Mechanical
and Materials Engineering
Abstracts 2002-2003
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479 EBU-II
Professor Charles R. Steele
Mechanics and
“Recent Progress Toward Understanding Hearing”
Three topics will be summarized.
(1) The function of the eardrum
and middle ear is to resolve the acoustic impedance mismatch between the air of
the outside world and the fluid of the inner ear. Without an impedance matching
device, very little acoustic energy would be absorbed by the inner ear and hearing
would be severely limited. Although the role of the middle ear is clear, it was
a mystery how the eardrum accomplishes this task over the audible frequency
range. In the present work, a computer simulation of the cat eardrum was
constructed. For the first time, the vibrations of the eardrum were fully
coupled to the acoustics of the ear canal and the dynamics of the middle ear
bones.
(2) A remarkable feature is the
feed back of energy in the inner ear, due to piezoelectric behavior of the
outer hair cells. This however saturates, which causes the generation of
nonlinear distortions, consisting of harmonics and combination tones. A model
for the distributed sensors and actuators as a simple "feed-forward" mechanism, does rather well in representing the effect.
(3) The specific fluid excitation
of the primary receptors, the inner hair cells, is investigated with a Fast4
simulation of the details of anatomy of the cochlea.
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479 EBU-II
Michael Paukshto, Ph.D.
Optiva, Inc.
“Cascade Crystallization of Nano-Film Optical Materials”
Optiva Inc. is a leading developer and manufacturer of advanced self-assembling materials for use in a wide range of optical applications. Benefiting from over two decades of successful large-scale research in supramolecular engineering, Optiva is the first to deliver a commercial mass-production process for optical nanomaterials. These materials are designed to self-assemble into liquid crystals in solution and then self-align into very thin crystalline films (TCF) when coated onto surfaces. Optiva's first family of TCF products is targeted at the flat panel display industry, offering significant cost and performance advantages over traditional optical film alternatives. It won the 2002 SID/Information Display Magazine Display Material or Component of the Year, Gold Award, for its Optiva Thin Crystal Film (TCF™) Polarizer. The Society of Information Display (SID), the largest international professional organization for the display industry, announced the award in the December issue of Information Display Magazine. Other category winners included, Dupont, Eastman Kodak, Samsung and Sony.
This presentation will provide an overview of TCF technology and will describe a new crystallization process as well as material structure, and associated coating equipment.
The materials are based on polycyclic aromatic compounds. Chemical modification of compounds changes hydrophobic-hydrophilic balance of disk-shaped molecules and makes them water-soluble with aggregation into rod-like supramolecules in aqueous solution and subsequent formation of supramolecular lyomesophases. Coating techniques provide control of crystallographic axes direction of the final crystal film. Shear force that is applied during deposition controls alignment of supramolecules. Structure of liquid material, wet coating and resulting 100-700 nm thin solid crystal films has been studied optically and by X-ray diffraction.
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479 EBU-II
ProfessorJoanna McKittrick
Mechanical and
“Surface Studies of Titanium for
Implant Applications”
Titanium has been successfully
used for decades in dental and orthopedic implants, but the exact mechanism of
successful osseointegration and biocompatibility has
not been determined. The hypothesis is
that the biocompatibility of titanium involves
an interaction between the surface layer of titanium dioxide on the metal
implant and reactive oxygen mediators of the inflammatory response in model
systems with varying degrees of complexity.
Different forms of surface oxide can exist on titanium, and the
relationship between defined surface oxides and anti-inflammatory response will
be discussed. We recently demonstrated that peroxynitrite,
a highly reactive compound and inflammatory mediator produced in vivo by the reaction of the free
radicals nitric oxide and superoxide, is
significantly degraded by the presence of titanium oxides. Furthermore, there appears
to be a strong correlation between the ability to degrade peroxynitrite
and ultimate in vivo
biocompatibility. The long-term goal is
to extend the knowledge gained from the titanium studies to develop new
biocompatible materials with superior mechanical and material properties than
that of titanium.
