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Dynamics of Lightweight Tensegrity-Inspired Metamaterials Fabricated with 3D-Printing
Get PDFTensegrity structures and lattices have been of interest in engineering applications for decades, with their dynamics becoming a thriving field of study. Tensegrities consist of structural members under purely axial loading, either tension or compression, and obtain their stability from prestress. They possess unique characteristics such as high strength-to-weight ratio, nonlinear behavior, and elastic response under severe deformation. Tensegrity lattices (or metamaterials) have been shown to exhibit appealing dynamic attributes such as continuous tunability with prestress, impact mitigation, energy trapping and lensing, and nonlinear wave propagation, to name a few. However, their pin-jointed and prestressed nature presents significant manufacturing limitations, especially in the formation of lattices with large numbers of tessellated unit cells. Therefore, experimental validation of the dynamics of tensegrity metamaterials has remained elusive. For lattices with tensegrity-like characteristics to be manifested for real-world applications, a method for producing tensegrity-like metamaterials at multiple length scales is needed.
In this thesis, we present a design for a 3D-printable tensegrity-inspired structure with the equivalent strain energy capacity and stress-strain response as a pin-jointed tensegrity. Using this structure as a building block for multidimensional lattices, we subject them to a range of dynamic loading conditions to study their response. First, we perform experiments and simulations to obtain the dispersion relations for 1D and 3D lattices. We demonstrate the lattices’ ability to continuously tune the dispersion characteristics (e.g., band gap and wave speed) under precompression. This trait shows potential for acoustic lensing and dispersive wave propagation. In 3D, we show that the lattice shows the same type of unique properties, such as faster shear speed than longitudinal speed, as pin-jointed tensegrity lattices. Next, we study the lattices under impact loading. Long-duration impact experiments on baseline unit cells and 1D lattices show their resilience to repeated deformation, elasticity, and load limitation behaviors. Short-duration impulse experiments and simulations exhibit a wealth of desirable properties, such as high force transmission reduction, highly dispersive wave propagation, tunable wave speeds, energy trapping, and redirection of energy. We demonstrate that these tensegrity-inspired metamaterials not only exhibit and experimentally demonstrate tensegrity-like characteristics, but open a new range of lightweight metamaterials with unprecedented dynamic properties.</p
Reduced-Order Model for Dynamic Soil-Pipe Interaction Analysis
Get PDFPipelines are very vulnerable infrastructure components to geohazard-induced ground deformation and failure. How soil transmits loads on pipelines and vice versa, known as soil-pipe interaction (SPI), thus is very important for the assessment and design of resilient pipeline systems.
In the first part, this work proposes a simplified macroelement designed to capture SPI in cohesionless soils subjected to arbitrary loading normal to the pipeline axis. We present the development of a uniaxial hysteresis model that can capture the smooth nonlinear reaction force-relative displacement curves (FDCs) of SPI problems. Using the unscented Kalman filter, we derived the model parameter κ that controls the smoothness of the transition zone from linear to plastic using published experimental data. We extended this uniaxial model to biaxial loading effects and showed that the macroelement can capture effects such as pinching and shear-dilation coupling. The model input parameters were calibrated using finite element (FE) analyses validated by experiments. The FDCs of the biaxial model were verified by comparison with FE and smoothed-particle hydrodynamic (SPH) simulations for different loading patterns: cyclic uniaxial, 0-shaped, 8-shaped, and transient loading. Accounting for smooth nonlinearity, hysteresis, pinching, and coupling effects, the proposed biaxial macroelement shows good agreement with FE and SPH analyses, while maintaining the computational efficiency and simplicity of beam-on-nonlinear-Winkler foundation models, as well as a small number of input parameters.
Next, this work presents analytical solutions for computing frequency-domain axial and in-plane soil impedance functions (SIFs) for an infinitely long rigid circular structure buried horizontally in homogeneous elastic half-space. Using Hankel— and Bessel—Fourier series expansion, we solved a mixed-boundary-value problem considering a harmonic displacement at the structure boundary and traction-free boundary condition at the half-space free surface. We then verified our analytical solutions using results obtained from FE simulations. The SIFs of a buried structure in a homogeneous elastic half-space calculated by these two approaches are in perfect agreement with each other. In addition, we used analytical solutions and FE simulations to comprehensively investigate factors that affect the SIFs in homogeneous and two-layered half-spaces, respectively. The parametric study shows that SIFs of buried structures in elastic half-space primarily depend on frequency of excitation, shear modulus and Poisson's ratio of the half-space, burial depth and radius of the structure. In a two-layered soil domain, SIFs depend also on material contrast and the distance from the structure location to the interface between soil layers.
