operating on infinite words and prove that the universality problem is undecidable for this new class of weighted automata. We show that the universality problem is undecidable by using a non-standard encoding of the infinite Post correspondence problem

Observation of Berry's Phase in a Solid State Qubit

In quantum information science, the phase of a wavefunction plays an important role in encoding information. While most experiments in this field rely on dynamic effects to manipulate this information, an alternative approach is to use geometric phase, which has been argued to have potential fault tolerance. We demonstrate the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit, manipulating the qubit geometrically using microwave radiation, and observing the accumulated phase in an interference experiment. We find excellent agreement with Berry's predictions, and also observe a geometry dependent contribution to dephasing.Comment: 5 pages, 4 figures, version with high resolution figures available at http://qudev.ethz.ch/content/science/PubsPapers.htm