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479 EBU-II
Luke
L. Hsiung
Lawrence
Chemistry and Materials Science Directorate
“A DYNAMIC DISLOCATION MECHANISM FOR SHOCK-INDUCED DISPLACIVE
TRANSFORMATIONS “
The occurrence of displacive transformations in tantalum [which has bcc
structure and exhibits no phase transformation under ambient and static
pressures (up to 100 GPa)]
induced by high strain-rate shock deformation has been investigated. While deformation twinning of {112}<111>-type was found to occur when shocked at 15 GPa, a shock-induced martensitic
transformation was discovered when shocked at 45 GPa. The martensite
phase has a hexagonal structure and the lattice parameters are ah »
0.468 nm and ch » 0.286 nm (ch/ah = 0.611). The orientation relationships between the martensite and parent phases are determined to be {10-10}h||{211}b,
[0001] h||<111>b
and <1-210>h||<0-11>b. Since both deformation twinning and martensitic transformation occurred in the {211}b planes associated with high resolved shear
stresses, it is suggested that both processes can be regarded as alternative
paths for the displacive transformations occurred in
shocked tantalum. While deformation
twinning occurs as a result of a/6<
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Monday, May 12, 2003
11:00 a.m.
479 EBU-II
H. Thomas Hahn
Raytheon Professor and
“Polymer Nanocomposites”
Because of their small size, nano-scale reinforcements are a much better choice than their micro-scale counterparts in improving composite properties. Such improvements are believed to be the combined result of benign stress distribution and synergistic interfacial interactions that are possible in the nanometer range.
Currently, a
wide variety of nanoreinforcements are
available and still more are coming into the market as the nanotechnology
progresses. Nanoparticles,
being 0-dimensional, do not have an efficient
reinforcement morphology but can be used to add other functionalities as
functional materials are readily available in particulate rather than bulk
form. An extensive research effort is
The present talk reviews the state of the art of nanocomposites, and discusses the potential of using graphite nanoplatelets (GNPs) as a new reinforcement phase. Being 2-dimensional, GNPs provide better reinforcement efficiency than 1-dimensional carbon nanotubes do. Unfortunately, however, the currently available GNPs are too thick to provide optimum reinforcement. Methods of synthesizing GNPs a few nanometers thick through intercalation, exfoliation and delamination will be discussed in the talk.
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Monday, April 21, 2003
11:00 a.m.
479 EBU-II
Professor Marc Madou
Chancellor Professor, UC Irvine
Department of Mechanical and Aerospace Engineering
Department of Biomedical Engineering
University of California, Irvine
“Nanotechnology: Icarus
Revisited ?”
The combination of natural polymers such as proteins and nucleic acids with MEMS and NEMS promises the advent of a totally new class of devices such as sensors and actuators with applications in diagnostics, responsive drug delivery, biocompatibility, self assembly etc. Proteins and nucleic acid are information rich molecules with structural and electrical properties making their incorporation in the human manufacturing arsenal an attractive proposition. This combination has become possible as today both top-down traditional manufacturing (e.g., MEMS and NEMS) and novel bottom-up manufacturing can realize components overlapping in size. Examples to illustrate the tremendous potential of merging top-down and bottom-up manufacturing techniques will be presented. These examples are culled from the fields of molecular diagnostics, responsive drug delivery systems, protein and DNA structural elements and sensors and actuators and molecular self-assembly. In molecular diagnostics we conclude that an important avenue to success is the merging of DNA arrays with microfluidics to achieve sample to answer systems. Future responsive drug delivery systems are seen as a culmination of results from genomics and proteomics coupled with implantable telemetric devices. This and other developments will cause a renewed interest in in-vivo diagnostics. Natural polymers will become part of our manufacturing arsenal and when using building blocs in the nanometer range a fundamental understanding and the use of molecular self assembly is a must for future progress. While biomimetics in the macro domain often has led to failure in the past (airplanes do not flap their wings as birds do, see Icarus legend), we believe that biomimetics in the nanodomain will succeed. Nature indeed has worked much longer on arriving at a biological cell than it did at making birds, trees or humans: nature excels at engineering in the nanodomain. While top-down manufacturing approaches will continue to prevail over the next two decades we will start seeing hybrid solutions, such as the use of flexible materials (e.g., hydrogels) rather than stiff building materials (e.g., steel and Si). There will be more emphasis on non-Si, modular and “beyond batch” techniques such as pick and place, drop delivery, lamination, etc. The next big breakthrough will be continuous manufacturing, perhaps rendered possible through molecular self-assembly (continuous fabrication rather than batch fabrication!).
