Yixiang Gan

Experimental Observations of Stress-Driven Grain Boundary Migration

Moving Boundaries Classical models of fine-grained metals view grain boundaries as static objects, but this view has been challenged by recent experimental observations. Drawing on techniques used by the fracture mechanics community, Rupert et al. (p. 1686) present experiments on freestanding aluminum films that show specific geometries cause either stress or strain concentrations on deformation. Confirming recent simulations, shear stresses were found to be a key driver of grain boundary motion. Shear stresses drive grain boundaries to move in a manner consistent with predictions of coupled grain boundary migration. In crystalline materials, plastic deformation occurs by the motion of dislocations, and the regions between individual crystallites, called grain boundaries, act as obstacles to dislocation motion. Grain boundaries are widely envisaged to be mechanically static structures, but this report outlines an experimental investigation of stress-driven grain boundary migration manifested as grain growth in nanocrystalline aluminum thin films. Specimens fabricated with specially designed stress and strain concentrators are used to uncover the relative importance of these parameters on grain growth. In contrast to traditional descriptions of grain boundaries as stationary obstacles to dislocation-based plasticity, the results of this study indicate that shear stresses drive grain boundaries to move in a manner consistent with recent molecular dynamics simulations and theoretical predictions of coupled grain boundary migration.

Scalable surface area characterization by electrokinetic analysis of complex anion adsorption.

By means of the in situ electrokinetic assessment of aqueous particles in conjunction with the addition of anionic adsorbates, we develop and examine a new approach to the scalable characterization of the specific accessible surface area of particles in water. For alumina powders of differing morphology in mildly acidic aqueous suspensions, the effective surface charge was modified by carboxylate anion adsorption through the incremental addition of oxalic and citric acids. The observed zeta potential variation as a function of the proportional reagent additive was found to exhibit inverse hyperbolic sine-type behavior predicted to arise from monolayer adsorption following the Grahame-Langmuir model. Through parameter optimization by inverse problem solving, the zeta potential shift with relative adsorbate addition revealed a nearly linear correlation of a defined surface-area-dependent parameter with the conventionally measured surface area values of the powders, demonstrating that the proposed analytical framework is applicable for the in situ surface area characterization of aqueous particulate matter. The investigated methods have advantages over some conventional surface analysis techniques owing to their direct applicability in aqueous environments at ambient temperature and the ability to modify analysis scales by variation of the adsorption cross section.

Effects of surface structure deformation on static friction at fractal interfaces

The evolution of fractal surface structures with flattening of asperities was investigated using isotropically roughened aluminium surfaces loaded in compression. It was found that asperity amplitude, mean roughness and fractal dimension decrease through increased compressive stress and number of loading events. Of the samples tested, surfaces subjected to an increased number of loading events exhibited the most significant surface deformation and were observed to exhibit higher levels of static friction at an interface with a single-crystal flat quartz substrate. This suggests that the frequency of grain reorganisation events in geomaterials plays an important role in the development of intergranular friction. Fractal surfaces were numerically modelled using Weierstrass– Mandelbrot-based functions. From the study of frictional interactions with rigid flat opposing surfaces it was apparent that the effect of surface fractal dimension is more significant with increasing dominance of adhesive mechanisms.

A particle-water based model for water retention hysteresis

A particle–water discrete element based approach to describe water movement in partially saturated granular media is presented and tested. Water potential is governed by both capillary bridges, dominant at low saturations, and the pressure of entrapped air, dominant at high saturations. The approach captures the hysteresis of water retention during wetting and drainage by introducing the local evolution of liquid–solid contact angles at the level of pores and grains. Extensive comparisons against experimental data are presented. While these are made without the involvement of any fitting parameters, the method demonstrates relative high success by achieving a correlation coefficient of at least 82%, and mostly above 90%. For the tested materials with relatively mono-disperse grain size, the hysteresis of water retention during cycles of wetting and drainage has been shown to arise from the dynamics of solid–liquid contact angles as a function of local liquid volume changes.

