Mechanical
and Materials Engineering
Abstracts 2003-2004
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479 EBU-II
Prof. Dr. Mehdi Farshad
Polymers/Composites Laboratory
Swiss Federal Laboratories for Materials Testing and Research
EMPA,
“Magnetoactive Polymer Composites”
This
presentation deals with the recent research and development in the area of magnetostrictive polymer composites and with the
exploration of some of their potential applications. In the course of this
research, magnetostrictive elastomers,
gels, fibers, and fabrics were produced and their mechanical and magnetic
properties were determined. As one of the potential application, a new
active barrier system was developed. The main concept was to induce a desired
mechanical vibration in the window or wall through electro-magnetic excitation
along its magneto-active boundary components. The magneto-elastic excitation
was used to produce desired vibrations to counteract the unwanted sound and
noise. In further developments, magnetoactive fibers
composed of thermoplastic resin (PE-LD) and carbonyl iron powder were produced.
Through an extrusion process, endless flexible fibers having diameters 0.2 and
0.5 mm were produced. The fibers showed a very high degree of bending
flexibility to the extent of knotting. The fibers with 20% t0 50% volume
fraction of carbonyl iron depicted, in respective fashion, a tensile strength
of ranging from 14 MPa to 8 MPa
and a maximum strain of about 20% to 6%. In a further work, the fibers were
woven into flexible fabrics with relatively high degree of flexibility.
Applications of this new material may include smart materials, electromagnetic
shielding, sensors, and high strain actuators.
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479 EBU-II
Professor Atul H. Chokshi
Department of Metallurgy
Indian Institute of Science,
*Currently on sabbatical leave at Caltech
“Criteria for Optimum Forming of Bulk
Metallic Glasses”
Bulk metallic glasses are a relatively new class of materials that can retain an amorphous structure after cooling from melt at slow rates of <100 K s-1. They have many attractive structural properties including high strength and high fracture toughness. Depending on the composition and temperature, such materials exhibit a wide range of ductilties from essentially brittle behavior at room and low temperatures to superplasticity at high temperatures. For structural applications, there is frequently a need to be able to form components into complex shapes. This talk will provide an overview of the high temperature mechanical behavior of bulk metallic glasses, and will develop criteria for optimum forming of such alloys.
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Monday,
May 24, 2004
11:00 a.m.
479 EBU-II
Professor Nicolas Triantafyllidis
Aerospace Engineering Department
The
“Bifurcation
and Stability of Multi-lattices with Applications to Martensitic
Transformations in Shape Memory Alloys”
Some of the most interesting and technologically
important solid—solid transformations are the first order diffusionless
transformations that occur in certain ordered multi—atomic crystals. These include the reconstructive martensitic transformations (where no group—subgroup
symmetry relationship exists between the phases) found in steel and ionic
compounds such as CsCl, as well as the thermally—induced,
reversible, proper (group—subgroup relationships exist) martensitic
transformations that occur in shape memory alloys such as NiTi. Shape memory alloys are especially
interesting, for engineering applications, due to their strong thermomechanical (multi—physics) coupling. The mechanism responsible for these
temperature—induced transformations is a change in stability of the crystal's
lattice structure as the temperature is varied.
To model these changes in lattice stability, a
continuum—level thermoelastic energy density for a
bi-atomic multilattice is derived from a set of
temperature—dependent atomic potentials. The Cauchy—Born
kinematic assumption is employed to ensure, by the
introduction of internal atomic shifts, that each atom is in equilibrium with
its neighbors. Stress—free equilibrium paths
as a function of temperature are numerically investigated, and an asymptotic
analysis is used to identify the paths emerging from multiple bifurcation points that are
encountered. The stability of each path
against all possible bounded perturbations is determined by calculating the phonon spectra of the crystal
(Bloch-wave method). The advantage of
this approach is that the stability criterion includes perturbations of all
wavelengths instead of only the long wavelength information that is available
from the stability investigation of homogenized continuum models. The above methods will be reviewed, and
results corresponding to both reconstructive and proper martensitic
transformations will be presented. Of particular interest is the prediction of
a transformation that has been experimentally observed in CuAlNi,
AuCd, and other shape memory alloys.