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479 EBU-II
Professor T. I. Zohdi
Department of
The focus of this work is to highlight some nonstandard inverse problems in micro-macro mechanics. Three objectives are addressed:
1) To design materials composed of randomly dispersed particulates suspended in a homogeneous binding matrix, where the objectives are to deliver prescribed macroscopic effective responses while simultaneously obeying local constraints that reflect the distortion of the microscale stress fields, as well as the likelihood for fatigue damage,
2) To design ``swarming fluid materials'', where the overall goal is to design autonomous self-correcting particulate groups whose goal is to reach a target, possibly ``guarded'' by obstacles, in a minimum amount of time and
3) To determine the ambient conditions under which flowing grains of interstellar dust in a gaseous, hydrogen-rich, atmosphere, can fuse, due to the high strain rates during binary particle impacts, coupled with thermochemical reactions.
All of the mentioned are treated with a general nonconvex optimization strategy, based on a nonderivative statistical genetic algorithm.
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479 EBU-II
Otis Walton
Grainflow Dynamics
“Effects of interparticle
forces on micron-scale pulmonary pharmaceutical powders”
Dry pulmonary powders are an attractive alternative to injections for systemic delivery of macro-molecules, proteins and peptides. Increasingly, pharmaceutical companies are also considering local pulmonary delivery of powders or aerosols for small molecules, anti-infectives or antibiotics, in order to increase the local concentration of the therapeutic substance while reducing potential systemic load. As particle size is reduced, the effects of surface forces can dominate the bulk powder behavior. Yet, dispersiblility and degree-of-agglomeration in aerosols appear to improve with decreasing particle size in these powders. Explanations of such counter-intuitive results, and a discussion of the sensitivity of fine powders to handling will be presented.
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Monday, March 10, 2003
11:00 a.m.
479 EBU-II
Tim Wright
Army Research Laboratory
Computational
solid mechanics (CSM) for impact situations, as it exists today, does not have
predictive capabilities that are comparable to those of computational fluid
dynamics (CFD). The most important reason for this is the lack of high quality
physical models for the damage and failure processes that occur during high
speed impact. Because these processes are expected, they must be part of design
and optimization.
An approach for modeling impact
damage in metals, based on physical modeling of adiabatic shear bands and void growth,
will be presented. Scaling laws for damage from adiabatic shear bands have been
developed over the past decade following detailed examination of the thermomechanical processes (1). Recently these laws have
been adapted for efficient use in large scale computations and show great
promise for capturing the essential aspects of diverse experiments ranging from
laboratory scale to full scale impacts (2). A similar approach for ductile void
growth and spall has also been initiated (3). A
careful analysis of these physical processes has revealed many new and
interesting features, which we hope to capture for large scale computations
through scaling laws, as well.
This approach is expected to lead to
damage and failure models that are based on the essence of the physics, rather
than on fitting of phenomenological models to large data bases. An added virtue
is that it tends to place a premium on accurate measurement and modeling of
homogeneous processes, rather than on failure as a separate phenomenon. Finally,
the approach also has implications for design and interpretation of
experiments.
1. Wright TW (2002). The physics and
mathematics of adiabatic shear bands.
2. Schoenfeld SE and Wright TW. A failure criterion based on material instability. To appear (2003) in International Journal of Solids and Structures.
3. Wu XY, Ramesh KT, and Wright TW. The dynamic growth of a single void in a viscoplastic material under transient hydrostatic loading. Journal of the Mechanics and Physics of Solids, 51 (2003) 1-26.
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Monday, February 24, 2003
11:00 a.m.
479 EBU-II
Professor
Panayiotis Papadopoulos
Department of Mechanical Engineering
University of California, Berkeley
"Experiments
and Modeling of the Superelastic Effect in
Shape-Memory Alloys"
This work concerns a combined experimental, theoretical and computational study of the superelastic alloys under multiaxial loading conditions. The experimental component involves a series of tension-torsion tests on thin-walled polycrystalline Nitinol tubes. A multivariant constitutive model is formulated in finite deformations and incorporates the effect of texture. The numerical implementation is based on the constrained minimization of the Helmholtz free energy with dissipation. Finite element-based simulations are conducted for thin tubes of Nitinol under tension-torsion, as well as for a simplified model of a biomedical stent.