Lastly, it demonstrates how the SIFs obtained previously can be incorporated into a reduced-order model to analyze SPI problems, specifically a straight pipe subjected to Rayleigh surface wave propagating through homogeneous and heterogeneous elastic half-spaces. Calculated displacement time histories at the control points are shown to agree well with those computed by direct two-dimensional FE analyses.</p
From Restoring Human Vision to Enhancing Computer Vision
Get PDFThe central theme of this work is enabling vision, which includes two subtopics: restoring vision for blind humans, and enhancing computer vision models in visual recognition. Chapter 1 first provides a gentle introduction to relevant high level principles of human visual computations and summarizes two fundamental questions that vision answers: "what" and "where." Chapters 2, 3, and 4 contain three published projects that are anchored by those two fundamental questions.
Chapter 2 introduces a cognitive assistant to restore visual function for blind humans by focusing on an interface powered by audio augmented reality. The assistant communicates the "what" and "where" aspects of visual scenes by a combination of natural language and spatialized sound. We experimentally demonstrated that the assistant enables many aspects of visual functions for naive blind users.
Chapters 3 and 4 develop data augmentation methods to address the data inefficiency problem in neural network based computer visual recognition models. In Chapter 3, a 3D-simulation based data augmentation method is developed for improving the generalization of visual classification models for rare classes. In Chapter 4, a fast and efficient data augmentation method is developed for the newly formulated panoptic segmentation task. The method improves performance of state-of-the-art panoptic segmentation models and generalizes across dataset domains, sizes, model architectures, and backbones.</p
"Otherness": Examining the Relationship Between Human and Non-Human
No full text[Introduction] Ursula K. Le Guin, in her essay “Deep in Admiration,” reminds us that “[l]ife … is symbiosis ‘all the way down’” (M5) and that “nothing is single in this universe” (M15). All relationships are reciprocal in nature. Human beings do not exist in a vacuum; the way we thoughtlessly consume the surrounding landscape before we even begin to consider what impact we might have on pre-existing ecosystems has consequences. Anna Tsing observes that “we are used to hearing music with a single perspective” (24), but in order to realize the multitude of chain reactions our decisions generate, we cannot settle with viewing complex ecological relationships through a singular perspective. The three following books allow us glimpses at the interactions and co-dependencies between human and non-human. From them, we come closer to understanding the reasons we have for dealing the damage we have done to this planet – but more importantly, we can begin to illuminate how we can cope with the destruction we have caused
Detection and Analysis of Martian Low-Temperature Geochemistry
Get PDFThe history of Mars is encoded in the geochemistry of ancient sedimentary Martian rocks and secondary phases. Recent landed missions have provided unparalleled datasets with which to investigate this geochemistry. Accordingly, this thesis is concerned primarily with the in-situ analysis of low-temperature Martian geochemical processes by landed missions, and the attendant Earth-based studies which enrich those in-situ investigations. There are five main studies reported here. The first is an analysis of a Mars-analog environment on Earth. Datasets similar to those that will be produced by the upcoming Mars-2020 rover are used to evaluate the ability of the rover to reconstruct a known paleoenvironment, to identify reference datasets that require further development, and to suggest operational modes that most efficiently use the rover’s resources. The second study is an in-situ noble gas analysis using the SAM instrument on the Curiosity rover to investigate a jarosite-containing sample using a two-step heating analysis for K-Ar dating. The jarosite likely formed at 2.12±0.36 Ga while plagioclase in the sample formed at 4.07 ± 0.63 Ga, indicating that liquid water interactions continued in Gale crater well past the end of the Hesperian period. The following chapter details another noble gas analysis, focusing on cosmogenic dating of surface exposure. In contrast to <100 Ma exposure ages observed at the floor of Gale crater, exposure ages exceeding 1 Ga are detected on the flanks of Mount Sharp. These ages indicate Mount Sharp formed during the Hesperian and has been largely unchanged in the intervening 3.1 Ga. The next study is a reevaluation of the data used to identify the presence of perchlorate in Gale crater. These data suggest that perchlorate is indeed present, but that it must be Amazonian in age, suggesting that rare surface wetting events have caused leaching of this soluble ion into the bedrock. The final study reports the development of a technique for measuring the isotopes in perchlorate using Orbitrap mass spectrometry on Earth, allowing investigation of the formation processes that impact the chlorine isotope ratio of this molecule on both Earth and Mars
Tracking Volatile Elements in Protoplanetary Disks and on Planetary Surfaces
Get PDFThe formation of planets begins with collisions of tiny, micron-sized, dust grains. These grains reside in structures known as protoplanetary disks, rotating disks consisting of gas and dust that encircle young protostars as a natural outcome of star formation. Although the processes of planet formation and evolution take place over millions and billions of years, in our limited view we can only see snapshots of the different stages. Many of the formative processes are difficult, if not impossible, to observe directly. However, evidence of these events exists in the chemical composition of the bulk material and surfaces of planets themselves, the gas and solid components of protoplanetary disks, and planetary debris such as asteroids and comets. This thesis utilizes modeling and observations of the carbon and nitrogen content of protoplanetary disks to shed light on key factors that control the formation and chemical composition of planets. In addition, this thesis advances techniques for the elemental analysis of planetary surfaces facilitating the detection of salts on the surface of Mars.
