Mechanical and Materials Engineering
Abstracts 2000-2001
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Monday, June 4, 2001
EBU-II 479
4:00 - 5:00 p.m.
Werner Goldsmith
Professor of the Graduate School
Departments of Mechanical Engineering and Bioengineering
University of California, Berkeley
"Biomechanics of Traumatic Brain Injury in Infants and Children"
A nationwide debate exists concerning the multitude of cases brought before criminal courts as to whether infantile traumatic brain injury occurring in situations not independently witnessed is the result of accident or deliberate abuse. Inflexible positions espoused by certain portions of the medical community specify that any time that, absent an otherwise obvious accidental cause, the existence of subdural bleeding (hematoma), often combined with hemorrhaging within the retina, is a definitive indication of abuse. Other physicians and biomechanicians have contradicted this stance by pointing to natural causes that can produce these same symptoms. These opposing views are further complicated by the possibility of the unwitnessed fall of a toddler where there is usually, but not invariably, an indication of contact.
This presentation will be concerned with the biomechanical aspects of head trauma in infants, their mechanical origins and the physical processes that produce visible symptoms of dysfunction. In particular, the consequences of falls on the head will be examined, and the mechanics of the so-called "Shaken Baby Syndrome" will be detailed. Kinematic quantities generated by fundamental research and others mandated by the federal government in defining tolerance levels of head injury will be compared to the results of analyses and tests involving child trauma incidents. Currently available data on the mechanical properties of infantile head components will be described that are used in evolving computational schemes converting information on motion such as acceleration to the much more accepted parameters of deformation, strain and stress used for an engineering description of damage and failure of systems. The nature of urgently needed additional research to provide more information on this critical topic will be indicated.
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Tuesday, April 24, 2001
EBU-II 479
11:00 – 12:15 p.m.
Patrick Veyssière
LEM, CNRS-ONERA
Chatillon, France
“Dislocation Patterning and Conservative Recovery in Metals Deformed Under
Single Slip"
One of the most unexpected if not disconcerting discoveries of the early TEM analyses of deformation microstructures is the widespread occurrence or irregular dislocation tangles and interspersed prismatic loops, especially in samples strained in single slip. It was nevertheless readily determined that dislocation aggregates consist mainly of dipolar configurations including elongated prismatic loops and the paramount role of cross-slip in generating such debris became rapidly acknowledged. The question of spontaneous patterning was found even more challenging after several extensive TEM studies revealed the degree of regularity of dislocation walls in fatigued crystals. On the other hand, in order to explain the saturation of the static dislocation density in microstructures of fatigue, one usually invokes a mechanism of recovery by spontaneous annihilation of dipoles below a certain dipole height. Despite significant efforts, the elementary dislocation manoeuvres that engender tangles have remained uncertain and the mechanism of spontaneous dipole disintegration by climb has received no alternatives. This seminar addresses the issues of dislocation spontaneous organization and recovery based on TEM observations conducted on gamma-TiAl (L10 ordered structure) deformed in single slip by the so-called ordinary dislocations (b = 1/2<110]): - when produced by conservative processes, prismatic loops are in general organized in strings where the end of a given loop is aligned with the beginning of the next loop in the screw direction. Strings interact with mobile dislocations forming a variety of configurations including tangles: - in loose tangles, strings can be totally or partially annihilated by a single impacting dislocation. In dense walls, loop refinement should take place upon impingement of loops of opposite kinds.
If time allows, other facets of the current work on gamma-TiAl will be addressed. This part of the talk will be centered on the question of the flow stress anomaly which will discussed both in terms of intrinsic and extrinsic dislocation locking properties: - from an extensive review of available experimental data and dedicated mechanical experiments, it will be shown that extrinsic effects greatly enhance the flow stress peak.
P. Veyssiere has held a permanent position (Director of Research) within the French National Agency for Scientific Research(CNRS) since 1988 and from 1988 to 1998 was assistant professor at the University of Poitiers. Veyssiere is head of the Laboratory for Microstructural Studies (LEM). Main fields covered in the LEM are plasticity (experiments, simulations, and modeling), transmission electron microscopy (TEM) (mostly analytical), phase transformations (experiments, simulations, modeling), and crystallography (quasi-crystals and topologically complex phases, nanotubes). Since 1983, Veyssiere’s main research interest has been in plastic properties of intermetallics with special emphasis on relating macroscopic mechanical behavior to properties of individual dislocations, preferably in model alloys.
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Monday, March 19, 2001
EBU-II 479
1:30 – 2:30 p.m.