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479 EBU-II
Peter Lomdahl
Theoretical Division
"Large-scale Parallel
Simulations of Phase-transitions in Metals"
Recent advances in computing technology has allowed direct simulations at the classical level of the the response of materials to shockwave loading and unloading. In dense monatomic, chemically unreactive fluids, the profile or structure of a shock wave is rather boring, being well described by viscous flow. In solids, on the other hand, the structure is far more complex, being dominated by plastic flow mechanisms as well as phase transformations. At Los Alamos National Laboratory we have developed a high performance parallel MD code (SPaSM) which has been designed to perform very large scale simulations with 10^8-10^9 atoms. I will discuss some of the recent advances we have made in simulations of shock waves and related phenomena, including plastic deformation, phase-transitions, and fragmentation. As experimental observations become more and more refined, and molecular-dynamics simulations become larger, even approaching the mesoscale, fruitful overlap is achievable in the near future.
Peter Lomdahl
is the Deputy Group Leader of the Condensed Matter and Statistical Physics
Group at Los Alamos National Laboratory. He has conducted research at
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479 EBU-II
Dr.
Syracuse University
"Fluid
mechanics and optical fiber defects"
Two experiments involving optical fibers, both of which occur at relatively high speeds, will be discussed. In the first experiment I will describe how air becomes entrapped into a coating via an extensional shear flow process that culminates in phenomenon known as tip streaming. Optical microscopy of the dynamic air-coating interface at framing rates of 500 per sec. reveal details of the entrainment process hitherto unknown. Methods for preventing the entrapment occurring will also be briefly described.
The second experiment concerns an event known colloquially as a fiber fuse. These events occur when fiber transmitting power densities greater than about 1 MW/cm2 are in some way physically abused. The result is a brilliant, highly visible, plasma-like disturbance that propagates toward the optical source at speeds of order 1 m.sec.-1 leaving in its wake a trail of voids. I will show that, as in the analogous electrical fuse, the resultant damage is a manifestation of a capillarity driven Rayleigh instability. Estimates for the fuse propagation speed and the local thermal field will be compared with observations.
Dr. Peter Simpkins did his
undergraduate work in aeronautical engineering at
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479 EBU-II
Ted Belytschko
Department of Mechanical Engineering
Northwestern University
"Arbitrary discontinuities and level sets in finite
elements methods"
A
technique for modeling arbitrary discontinuities in finite elements is
presented. Both discontinuities in the function and its derivatives are
considered. Methods for intersecting and branching discontinuities are
given. Discontinuities in functions are applicable to phenomena such as
cracks, shear bands, and shocks in compressible flow. Discontinuities in
derivatives of functions are needed when the element edges do not coincide with
interfaces between material or between different
phases in fluid flow. The discontinuous approximation is constructed in
terms of a signed distance functions, so level sets can be used to update the
position of the discontinuities. A standard displacement Galerkin method is used for developing the discrete
equations. For the special case of cracks, the geometry is defined by two sets
of orthogonal level set functions or by vector level sets. The following
applications are given: three dimensional crack growth,
a representative volume element of a composite material for determining its
material properties, a jointed rock mass, growth of a bubble in a liquid and
solidification of a liquid. Some applications to the mechanics and
fracture of nanotubes are also described.
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CMRR Auditorium
Dr. Robert S. Vecchio
Lucius Pitkin, Inc.
"
Following
the collapse of the
Dr.
Vecchio is a partner at Lucius Pitkin,
Inc. (LPI), a 30 employee engineering consulting firm, which has been located
in
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479 EBU-II
Professor G. Ravichandran
Graduate Aeronautical Laboratories
California Institute of Technology
The development
of bulk metallic glasses, with
desirable mechanical properties such as strength (~2 GPa)
and elastic limit (~2%), as well as good glass forming and shaping abilities,
offers opportunities to utilize this class of solids as structural amorphous
materials. In this talk the mechanical
behavior of a bulk metallic glass Zr41.2Ti13.8Cu12.5Ni10Be22.5
(Vitreloy 1) and its composite (b-phase Vitreloy composite, i.e., Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5)
is discussed. The stress-strain relations for Vitreloy
1 over a broad range of temperatures (up to the crystallization temperature)
and strain rates (10-4 to 2x103 s-1) were established in uniaxial
compression. The effect of strain rate
and temperature on steady state flow stress, viscosity, and peak stress, as
well as the effect of jump-in-strain-rate on the stress-strain behavior, are
discussed. Based on the experimental
results, boundaries between the main deformation modes are proposed involving
Newtonian flow and nonlinear flow resulting in homogeneous deformation and
shear-localized failure. A fictive stress model is shown to capture the
stress-strain behavior over the entire range of strain rate and temperature. A
dynamic indentation experimental setup was developed to evaluate the
high-strain-rate inelastic post yield deformation behavior of Viteloy 1 and its b-phase composite. Both materials are
found to be strain rate insensitive up to 2,000 s-1. Numerical simulations of the indentation
experiments reveal that both materials are pressure (or normal stress)
dependent. To further examine the inelastic deformation of amorphous alloys at
room temperature, multiaxial compression experiments
using a confining sleeve technique were performed. It is found that the
behavior of Vitreloy 1 follows a pressure dependent Tresca criterion. In contrast to the catastrophic shear
failure behavior observed in uniaxial compression, Vitreloy 1 accommodates large (>10%) inelastic
deformation through the formation of multiple shear bands.