Chapter 2 estimates the maximum potential destruction of solid, refractory carbon in protoplanetary disks in an effort to explain the lack of carbon found in meteorites and the bulk silicate Earth relative to the interstellar materials that seeded their formation. In a T-Tauri disk assuming uniform turbulence and passive heating from stellar photons destruction of refractory carbon sources via oxidation and UV photolysis is limited to the warm, photochemically-active disk surface layers. Exploration of distinct disk environments, considering non-idealized mass transport or enhanced disk heating due to active stellar mass accretion, is needed to explain the widespread lack of carbon in rocky solar-system bodies.
Chapters 3 and 4 present spectral observations by the Atacama Large Millimeter/submillimeter Array (ALMA) of mature, 5-11 Myr-old, protoplanetary disks in the Upper Scorpius region that indicate diverging behavior of the key carbon and nitrogen species in the disk gas as disks evolve. Selective depletion of CO from the gas may cause disk gas masses to be underestimated if based on CO measurements alone and further investigation of additional gas tracers is warranted.
Depletion of CO from the gas in the outer regions of disks observed by ALMA may be the result of sequestration of carbon into less volatile species such as CO2 and CH3OH. Chapter 5 explores the fate of CO2 and CH3OH ices entering the inner regions of protoplanetary disks. Carbon returns to CO in unshielded transparent regions of the inner disk surface, consistent with infrared observations, but carbon reservoirs in the disk midplane may be distinct depending on the efficiency of mass transport in the disk.
Chapter 6 examines the abilities of the Laser-Induced Breakdown Spectroscopy (LIBS) instrument ChemCam on the Mars rover Curiosity in regards to the detection of salts. LIBS analysis of a set of prepared sample pellets containing decreasing concentrations of salt identifies elemental emission lines of Cl, C, and S that are sensitive to changes in chloride, carbonate, and sulfate salt concentrations, respectively, and provides detection limits for ChemCam measurements of these salts.</p
Seismic Waveform Modeling of Natural Hazards and Sharp Structural Boundaries
Get PDFSeismic waveform modeling is a powerful tool for seismologists to learn about the Earth’s dynamics, either how a natural hazard evolves with time, or the long-term deformation process governed by fine-scale structures along boundaries inside the Earth. Knowing that the recorded seismograms reflect the cumulative effects of the source, the earth structure, and the instrument response, I carefully study the characters of the seismograms such as the arrival time, amplitude, frequency content, and multipathing, for several settings, with the goal of improving our description of either the source or the structure.
Part 1 focuses on source characterization for non-earthquake natural hazards. I perform moment tensor inversions for the large seismic events at the Kilauea summit to infer the triggering mechanisms for the explosive eruptions and caldera collapse during the 2018 eruption sequence. The addition of infrasound data is crucial to resolve the uncertainties in the moment tensor solutions, particularly the depth and the necessity of the isotropic component. I also present a new mechanistic model to describe the seismic signal from debris flow and apply to the 2018 Montecito debris flow in which key parameters such as boulder size and flow rate and their evolution during the event can be determined using a single seismic station.