Thermo-mechanics of pebble beds in fusion blankets

Thermo-Mechanik von Schuttbetten in Fusionsreaktorblankets Heliumgekuhlte Schuttbetten (HCPB: Helium Cooled Pebble Beds) werden in der Ummantelung von Fusionsreaktoren, dem sogenannten Blanket, zur Tritiumerzeugung und als Neutronenmultiplikator verwendet und unterliegen somit harten Einsatzbedingungen. Die Schuttbetten bestehen aus nahezu kugelformigem Granulat und weisen aufgrund dieser diskreten Beschaffenheit ein komplexes Materialverhalten auf. Eines der wichtigsten Forschungsthemen bei HCPB-Blankets ist die Abhangigkeit der Warmeleitfahigkeit von der Druckspannung, die durch die thermische Ausdehnung der Schuttbetten im Betrieb hervorgerufen wird. Um den Anforderungen in Hinblick auf Design und Analyse eines HCPB-Blankets gerecht zu werden, wird ein Materialmodell benotigt, das die thermo-mechanische Antwort auf eine ausere Anregung vollstandig gekoppelt beschreibt. In der vorliegenden Dissertation wurden ein numerisches Simulationsverfahren fur Schuttbetten unter fusionstypischen Einsatzbedingungen entwickelt. Die Schuttbetten aus Brutkeramik und Beryllium werden dabei mittels der Diskrete-Elemente-Methode und phanomenologischer Ansatze modelliert. Daruber hinaus wird gezeigt, wie vorhandene experimentelle Ergebnisse im Rahmen dieser Vorgehensweise ausgenutzt werden konnen. Bei der Diskrete-Elemente-Methode werden die einzelnen Granulatkorner unter Gleichgewichtsbedingungen betrachtet. Hierbei wird neben der Anordnung der einzelnen Partikel auch das globale Bauteilverhalten unter Einwirkung makroskopischer Druckbelastung untersucht. Ausgehend von einer zufalligen Packungsdichte als Anfangsbedingung liefert die Simulation die Verteilung der Kontaktbelastung zwischen den einzelnen Partikeln. Die Simulation eines einachsigen Drucktests mit Hilfe der Diskrete-Elemente-Methode ergab dabei eine quantitative Ubereinstimmung mit experimentellen Ergebnissen. Daruber hinaus wurden die Beziehungen zwischen mikroskopischen Grosen, wie z.B. der maximalen Kontaktbelastung oder der Koordinationszahl im Bauteil zu makroskopischen Belastungsgrosen untersucht. In einem zweiten Ansatz wurde das globale Materialverhalten unter fusionsahnlichen Bedingungen durch ein phanomenologisches Modell beschrieben, welches die Schuttbetten als kontinuierliches Material betrachtet. Ziel ist die Entwicklung eines Materialgesetzes, das in eine Finite-Elemente-Simulation des Gesamtbauteils eingebunden werden kann. Das thermo-mechanische Materialverhalten wird dabei durch ein nichtlineares Elastizitatsgesetz abgebildet, welches ein modifiziertes Drucker-Prager-Cap Modell sowie eine dehnungsabhangige Warmeleitfahigkeit beinhaltet. Die benotigten Materialparameter wurden aus vorhandenen experimentellen Ergebnissen abgeleitet. Dieses Vorgehen wurde anhand verschiedener Schuttbett-Varianten angewendet und fur unterschiedliche Temperaturniveaus verifiziert. Daruber hinaus wurde das phanomenologische Modell in eine benutzerdefiniterte Materialroutine implementiert, um vollstandig gekoppelte thermo-mechanische FE-Analysen durchfuhren zu konnen. Die Grenzflache zwischen Granulat und Behalterwand wird durch ein Warmeubergangsmodell dargestellt, welches die Warmeleitung im Kontaktbereich bei unterschiedlichen Spannungen und Temperaturen berucksichtigt. In einer Vergleichsstudie wurden die Ergebnisse der Simulation auf Basis des phanomenologischen Modells mit experimentellen Ergebnissen verglichen. Dabei hat sich gezeigt, dass der vorliegende Modellierungsansatz fur die thermo-mechanische Analyse eines Fusionsreaktorblankets geeignet ist. Abschliesend wird gezeigt, dass die in der vorliegenden Dissertation entwickelten numerischen Methoden eine effiziente Analyse von HCPB-Blankets ermoglichen und somit ein wichtiges Werkzeug in Hinblick auf das Design derartiger Bauteile darstellen. Daruber hinaus liefert die vorliegende Arbeit die Grundlage, um weitere experimentelle Daten, wie z.B. zum Schwellen oder zur Degradation durch Bestrahlung in das vorhandene Materialmodell fur Schuttbetten zu implementieren.