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479 EBU-II
Professor Frederick Milstein
Mechanical & Environmental
“Atomic
Pattern Formation at the Onset of Stress Induced Elastic Instability: Fracture
or Phase Change?”
Numerous studies, dating to pioneering work by Max Born and associates, aim to evaluate theoretical crystal-strength in the context of elastic stability at finite strain. From another viewpoint, crystalline phase change, rather than fracture, is the considered result of stress induced elastic instability. “What are the prevailing atomic mechanisms at the onset of instability under stress?” is a classic, largely unanswered, question. This presentation first reviews stability criteria expressed as inequalities among second order elastic moduli. Theoretical modes of bifurcation at the onset of instability, as well as post-bifurcation behavior leading to fracture or phase change, are then explored. Theoretical concepts are illustrated and verified by means of lattice statics and molecular dynamics computational examples. Specific topics include the roles of crystal symmetry, mode of loading, higher order elastic moduli, and thermal activation.
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479 EBU-II
Professor Luis Dorfmann,
Ph.D., P.E.
Institute of
“Modelling the Mechanical response of Elastomers: Elasticity, Inelasticity and Magnetoelasticity”
The seminar starts by reviewing the mechanical behavior and important aspects of material modeling of filled and unfilled natural rubber. We examine the change in material response associated with the Mullins effect and with cavitation nucleation arising from tensile hydrostatic stresses of sufficient magnitude. The second part of the seminar focuses on the magnetoelastic theory and on the development of constitutive equations for the nonlinear response of elastomers. We first summarize the equilibrium equations for magnetic materials and the mechanical balance laws. To complete the system of governing equations we propose a free energy function, which depends on the deformation gradient and the magnetic induction. The general constitutive law for an isotropic material in the presence of a magnetic field is obtained and expressed in a compact form. Then, these equations are applied and solutions obtained, in the case of an incompressible material, for certain representative boundary-value problems involving circular cylindrical geometry. It is shown that the magnetic field stiffens the response of the material.
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479 EBU-II
Professor Lallit Anand
Department of Mechanical Engineering
Massachusetts Institute of Technology
“Superelasticity of Crystalline Shape Memory
Materials: Application to Ti-Ni Polycrystals”
A three-dimensional crystal-mechanics based theory for the thermo-mechanically coupled superelastic response of polycrystalline shape-memory materials is developed and used to simulate the response of a Ti-Ni (Nitinol) shape-memory alloy. Both manifestations of superelasticity: stress-strain response at a fixed temperature, and strain-temperature response at a fixed stress have been experimentally studied. The model, when suitably numerically implemented and calibrated, is shown to accurately predict the superelastic response of the material. Also, the strain-temperature cycling experiments under different constant axial stresses are predicted with reasonable accuracy. The effects of self-heating and cooling due to the exothermic and endothermic nature of the austenite-to-martensite and martensite-to-austenite transformations were investigated by performing superelastic tension experiments at strain rates which are high enough to result in non-isothermal testing conditions. The thermo-mechanically coupled theory is able to capture the resulting inhomogeneous deformation associated with the nucleation and propagation of transformation fronts, and also the “apparent hardening” of the nominal stress-strain curves observed in the experiments.