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479 EBU-II
"Ionic Polymer-metal Composites as Soft Actuators and
Sensors: Experiments and Nanoscale Modeling"
Ionomeric polymer-metal composites (IPMCs)
are soft actuators and sensors. They generally consist of a thin
polyelectrolyte membrane, plated on both faces by some noble metal, generally
platinum with a layer of finishing gold, and neutralized with necessary amount
of counter-ions, balancing the charge of anions covalently fixed to the
membrane. When a thin strip of an IPMC membrane in the hydrated state is
stimulated by an application of a small (1–3 V) alternating potential, it
undergoes a bending vibration at the frequency of the applied voltage,
generally no more than a few tens of Hertz.
Under an applied direct current (DC), the composite quickly bends toward
the anode, then slowly relaxes in the opposite
direction. When the same membrane is suddenly bent, a small voltage of the
order of millivolts is produced across its faces.
Hence, IPMCs can serve as soft actuators and sensors.
Through a systematic experimental characterization of these composites under
various conditions, potential nano-scale coupled electrical-chemical-mechanical
mechanisms responsible for the observed behavior of the materials, are
identified and mathematically modeled.
Examples of the model results will be presented for illustration.
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479 EBU-II
"Cellular metals for lightweight multifunctional
structures and platform protection"
Cellular
metal structures have mechanical and thermal attributes that promote their use
as core elements in lightweight structural panels, with application to advanced
aerospace platforms, expeditionary vehicles, surface ships and underwater naval
vehicles. These materials, comprising either stochastic foams (open-cell or
closed-cell) or open period truss networks, have been implemented in sandwich
panel construction. New approaches to the fabrication of cellular metal
structures have been developed, with cost, performance and durability
advantages. Novel core fabrication technologies utilizing in situ foaming,
metal textile, perforated sheet and discrete pin approaches will be
discussed. The mechanics of the various
structures will be described, and experimental measurements will be compared
with theoretically-derived optimal designs, for panels subjected to both static
and dynamic loading. Performance metrics
are utilized to compare various core topologies, showing that open cellular
systems with topologically optimized core elements may outperform existing
sandwich panel construction techniques (honeycomb panels and stringer-stiffened
plates), while enabling other functionalities such as integrated sensing and
thermal management. A key concept in the
design of such systems is how the core elements respond to various loading scenarios
(aerodynamic pressure loading vs. blast loading). Tailoring the response of the
core truss elements in terms of bending dominated behavior or
stretching/compression dominated behavior is a critical consideration. The
mechanical performance is examined as a function of design variables including
core density, truss element geometry, and geometrical aspects of the periodic
structure. The ability to locally tailor the core geometry, and the resulting
effects on the failure modes under loading, will be described. Recent
developments (such as the bombing of the USS Cole) have shifted the focus of
our work from lightweight construction to the use of cellular metal systems for
platform protection (both ballistic and blast environments). Differences in the optimal periodic core
topologies for lightweight structures blast-resistant materials will be
discussed, and
concepts for the development of integrated armor systems will be
introduced. The talk will also include a
brief discussion of the development of novel morphing structures utilizing
similar cellular metal fabrication and topological design concepts.
After completing undergraduate studies at the
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1:00 p.m.