Thermal Discrete Element Analysis of EU Solid Breeder Blanket Subjected to Neutron Irradiation

Abstract Due to neutron irradiation, solid breeder blankets are subjected to complex thermo-mechanical conditions. Within one breeder unit, the ceramic breeder bed is composed of spherical-shaped lithium orthosilicate pebbles, and as a type of granular material, it exhibits strong coupling between temperature and stress fields. In this paper, we study these thermo-mechanical problems by developing a thermal discrete element method (Thermal-DEM). This proposed simulation tool models each individual ceramic pebble as one element and considers grain-scale thermo-mechanical interactions between elements. A small section of solid breeder pebble bed in a helium-cooled pebble bed (HCPB) is modelled using thousands of individual pebbles and subjected to volumetric heating profiles calculated from neutronics under ITER-relevant conditions. We consider heat transfer at the grain scale between pebbles through both solid-to-solid contacts and the interstitial gas phase, and we calculate stresses arising from thermal expansion of pebbles. The overall effective conductivity of the bed depends on the resulting compressive stress state during the neutronic heating. The Thermal-DEM method proposed in this study provides access to the grain-scale information, which is beneficial for HCPB design and breeder material optimization, and a better understanding of overall thermo-mechanical responses of the breeder units under fusion-relevant conditions.

Evaporation Limited Radial Capillary Penetration in Porous Media

The capillary penetration of fluids in thin porous layers is of fundamental interest in nature and various industrial applications. When capillary flows occur in porous media, the extent of penetration is known to increase with the square root of time following the Lucas-Washburn law. In practice, volatile liquid evaporates at the surface of porous media, which restricts penetration to a limited region. In this work, on the basis of Darcys law and mass conservation, a general theoretical model is developed for the evaporation-limited radial capillary penetration in porous media. The presented model predicts that evaporation decreases the rate of fluid penetration and limits it to a critical radius. Furthermore, we construct a unified phase diagram that describes the limited penetration in an annular porous medium, in which the boundaries of outward and inward liquid are predicted quantitatively. It is expected that the proposed theoretical model will advance the understanding of penetration dynamics in porous media and facilitate the design of engineered porous architectures.

The pore-load modulus of ordered nanoporous materials with surface effects

Gas and liquid adsorption-induced deformation of ordered porous materials is an important physical phenomenon with a wide range of applications. In general, the deformation can be characterized by the pore-load modulus and, when the pore size reduces to nanoscale, it is affected by surface effects and shows prominent size-dependent features. In this Letter, the influence of surface effects on the elastic properties of ordered nanoporous materials with internal pressure is accounted for in a single pore model. A porosity and surface elastic constants dependent closed form solution for the size dependent pore-load modulus is obtained and verified by finite element simulations and available experimental results. In addition, it is found to depend on the geometrical arrangement of pores. This study provides an efficient tool to analyze the surface effects on the elastic response of ordered nanoporous materials.

Size-Dependent Crush Analysis of Lithium Orthosilicate Pebbles

Abstract The crushing strength of the breeder material [lithium orthosilicate (Li4SiO4 or OSi)] in the form of pebbles to be used for EU solid breeder concept is investigated. The pebbles are fabricated using a melt-spray method, and hence, a size variation in the pebbles produced is expected. Knowledge of the mechanical integrity (crush strength) of the pebbles is important for a successful design of a breeder blanket. In this paper, we present the experimental results of the crush (failure) loads for spherical OSi pebbles of different diameters ranging from 250um to 800um. The ultimate failure load for each size shows a Weibull distribution. Furthermore, the mean crush load increases with increase in pebble diameter. It is also observed that the level of opacity of the pebble influences the crush load significantly. The experimental data presented in this paper and the associated analysis could possibly help us to develop a framework for simulating a crushable polydisperse pebble assembly using discrete element method.

Phase transitions and cyclic pseudotachylyte formation in simulated faults

Field evidence from faults containing pseudotachylytes has revealed cyclical episodes of frictional melting, ductile deformation, and overprinting at a later stage by a new generation of pseudotachylytes. Here we connect these cycles to earthquake dynamics using a development of a discrete element model with solid grains that can melt during frictional heating and viscous melts that can bond through solidification during cooling. A new earthquake episode initiates with the crushing of bonded clusters once the bond strength is exceeded, with frictional shear heating being activated again. We explore the competition between melting and solidification in terms of phase transitions using scaling laws dependent on the characteristic times for melting, thermal diffusion and loading rates. A phase diagram is constructed that is capable of explaining the tendencies towards pseudotachylytes associated to cataclasites or mylonites, depending on the fault conditions (its depth and thickness, crust motion and ambient temperature) and the mechanical and thermal parameters defining the grains within the fault and the host rock.