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479 EBU-II
Professor J. Tinsley Oden
Associate Vice President for Research
Director, Institute for Computational Engineering & Sciences (ICES)
Cockrell Family Regents' Chair #2 in
“ESTIMATION AND CONTROL OF
MODELING AND APPROXIMATION ERROR IN MECHANICS: Applications to Multiscale
Models of Heterogeneous Materials”
The use of computational methods to study physical events or the behavior of engineering systems involves the selection of mathematical models of the events and the construction of appropriate discretizations of the models so that they can be analyzed using digital computers. Both processes, model selection and discretization, are based on the judgment and experience of the modeler, and both invariably lead to errors in predictions of the response of real physical systems. In this lecture, a general mathematical framework is developed for characterizing modeling and discretization error and in adapting the model and its approximation so as to control these errors. The canonical application of the theory is the study of micromechanical effects in heterogeneous materials, particularly composites. But the theory has far ranging generalizations, including dimensional reduction, random media, wave propagation, and in analyzing the transition from models of continua to those based on molecular dynamics or other theories. Several applications are presented. The relevance of the theory to issues of validation and verification of computer predictions is also discussed.
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479 EBU-II
Professor Nicholas Kioussis
Department of Physics
Computational
“Effect
of chemistry on the electronic and mechanical properties materials: Linking ab initio and continuum
approaches"
Recent progress has made possible the modeling of material properties across various length and time scales with ab initio calculations. Such approaches are capable to predict materials behavior at macroscopic scales based on microscopic physics that ab initio calculations provide. I will review an ab initio based multiscale approach that seeks to link different length scales sequentially. The approach relies on a variational continuum formulation of dislocations, in the spirit of the Peierls-Nabarro model. The key ingredient in this approach is the ab initio determined Gamma surface, which can capture the effect of chemistry on mechanical properties of materials. The application of this approach to interesting material problems will be presented, such the hydrogen enhanced local plasticity in metals and the hydrogen-induced unzipping of carbon nanotubes.
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479 EBU-II
“The Big Spin: Integration
Algorithms for Rigid Body Motion”
At present, it seems to be an
accepted fact that the Newmark (Verlet,
Stoermer, ...) algorithm in vector space mechanics
has good properties at least partially due to its geometrical implications and
associations (viz some recent work on Lie-group ODE's). It is also more or less common knowledge that a
direct transfer of those properties to algorithms for rigid body rotation is
not quite obvious. We present some examples of explicit time-stepping
algorithms, including variants of Newmark's, and
compare their performance with other well-known approaches, including some implicit
algorithms.
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479 EBU-II
Professor Ali S. Argon
Massachusetts Institute of Technology
“Plasticity of Amorphous Silicon:
A Space Network Solid”
Dislocations with virtually
limitless ability for long range glide are the principal “carriers” of
plasticity in crystalline solids. These, however, can also deform plastically
to a much lesser extent by shear transformations such as by deformation
twinning or martensitic shears.
Amorphous solids, possessing no long range crystalline coherency lack the ability to deform plastically by dislocation motion, but instead, deform by recurring local shear transformations in fertile atom clusters possessing some “free volume”. In glassy metals having no directional bonding such clusters are relatively small with a size of c.a. 3-4 nm. In glassy polymers with long chain molecules with high back bone stiffness and little bond angle flexibility, but with relative ease for intermolecular rearrangements, the clusters undergoing shear transformations are considerably larger with dimensions of c.a. 10-15 nm. In space network glasses with strong covalent 3-D directional bonding the character of the basic unit plastic event had so far not been well understood.
In this lecture the character of plastic behavior of amorphous silicon, as a representative of a covalently-bonded space network glass will be discussed, where the shear transformations occur preferentially in relatively large atom clusters possessing a high concentration of a “liquid-like” component. Unlike in metallic glasses where the preferentially transforming sites possess “free volume” the “liquid-like” component of silicon in a deforming atom cluster is of higher density and has a larger atomic coordination. Nevertheless, there is an interesting parallel between the steady state concentration of “liquid-like” component producing steady state plastic flow in amorphous silicon and the “mobile dislocation densities” in plastically flowing crystalline solids.
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CANCELLED
479 EBU-II
David H. Lassila
Materials Technology Leader
Engineering Directorate
Over the past 6 years LLNL researchers and collaborators have been engaged in a comprehensive multiscale modeling program that is ultimately aimed at predicting the plastic deformation response of bcc metals. Many of the simulation tools that have been employed and further developed under this program, such as “dislocation dynamics” computer codes and 3-D molecular dynamics, have produced exciting and, in some cases, unexpected results related to dislocation mobility and hardening behaviors in bcc metals. In addition to the multiscale modeling and simulations efforts, experimental techniques have been developed specifically to validate the simulation results.