Reissner
Conference Room, SERF 341
"Modern Developments in
E. Reissner's Tension Field Theory"
In one of his first papers, E. Reissner developed a modified version of membrane theory
called 'tension-field theory' to account for wrinkling in thin sheets under
twist and shear. His efforts gave rise
to a specialized literature in Structural Mechanics. In this talk we reconsider
this subject using the modern tools of Nonlinear Elasticity. We show that
tension fields possess a number of unique physical and mathematical properties.
Some combined analytical and numerical results are obtained for complex
three-dimensional finite deformations of thin membranes with tension fields
included. These are shown to exhibit many of the complex qualitative features
observed in simple experiments.
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479 EBU-II
George T. (Rusty) Gray III
Materials Science and Technology Division
"Dynamic Deformation and Modeling of Inert and
Energetic Materials"
The
mechanical response of materials has become of increasing importance in a range
of problems. To attain a truly
predictive capability to enable accurate simulations of dynamic impact, shock,
and high-rate loading phenomena applications
require a linked experimental, modeling, and validation program. The
derivation of physically-based models is only achieved via close collaboration
between experimentalists and modelers.
Small-scale experimentation is validated through the use of integrated
scaled-up tests, which are tied to finite-element (FE) simulation and analysis.
This cross-linked process affords the chance to build the necessary bridges
across the length scales from microstructure to full-scale structures through a
linked and validated scheme to match physical phenomena modeling to engineering
response. Efforts to attain these objectives are assessed and new advances
highlighted in approaching this integrated response.
Dr. Gray is a Laboratory Fellow and Team Leader in the Dynamic Materials Properties Section in MST-8 at Los Alamos National Laboratory. His research is focused on structure/property relationships during the deformation of materials, in particular in response to high-strain-rate and shock deformation. The scope of the research on substructure evolution and mechanical response under dynamic loading conditions has included: metals, alloys, intermetallics, composites, and polymers. His research has focused on utilizing high-rate Split-Hopkinson bar and shock recovery experiments as part of an interdisciplinary research team combining real-time experiments, theoretical modeling, and post-shock material studies to investigate defect generation and storage during high-strain rate and shock loading. Dr. gray has developed and promoted the use of "soft" shock recovery techniques for systematically studying the influence of shock-wave loading parameters on post-shock material response. The generation of unique defect structures, such as deformation twins, microbands, and pressure-induced phase products (such as w-phase in Ti), remains an active topic of his research.
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479 EBU-II
Professor Kaushik Bhattacharya
California
Institute of Technology
"Shape-memory
alloys: From microstructure to macroscopic properties"
Shape-memory alloys
are interesting materials due to their unusual ability to "remember"
particular shape. This ability arises
from the characteristic microstructure that they form. This talk will review the progress that has
been made in understanding why these materials form these microstructures, how
these microstructures give rise to the macroscopic properties and how one can
use this understanding for the development of new materials and
applications. Theoretical tools that
enable this multi-scale analysis will be described and experimental validation
of theory will be discussed.
Kaushik
Bhattacharya is currently a Professor of Applied Mechanics and Mechanical
Engineering at the California Institute of Technology (Caltech), and has been on
the faculty there since 1993. He
received his B.Tech degree in Mechanical Engineering
from the Indian Institute of Technology,
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479 EBU-II
Professor Nasr M. Ghoniem
Mechanical and Aerospace Engineering Department
UCLA
"DESIGNING MATERIALS WITH POWERFUL
COMPUTERS AND THE
MULTISCALE MODELING APPROACH"
Computers are becoming ever more powerful, theory and software more sophisticated and at the same time, material volumes in engineered devices are shrinking! This state of affairs is presenting vast challenges to the science and engineering community. Recently, a “multiscale modeling strategy” has been proposed, in which material behavior is computationally simulated at a hierarchy of length and time scales. The promise of this approach is that we can “engineer” materials from the atoms up for more exotic applications, with new research opportunities to meet this formidable challenge. In this talk, we describe the strategy of multiscale modeling in the context of plasticity and fracture of structural materials in future fusion energy systems. We briefly discuss the hierarchy of computational methods employed in this approach, and then focus on the role of dislocations as the elementary carriers of plasticity and fracture. The method of 3-d dislocation dynamics, which is based on the equations of motion of space curves, is derived from variational principles. Computer simulations for the plastic deformation of nano- and micro-volumes of materials will be shown to illustrate applications of computational dislocation dynamics to the study of plastic instabilities and shear bands in irradiated materials. We will finally discuss a number of unsolved problems and challenges in this emerging field of multiscale materials modeling.