Professor Raymond W. Ogden
Department of Mathematics
University of Glasgow, Scotland
"The Dynamics of
Residually-Stressed Thin-Film Surface-Coated Elastic Solids"
In this talk we discuss a theory for an elastic thin-film surface coating of an elastic solid under finite deformations and the effect that the coating has on the propagation of waves in the bulk material. The theory is illustrated by examining the propagation of surface waves in a deformed thin-film coated half-space and associated questions of stability. The important influence of residual stresses in the coating material is highlighted.
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Tuesday, February 27, 2001
EBU-II 479
2:00 PM
Gregory J. Rodin
Texas Institute for Computational and Applied Mathematics
The University of Texas at Austin
"Rapid Solution of Large
Problems in Micromechanics"
This talk is concerned with hierarchical solution methods for large problems in micromechanics. Most of the talk will be dedicated to O(N) boundary element methods, in which the governing equations are solved iteratively, using the fast multipole method for matrix-vector multiplication. At the end of the talk, several open issues related in hierarchical homogenization will be discussed.
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Friday, February 23, 2001
EBU-II 479
10:00 – 11:00 A.M.
Ricardo B. Schwarz
Materials Science and Technology Division
Los Alamos National Laboratory
"Bulk Metallic Glasses"
Metallic glasses have many attractive properties not found when the same alloys are in their stable crystalline states. These include high strength, good corrosion resistance and, in the case of ferromagnetic alloys, low hysteresis losses under cyclic magnetic excitation. During the last decade, researchers have developed methods to prepare bulk metallic glasses with dimensions of several centimeters. These glasses are being used as such, and as precursor for crystal-amorphous composites having attractive toughness and wear resistance properties.
To form the metallic glass, crystallization must be bypassed during the undercooling of the melt. We prevent heterogeneous crystallization (at inclusions such as oxides) by fluxing the melt. Homogeneous nucleation is avoided by selecting alloy compositions near deep eutectics. Ferromagnetic glasses, containing at least 64 at.% Fe, have been prepared in the form of 4-mm-diameter rods. For all these glassy alloys, the difference between the crystallization temperature and the glass transition temperature, T=T Txg, is between 60 and 100 K. This large T range enables the fabrication of near-net-shape components. We will discuss the synthesis, properties, and potential applications of these novel materials.
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Monday,
January 29, 2001
EBU-II 479
3:00 p.m. – 5:00 p.m.
Professor Mikhail Moglivesky
Russian Academy of Sciences, Siberian Branch, and Lavrentyev Institute of
Hydrodynamics
This seminar will be divided into two parts, one hour each. The first part is focused on high strain-rate deformation, while the second is of a broader interest.
Part I: Mechanisms of Defect Generation Under Extreme Deformation
Conditions
The processes of defect generation and interaction under extreme stresses,
temperature or duration will be analyzed. The following topics will be
included:
1. Nature of thermal fluctuations in solids; Collective thermal
displacements of atoms and their role in defect generation (point defects;
shear; melting.)
2. Successive stages of shear generation in defect-free lattice.
3. Role of point defects and their complexes in shear generation under shock
loading.
4. Dislocation reactions under shock loading (successive stages; uniaxiality of
the process; formation of dislocations in "neutral" systems.)
5. Critical conditions for adiabatic shear generation.
6. Specific features of defect structure development in meteorites (successive
stages of life; extremely long low-temperature diffusion.)
Part II: The Scientific Method of Leonardo da Vinci
An attempt to understand why he was so astonishingly successful in many
fields. This will include:
1. Origin of problems
2. Formulation of problems
3. Mistakes of Leonardo
4. Examples of the use of method
Some of the brightest achievements of Leonardo da Vinci in physics are also
presented.
1. Principle of impossibility of perpetuum mobile. For seven years, he tried to
construct a PM; he invented several original schemes and arrived, at last, to
the formulation of this principle, which is actually equivalent to the
Principle of Energy Conservation.
2. Friction: Classical formulation of the friction coefficient. The case of
friction force, proportional to the surface. Methods for decreasing friction
(different bearings.) Energy absorber.
3. Some comments on his achievements in a number of other fields: wave nature
of light; blue color of atmosphere; phase transitions in water; reflection of
jets under oblique collision.
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Thursday, September 28, 2000
EBU-II 479
2:00 - 3:00 p.m.