An overview of some simulation results will be presented, leading into a description of some of the experimental challenges for detailed validation efforts. Recent experimental results on the crystallographic slip behavior of Mo single crystals that have been obtained using a unique “6-degree of freedom” (6DOF) deformation experiment are presented. In this experiment, the full stress and strain tensors are determined which allows “slip system activity” during axial compression of single crystal test samples to be calculated for the first time. The results of these experiments show that slip behavior is in substantial deviation from the expected “Schmid” behavior. These experimental results bring into question some of the fundamental assumptions used in both the construction of crystal plasticity constitutive relationships and rules for dislocation mobility use in 3-D dislocation dynamics simulations.
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479 EBU-II
Professor G. Ravichandran
Aeronautics and Mechanical Engineering
California Institute of Technology
“Large Electrostrictive
Actuation of Ferroelectric Perovskite Single
Sensors
and actuators based on ferroelectric materials are finding increasing use in
applications related to mechanical, aerospace and biomedical fields. Most of the current devices rely on the
linear piezoelectric behavior of formulations of PZT which offer high
bandwidth, linear actuation and strains of up to 0.2%. The nonlinear electromechanical behavior of
ferroelectric materials is largely governed by the motion of domains and is
affected by stress as well as electric field. Results from continuum modeling
based on energy minimization of ferroelectric single crystals for the structure
and behavior are presented. A principle
result of the modeling is the mode of actuation in ferroelectric single
crystals by 90o domain switching that could result in large strains.
An experimental setup has been designed to investigate large strain actuation
in single crystal ferroelectrics under combined electrical and mechanical
loading. Experiments have been performed
on single domain bulk crystals of barium titanate with (100) and (001)
orientation. The electrostrictive
response is shown to be highly dependent on the level of applied stress with a maximum
strain of 0.9% measured at a compressive stress of about 2 MPa.
The in situ observations of the
domain patterns using polarized light microscopy are presented. Continuum modeling and simulations are used
to gain insights in to the mechanics and mechanisms of large strain actuation
and dynamics of domain switching in ferroelectrics.
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479 EBU-II
“Shock Waves in Granular Flows”
A granular media is any collection of particles that interact through inelastic collisions and for which the thermal energy, kT is small compared to other energy scales in the problem. Examples include sand, sugar, crushed coal, cereals, pills, cosmetics, and asteroids. The transport, mixing and segregation of granular media is important in industrial applications, however a basic understanding of the bulk flow properties is lacking. Because granular flows continuously dissipate energy in inelastic collisions, the speed of a pressure wave is often small compared to the mean velocity of the system. Shock waves form easily in granular materials when the flow encounters an obstacle. I will present experiments and simulations of shocks formed in two geometries: granular flow past a wedge in a two-dimensional cell and flow past a cylinder in a vibrated granular layer. I compare results to a proposed continuum description for granular flows. I will discuss the application of jump conditions, similar to the Rankine-Hugoniot equations, to granular flows.
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479 EBU-II
Dr. Peter Lomdahl
and
Mechanical and Aerospace Engineering, UCSD
“Large-Scale Molecular Dynamics
Studies of Complex Phenomena in Solids and Fluids”
Recent advances in high performance parallel computing hardware as well as maturing integration, analysis and visualization capabilities for very large-scale molecular dynamics (MD) simulations, have allowed new insight for classical problems in the physics of solids and fluids. I will summarize two such developments from our work at Los Alamos National Laboratory. The first is the analysis of shock-induced phase transitions in iron and the second is the investigation of the classical Rayleigh-Taylor (RT) instability in fluids via MD. In both cases comparison with experiments will also be presented. The basic principles of MD simulation techniques will also be covered.
