SIZE-SENSITIVE CRYSTAL PLASTICITY FINITE ELEMENT FRAMEWORK FOR SIMULATING BEHAVIOR OF LAMELLAR METAL-METAL COMPOSITES

Growing demands for materials with enhanced and superb characteristics increase the difficulty and the amount of research necessary to be conducted in many different areas of expertise. The vast field of computational mechanics represents a significant source of valuable solutions to many of these challenges and can provide a smoother transition in the process when a new material is introduced. Experimental techniques are not always able to measure the localized material features due to the very complicated deformation conditions. As an alternative approach, full-field models are developed, such the ones contained in this dissertation that can bridge this gap and provide source of significant insights. The crystal plasticity finite element models (CPFEM) developed under this dissertation are presented and discussed through several specific case studies, which establish the fundamental microstructure-property relationships that describe in particular the deformation behavior of novel multilayer metallic lamellar microstructures composed of Zirconium-Niobium and Magnesium-Niobium layers. These lamellar material systems exhibit extraordinary strength while preserving ductility and they are promising candidates for application in many industries, such as nuclear and automotive. Different formulations of the 3D multiscale models were numerically implemented to investigate the origin and the development of the microstructural features that occured during the fabrication process of these lamellar composites. In particular, the orientation stability of nanocrystalline Zirconium and the formation of strain localizations were investigated during accumulative roll bonding process. Furthermore, the work contained in this dissertation describes the first attempt to incorporate the confined layer slip (CLS) model into CPFE, which greatly contributes to fundamental understanding of how Magnesium-Niobium nano-layered composites deform elastically and plastically at nanometer length scales. Next, significant efforts were put into investigating a mechanism of deformation twinning. This deformation mechanism governs the mechanical behavior of many polycrystalline metals, particularly those with low symmetry crystal structures. Deformation twins are represented as lamellar inclusions in the granular microstructures, and overall the material behaves as a composite. Hence, a novel modeling approach, which explicitly models the formation and thickening of a twin lamella within a crystal plasticity finite element framework was developed. The model represents a unique numerical procedure which is able to relate spatially resolved fields of stress and strains with microstructural changes during a twin formation and thickening. This approach was applied to study the twin formation and thickening in cast Uranium and Magnesium alloy AZ31. In AZ31 the effects of dislocation density on a twin propagation were investigated, as well as the influence of the double twin formation on the material’s fracture behavior. Overall, the presented work in my dissertation provides a powerful predictive simulation tool that could be used in many subsequent studies contributing to the further advancements in the field of computational material science

Anisotropic shock response of 1,3,5-Triamino-2,4,6-Trinitrobenzene (TATB)

All-atom molecular dynamics simulations were used to study shock wave loading in both oriented single crystals and two orientations with the presence of a grain boundary of the highly anisotropic triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The crystal structure consists of planar hydrogen-bonded sheets of individually planar TATB molecules that stack into graphitic-like layers. In the oriented single crystal case, shocks were studied for seven systematically prepared crystal orientations with limiting cases that correspond to shock propagation exactly perpendicular and exactly parallel to the graphitic-like layers. The simulations were performed for initially defect-free crystals using a reverse-ballistic configuration that yields explicit, supported shocks. In the grain boundary case, shocks with opposite directions were studies for two crystal orientations joined by a grain boundary. Results from both studies indicate that TATB shock response is highly sensitive to crystal orientation, with significant qualitative differences for the time evolution of stress and temperature, elastic/inelastic compression response, and defect formation and growth.Includes bibliographical reference

Anisotropic shock response of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)

The thermo-mechanical response of shock-induced pore collapse has been studied using non-reactive all-atom molecular dynamics (MD) and Eulerian continuum simulations for the molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Three crystal orientations, bracketed by the limiting cases with respect to the crystal structure anisotropy in TATB, are considered in the MD simulations, while an isotropic constitutive model is used for the continuum simulations. Simulations with three impact speeds from 0.5 km s[superscript -1] to 2.0 km s[superscript -1] are investigated. Results from MD and continuum simulations are in agreement in terms of shock wave speeds, temperature distributions, and pore-collapse mechanisms. However, differences arise for other quantities that are also important in hotspot ignition and growth, for example, the skewness of high-temperature distributions and the local temperature field around the post-collapse hotspot, indicating the urgent need to incorporate anisotropic crystal plasticity and strength models into the continuum descriptions. The deformation mechanisms of TATB crystals in the shock-induced pore collapse MD simulations were studied using Strain Functional Analysis. This new approach maps discrete quantities from atomistic simulations onto continuous fields via a Gaussian kernel, from which a unique and complete set of rotationally invariant Strain Functional Descriptors (SFD) is obtained from the high-order central moments of local configurations, expressed in a Solid Harmonics polynomial basis by SO(3) decomposition. Coupled with unsupervised machine learning techniques, the SFD successfully identifies and distinguishes the deformations presented in the MD simulations of shock-compressed TATB crystals. It enables automated detection of disordered structures in the system and can be readily applied to materials with any symmetry class.Includes bibliographical references (pages 142-168)