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479 EBU-II
Michael Paukshto,
Ph.D.
Optiva, Inc.
"New Nano-Film
Optiva Inc. has pioneered the development of thin
crystalline film (TCF) optical coatings for use in information displays and
other applications. TCF technology has
now migrated out of the R&D stage into manufacturing and is currently being
incorporated into new display products. This presentation will provide an
overview of TCF technology and will describe material structure, optical properties
and characterization, material processing and associated coating equipment.
The materials are based on polycyclic
aromatic compounds. Chemical modification of compounds changes
hydrophobic-hydrophilic balance of disk-shaped molecules and makes them water-soluble
with aggregation into rod-like supramolecules in
aqueous solution and subsequent formation of supramolecular
lyomesophases. Coating techniques provide control of
crystallographic axes direction of the final crystal film. Shear force that is
applied during deposition controls alignment of supramolecules.
Structure of liquid material, wet coating and resulting 100-700 nm thin solid
crystal films has been studied optically and by X-ray diffraction.
Michael V. Paukshto
received Ph. D. and Dr. Sci. from
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479 EBU-II
E.M. Bringa
Chemistry and Materials Science Directorate
Lawrence
"Simulating shock-induced plasticity in
single crystal Cu"
Large scale molecular dynamics simulations with the embedded atom method
(EAM) potentials are used to simulate shock propagation in single crystal Cu
for different crystal orientations. Simulations were performed for a wide range
of pressures (2-300 GPa) and
agree well with recent experimental data. Large anisotropies are found for
shock propagation in Cu, with different plasticity mechanisms along different
directions. We have observed for the first time the formation of nano-twins in
shocked Cu. Due to post-shock evolution they could transform into the
micro-twins seen in experiments at similar shock pressures. Experiments for
"single crystals" deal with samples having a small initial defect
density, including point defects, dislocations, etcetera. Therefore,
simulations were also run for crystals with dislocation sources, vacancies and
nano-void distributions before the shocks. Large vacancy concentrations (up to
0.5 %) do not change significantly the Hugoniot nor the Hugoniot elastic limit
(HEL); however, the inclusion of voids decreases the HEL significantly.
Simulations of the activation of a Frank-Read dislocation source also give
insights into shock-induced dislocation multiplication process.
This work was performed under the auspices of the U. S. Department of
Energy by the
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479 EBU-II
"DYNAMIC
BEHAVIOR OF MATERIALS AT RMCS"
A comprehensive,
multi-disciplinary centre at RMCS is being created to assess or construct
models for materials and structures under extreme dynamic loads. The Centre
creates an interlocking set of tools, methods and knowledge bases to enable
accurate simulation of the complete loading and failure history of land, air,
sea and space structures. Recent work has continued on the shock response of a
range of materials including metals, polymers and brittle materials. The work
is aimed principally to look at mechanisms operating on these classes of
material with the hope that better models might be constructed to describe
constitutive behavior. There have been various recent studies of strength
effects in metals including conventional high-impedance alloys and in some intermetallics (for use in jet turbine engines). The
results presented show some new interpretations of the response of these
materials to shock loading. The classical view of high-rate tensile failure is
seen to only describe aspects of the phenomenon of spallation
in a first-order manner. In particular, the correlation between the reload
signals observed with the microstructure of recovered targets means that more
advanced theoretical descriptions must be constructed. Also,
assumptions as to the effects seen when realistic pulse shapes are applied
relative to the top-hat pulse of plate impact need quantifying. An
overview of recent results is presented with indications of theoretical work
necessary. Brittle materials respond to plate impact loading in a variety of
ways. In particular, it was noticed that glasses failed later behind the shock
front in a failure wave. A range of polycrystalline ceramics has been tested
and they too appear to fail in this manner. Further, the strengths measured indicate
that there are perhaps two different classes of material; those that fail in a
purely ductile manner, and those that fail brittle manner. The work done is
reviewed and explanations are suggested to explain brittle response under shock
loading. Polymers have received less attention than ductile or brittle
materials. We have tested a range of materials over the past few years to
determine their equation of state but also their strength behavior under shock.
An overview of the trends observed is presented and new results for
shock-recovered materials are presented.
Illustrations of shock and dynamic fracture work on each
of these classes of material are given to show the range of the work underway.
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