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479 EBU-II
Dr. Robert Lucas
Director of the Computational
Sciences Division
Information Sciences Institute (ISI)
“Perspectives on
Supercomputing: Applications,
Programming Tools, and the Future”
Bob Lucas has spent nearly twenty years developing code for many supercomputing applications. In that time he has used a wide variety of programming languages, algorithms, and parallel systems. In this talk, he will revisit this history, drawing lessons about research in applications, parallel programming tools, and high-end systems.
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479 EBU-II
Ricardo B. Schwarz
Structure/Properties Relations Group
"Physics of Hydrogen, Deuterium, and Tritium in Palladium"
Atomic hydrogen is the simplest solute in a metal one can think of. We have used an ultrasonic spectroscopy technique to measure the three independent elastic constants of Pd-H, Pd-D, and Pd-T single crystals as a function of protium, deuterium and tritium concentration, respectively. Within the the two-phase Pd-(Pd-hydride) phase field, C' shows a strong softening (whereas C44 shows none), which we explain in terns of Zener-type anelastic relaxations affecting the shape of the hidride precipitates. In the single beta-phase, the shear elastic constant C' exhibits a strong isotope effect (again, C44 shows none) that is attributed to optical phonons, which are excited differently in Pd-H, Pd-D and Pd T. We use a simple spring model for the Pd-H atomic interactions to explain most of these observations.
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479 EBU-II
Professor Shu Chien
Chair of the Department of Bioengineering
"Bioengineering at UCSD and Beyond"
During the last two years, our department has discussed research directions and possible collaborations with other departments in the area of bioengineering and biomedical applications. To some extent, these discussions have been limited by our own understanding of what constitutes bioengineering and how it relates to the biomedical applications that interest many MAE faculty.
To educate all of us, I have
asked Prof. Shu Chien,
Chair of the Department of Bioengineering, and one of the founders of
bioengineering at UCSD, to present to us his perspective on bioengineering at
the Mechanics and Materials Seminar on Jan. 26 at
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CANCELLED
479 EBU-II
David H. Lassila
Materials Technology Leader
Engineering Directorate
Over the past 6 years LLNL researchers and collaborators have been engaged in a comprehensive multiscale modeling program that is ultimately aimed at predicting the plastic deformation response of bcc metals. Many of the simulation tools that have been employed and further developed under this program, such as “dislocation dynamics” computer codes and 3-D molecular dynamics, have produced exciting and, in some cases, unexpected results related to dislocation mobility and hardening behaviors in bcc metals. In addition to the multiscale modeling and simulations efforts, experimental techniques have been developed specifically to validate the simulation results.
An overview of some simulation results will be presented, leading into a description of some of the experimental challenges for detailed validation efforts. Recent experimental results on the crystallographic slip behavior of Mo single crystals that have been obtained using a unique “6-degree of freedom” (6DOF) deformation experiment are presented. In this experiment, the full stress and strain tensors are determined which allows “slip system activity” during axial compression of single crystal test samples to be calculated for the first time. The results of these experiments show that slip behavior is in substantial deviation from the expected “Schmid” behavior. These experimental results bring into question some of the fundamental assumptions used in both the construction of crystal plasticity constitutive relationships and rules for dislocation mobility use in 3-D dislocation dynamics simulations.
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479 EBU-II
Thomas J. Ahrens
Professor of Geophysics
Seismological Laboratory
California Institute of Technology
"Effect
of Phase
Changes on the Attenuation of Shock Waves and Impact Cratering"
Hysteretic phase changes associated with transformation of 4- to 6-fold coordination of Si+4 or Ge+4 such as occurring in SiO2 and GeO2, and transformation of H2O - Ice I to Ice VI, or, Ice VII result in density increases of ~ 30 % over a small pressure range. Pertinent shock wave (Hugoniot) data for GeO2 and SiO2 are reviewed and new data for H20 –ice is presented. Upon propagation of unsupported spherical shocks at stresses in the regimes of, phase changes- involving marked density increases, enhanced attenuation occurs. In the case of planetary H2O ice the resultant entropy production in the Ice I to Ice VI or IceVII results in partial melting at much lower shock stresses than previously inferred. As a result, the shock-induced melting, in-turn, results in the ground-hugging ejecta flows such as seen in many Mars impact craters. Thus, ‘rampart' craters are characteristically produced on planetary surfaces containing water ice.