Part 2 consists of three studies spanning from the crust to the core, where forward waveform modeling is used to improve our understanding of the sharp structural boundaries and their role in observed ground motion and long-term dynamics. Numerical simulation and dense array analysis are used to model the direct effect of shallow basin structures in Los Angeles on shaking duration and reveal the importance of basin edges and attenuation model for predicting ground motion during large shallow ruptures. I also identify a strong velocity contrast in the lower crust – upper mantle structure across the San Andreas plate boundary system and, given velocity is a proxy to lithospheric strength, the sharp contrast can have a significant role in modulating the long-term plate deformation. Lastly, we observe strong waveform anomalies at the edge of the Pacific Large Low Shear Velocity Province (LLSVP) which have great importance in governing deep mantle convection. To fit the observation, I propose a model of ultra-low velocity zone (ULVZ), plume and slab interacting at the edge of the LLSVP. The configuration and location of this ULVZ-plume-slab interaction is important in inferring the mechanism behind plume generation which gives rise to the Hawaii-Emperor Seamount chain.</p
Small Molecule Reactivity of Trisphosphine-Supported Iron and Cobalt Complexes
Get PDFThe work described in this thesis emphasizes accessing novel reactivity patterns in the activation of carbon monoxide, dinitrogen, and dihydrogen by leveraging phosphine-supported iron and cobalt complexes. In Chapters 2 and 3, systems that access CO and N2 reductive functionalization from highly reduced Fe-hydride precursors are described. These systems access productive C-H and N-H bond forming steps from hydride precursors that ultimately allows for the liberation of four-electron reduced products through novel chemical pathways. Chapter 4 describes an unusual example of a terminal cobalt carbyne through the O-functionalization of a carbonyl complex. Next, in Chapter 5, we consider electronically unusual examples of dihydrogen complexes and explore their propensity for accessing H-atom and hydride transfer. Finally, to conclude, Chapter 6 details our synthetic efforts targeting the synthesis of a terminal Fe-carbide complex through the cleavage of a thiocarbonyl precursor.</p
Fracture and Toughening of Brittle Structures with Designed Anisotropy
Get PDFDespite good thermal and chemical properties, the use of ceramic materials in structural applications is limited by their inherently brittle nature. Efforts have been made to improve the toughness of ceramics through composite design, but recent developments in net shape processing such as additive manufacturing have significantly expanded this design space. Where composite topologies and morphologies were previously limited by material composition and thermodynamics, tools like 3D printing now allow for the design of composite structures of nearly any shape or arrangement.
This work seeks to understand how these processing advances might be utilized to improve the toughness of brittle composites by exploring how previously inaccessible anisotropic inclusion structures might influence fracture behavior. The study begins with the evaluation of printed photopolymer structures as model brittle materials. First, printed structures are used to explore how elastic contrast between inclusions and matrix can affect crack propagation and improve toughness. Here, anisotropy presents an opportunity to achieve similar toughness to isotropic structures at smaller volume fractions by virtue of topologies that only exhibit toughening only in a singular direction, but require significantly less material to do so. Next, the effect of anisotropic voids is explored as a means of controlling crack nucleation and growth. With consideration of both compliance and directional propagation, a "fracture diode" that exhibits controlled, predictable fracture 100% of the time can be realized.
After exploring brittle polymers, ceramics systems with similar toughness and higher stiffness are considered. First, a model layered system of mica is explored, where wedge splitting can be used achieve stable crack growth. This allows for the evaluation of how changes in compliance can improve the interlayer toughness without directly interacting with the crack. Finally, this study extends further into ceramics by exploring silicon oxycarbide (SiOC) truss structures and truss elements produced from 3D printed preceramic polymers. In addition to considering the material itself, changes in truss structure are explored as a means of changing deformation mode, and by consequence, failure strength. These model experiments suggest that if trusses are compatible, they can be interchanged to control failure of the bulk structure.
This study demonstrates how designed heterogeneities with anisotropic structure can be used to both enhance the toughness of brittle composites as well achieve a greater degree of control over both crack nucleation and propagation in brittle systems where predicting failure is otherwise difficult. Looking forward, new processing tools like additive manufacturing present major opportunities for expanding the design space of brittle composites to achieve higher toughness and better fracture control than previously available. These new techniques may be able to expand the mechanical viability of ceramics, and make them better suited to mechanically demanding applications in the future.</p
Kinetic Studies of Hydrogen Oxidation by Cobaloximes and Synthesis, Spectroscopy and Boronation of a New Heteroleptic Ruthenium Cyanide Complex
Get PDFCobaloximes are macrocyclic complexes well-studied as homogeneous catalysts for hydrogen evolution, but they are also competent at the microscopic reverse reaction, hydrogen oxidation. Kinetic studies of Co(dmgBF2)2L2 reacting with hydrogen and base reveal a rate law that is second-order in cobalt and first-order in H2, indicating that the mechanism of H-H bond breaking is homolytic.
The reduction potentials of metal cyanide complexes can be tuned by appending boranes to the N-terminus. By boronating ruthenium cyanide complexes containing diimine ligands, the RuII/III couple can be tuned without drastic modification of the diimine0/- couple. A new member of the [Ru(dimmine)(CN)4]2- family is synthesized with the ligand 4,4’-bis(trifluoromethyl)-2,2’-bipyridine (CF3bpy) and boronated, resulting in a molecule with two reversible redox events separated by 3.2 V.</p