Modeling of photonic band gap crystals and applications

In this work, we have undertaken a theoretical approach to the complex problem of modeling the flow of electromagnetic waves in photonic crystals. Our focus to address the feasibility of using the exciting phenomena of photonic gaps (PBG) in actual applications.;We start by providing analytical derivations, as well as the underlying physical principles, of the computational electromagnetic methods used in our work. A comparative study of the strengths and weaknesses of each method is provided. The Plane Wave expansion, Transfer Matrix, and Finite Difference Time Domain Methods are addressed. We also introduce a new theoretical approach, the Modal Expansion Method.;We then shift our attention to actual applications. We begin with a discussion of 2D photonic crystal wave guides. The structure addressed consists of a 2D hexagonal structure of air cylinders in a layered dielectric background. Comparison with the performance of a conventional guide is made, as well as suggestions for enhancing it. Our studies provide an upper theoretical limit on the performance of such guides.;Next, we study 3D metallic PBG materials at near infrared and optical wavelengths. Our main objective is to study the importance of absorption in the metal and the suitability of observing photonic band gaps in such structures. We study simple cubic structures where the metallic scatterers are either cubes or interconnected metallic rods. The effect of topology is also addressed. Our results reveal that the best performance is obtained by choosing metals with a large negative real part of the dielectric function, together with a relatively small imaginary part. Finally, we point out a new direction in photonic crystal research that involves the interplay of metallic-PBG rejection and photonic band edge absorption. We propose that an absolute metallic-PBG may be used to suppress the infrared part of the blackbody emission and, emit its energy only through a sharp absorption band. Potential applications of this new PBG mechanism include highly efficient incandescent lamps and enhanced thermophotovoltaic energy conversion. The suggested lamp would be able to recycle the energy that would otherwise go into the unwanted resulting in a 40% increase in efficiency

Some Critical Thoughts on Computational Materials Science

1. A Model is a Model is a Model is a Model The title of this report is of course meant to provoke. Why? Because there always exists a menace of confusing models with reality. Does anyone now refer to “first principles simulations”? This point is well taken. However, practically all of the current predictions in this domain are based on simulating electron dynamics using local density functional theory. These simulations, though providing a deep insight into materials ground states, are not exact but approximate solutions of the Schrödinger equation, which - not to forget - is a model itself [1]. Does someone now refer to “finite element simulations”? This point is also well taken. However, also in this case one has to admit that approximate solutions to large sets of non-linear differential equations formulated for a (non-existing) continuum under idealized boundary conditions is what it is: a model of nature but not reality. But us let calm down and render the discussion a bit more serious: current methods of ground state calculations are definitely among the cutting-edge disciplines in computational materials science and the community has learnt much from it during the last years. Similar aspects apply for some continuum-based finite element simulations. After all this report is meant to attract readers into this exciting field and not to repulse them. And for this reason I feel obliged to first make a point in underscoring that any interpretation of a research result obtained by computer simulation should be accompanied by scrutinizing the model ingredients and boundary conditions of that calculation in the same critical way as an experimentalist would check his experimental set-up

Liquid Crystal Anchoring Control and its Applications in Responsive Materials

Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of interests not only in the display industry, but also in the soft matter community, e.g., to direct colloidal assembly and phase separation of surfactants, and to actuate two-dimensional (2D) sheets into three-dimension (3D). The functionality and performance of LC materials extensively rely on the molecular ordering and alignment of LCs, which are dictated by LC anchoring at various boundaries. Therefore, this thesis focuses on the study of LC anchoring from both small molecule LCs and liquid crystal monomers (LCMs), which in turn guides my design of surface topography and surface chemistry to control formation of uniform LC defect structures over cm2 samples under complex boundary conditions. The ability to precisely embed defect structures in a LC material also allows me to exploit the responsiveness of LCs to create actuators and scaffolds to (dis)assemble nano- and micro-objects. Specifically, by exploiting the bulk disclinations formed in the nematic phase of 4-octyl-4’-cyanobiphenyl (8CB) surrounding the micropillar arrays, we demonstrate (dis)assembly of gold nano-rods (AuNRs) for dynamic tuning of surface plasmon resonance (SPR). Due to the highly temperature-sensitive elastic anisotropy of 8CB, the bulk disclinations and consequently the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling the LC system. Then we design and synthesize a new type of nematic LCMs with a very large nematic window. Therefore, they can be faithfully aligned at various boundary conditions, analogous to that of small molecule LCs. After crosslinking LCMs into liquid crystal polymers (LCPs), we are able to study the LC assembly, director field, and topological defects using scanning electronic microscopy (SEM) at the 100 nm resolution. We then turn our attention to direct LCM alignment through controlling of surface chemistry and topography. We demonstrate the essential role of surface chemistry in the fabrication of liquid crystal elastomer (LCE) micropillar arrays during soft lithography. A monodomain LCM alignment is achieved in a poly(2-hydroxyethyl methacrylate) coated polydimethylsiloxane (PDMS) mold. After crosslinking, the resultant LCE micropillars display a large radial strain (~30%) when heated across the nematic-isotropic phase transition temperature (TNI). The understanding of surface alignment in LCMs is then transferred to LCEs with embedded topological defects. On micron-sized one-dimensional channels with planar surface chemistry, LCMs can be faithfully oriented along the local channel direction. After crosslinking, the 2D LCE sheets show pre-programmed shape transformation to complex 3D structures through bending and stretching of local directors when heated above TNI. Last, we control LC alignment and defect formation on a flat surface simply by using chemical patterns. Planar anchored SU8 is photopatterned on homoetropically anchored dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) coated glass. By exploiting the pattern geometry, thus, boundary conditions, in combination with anisotropy of LC elasticity, we show that LC orientation can be precisely controlled over a large area and various types of topological defects are generated. Such defect structures can be further used to trap micro- and nanoparticles