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479 EBU-II
Arun Majumdar
Department of Mechanical Engineering,
Materials Science Division,
“Mechanics and Dynamics at the Interface of Hard and Soft Materials”
Hard and soft materials are characterized by the ratio of their respective binding energies (Eb) with respect to thermal fluctuations that are characterized by kT. Mechanics and dynamics of hard materials (Eb >> kT) are generally unaffected by kT, except when undergoing irreversible processes such as transport phenomena or inelastic deformations. On the other hand, fluctuations dominate the behavior of soft materials (Eb ~ kT) such as liquids and biomolecules, where entropic forces are critical in their mechanics. As part of this lecture, I will focus on two topics, both of which relate to the interplay between entropic and elastic forces: (i) Transport of heat in solid nanostructures such as nanotubes, nanowires and superlattices. I will share some of our recent discoveries of how heat transport in such nanostructures can be manipulated by size confinement and interface engineering; (ii) Actuation of mechanical devices such as cantilever beams using reactions of biomolecules (eg. DNA hybridization, antigen-antibody binding). I will also discuss the implications of our work on energy conversion and biomedical technologies. Using these two topics as examples, I will attempt to highlight the role of nanoengineering both in the development of technology as well as on our education system.
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479 EBU-II
Professor Marc André Meyers
Dept. of Mechanical and
“GROWTH, STRUCTURE, AND MECHANICAL RESPONSE OF ABALONE
SHELLS”
Many properties of biological systems are far beyond those that can be achieved in synthetic materials with present technologies. Biological organisms produce complex composites that are hierarchically organized in terms of composition and microstructure, containing both inorganic and organic components in complicated mixtures. These mollusks owe their extraordinary mechanical properties (much superior to monolithic ca C03) to a hierarchically organized structure, starting with single crystals of the aragonite polymorph of CaCO3, consisting of “bricks” with dimensions of 0.5 vs.10 mm (microstructure), and finishing with layers approximately 0.2 mm (mesostructure). This is a model material for future hierarchically structured composites.
The growth of the nacreous layers (aragonite) was observed by inserting glass plates in the extrapallial layer for different time periods, removing them, and observing them by scanning electron microscopy. Details of the growth sequence were revealed.
Quasi-static and dynamic compression and three-point bending tests have been carried out. The mechanical response of the abalone is correlated with the microstructure and damage mechanisms. The mechanical response is found to vary significantly from specimen to specimen and requires the application of Weibull statistics in order to be quantitatively evaluated. The abalone exhibited orientation dependence of strength as well as significant strain-rate sensitivity; the failure strength at loading rates of 104 GPa/s was approximately 50% higher than the quasi-static strength. The abalone compressive strength when loaded perpendicular to the shell surface was approximately 50% higher than parallel to the shell surface. The compressive strength of abalone is 1.5 – 3 times the tensile strength (as determined from flexural tests), in contrast with monolithic ceramics, for which the compressive strength is typically an order of magnitude greater than the tensile strength. Quasi-static compressive failure in both shells occurred gradually, in a mode sometimes described as “graceful failure”. The shear strength of the organic/ceramic interfaces of Haliotis Rufescens has been determined by means of a shear test and was found to be approximately 30 MPa. Considerable inelastic deformation of these layers (up to a shear strain of 0.4) preceded failure. Crack deflection, delocalization of damage, plastic microbuckling (kinking), and viscoplastic deformation of the organic layers are the most important mechanisms contributing to the unique mechanical properties of these shells.
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Information: (858) 534-3980