Eliot Fried
Department of Theoretical and Applied Mechanics
University of Illinois at Urbana-Champaign
"Surface Defects and Associated Microstructures in Nematic
Elastomers"
A nematic elastomer is a rubber-like analogue of a nematic liquid crystal. These materials are formed by cross-linking a polymeric fluid consisting of chains that include nematic molecules as spacers or as pendant side-groups. Associated with the coupling between macroscopic distortion and macroscopic order that is present in these materials is a wide range of unusual phenomena, including mechanically-induced optical switching, memory effects, piezoelectricity, and spontaneous shape changes during phase transitions. Many of these phenomena are accompanied by the presence of evolving microstructures, known as "striped domains," which involve surface defects across which the nematic orientation and the deformation gradient are discontinuous. We will present a molecular-statistical framework that determines a class of continuum-level free-energy densities for nematic elastomers. Included in this class of energy densities are nematic generalizations of the neo-Hookean and the Mooney-Rivlin expressions for conventional rubber. Significantly, each of these free-energy densities is found to possess a continuous spectrum of energy minima. We will explain how this degeneracy allows for the existence of equilibrated striped domains and discuss a elastomer did not occur until 1981, commercial applications of nemaatic elastomers are still under development. Areas that show promise include MEMS devices, integrated optics, and artificial muscle. Our research on nematic elastomers is part of multidisciplinary effort involving experimentation and synthesis. the long-term goal of this effort is to produce nematic, smectic, cholesteric, and other optical elastomers with optimal properties.
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Wednesday, September 6, 2000
EBU-II 479
11:00 a.m. - 12:00 p.m.
Professor L. V. Nikitin
Lomonosov Moscow State University
Russia
"Behavior of a Softening Solid in a Displacement Controlled Test"
Some metallic materials in tension and geomaterials in compression exhibit a descending segment on the load-displacement diagram. The model of the strain-softening rate insensitive elasto-plastic solid conventionally describes constitutive behavior of such materials. If the process of deformation took place at the descent segment of the stress-strain diagram the governing equations would change their type and the initial boundary-value problem become ill-posed. However it never happens in the properly posed physical problems. It is proved that continuous deformations in the region of softening are unstable. Instability leads to a dynamical process even in the case of quasistatic external loading. Deformations corresponding to the descent part of the stress-strain diagram are localized at the surfaces, lines or points depending on the dimension of the problem under consideration. Localization of deformations produces unloading that provokes propagation of shock waves of unloading and secondary hardening. The fundamental laws of conservation and balance combined with the initial and boundary conditions constitute well-posed mathematical problem. As an illustrative example the problem of quasistatic displacement controlled stretching of an elasto-plastic bar with partial softening is considered. Usage of the model of an elast-visco-plastic softening solid permits to study evolution of strain localization. In this case the post critical process is also dynamic but the governing equations are hyperbolic and problem is well posed even for deformation on the descent segment. Stretching with of constant velocity of the end of a rod leads to localization of deformation in a small length depending on velocity and approaching zero when velocity vanishes. Stretching with constant stress of the end of a rod also leads to localization of deformation but region of localization linearly grows with time. As an alternative to a softening solid a model of material with structural transformation of a solid into another solid with different mechanical properties and natural stress state is proposed and analyzed. Conditions at the front of a shock wave of structural transformation include equation of energy. It is shown that depending on material properties structural transformation may be both static and dynamic. J.R. Rice discovered, during computer simulation, the existence of a wave that could propagate, apparently without change of form and without attenuation, along the edge of a propagating crack in a brittle material. The edge of the propagating crack is nominally straight; the wave corresponds to a small perturbation from straightness. Symmetric (Mode I) loading is assumed. The fracture criterion is the Griffith energy balance (so that energy is conserved). e Sharon observed traces on fracture surfaces in glass that definitely correspond to long-lasting disturbances, and thus provide supporting evidence for the existence of a crack front wave. The same experiment in plexiglass shows traces that attenuate, suggesting that a crack front wave in such material be modified by a dissipative mechanism. Analytic confirmation of the existence of crack front waves was provided by Ramanathan and Fisher, who verified the presence of a pole in a transfer function relating crack shape perturbation to energy flux perturbation. The transfer function followed directly from the complete solution to the problem of the dynamic perturbation of a moving crack front by Movchan and Willis.
The theory of Movchan and Willis will be summarized, and the transfer function developed. A more recent extension, not yet published, provides the corresponding solution for propagation in a viscoelastic medium. this will also be outlined (the calculation for the attenuation of the crack front wave is in progress). The Movchan/Willis theory has also been developed for mixed Mode II/III propagation. It is again possible to seek possible crack front waves in this situation, which might be relevant, for example, in the seismic context. Results obtained so far are surprising and need detailed checking before any definite pronouncement can be made.
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Monday, August 28, 2000
EBU-II 479
2:00 - 3:30 p.m.
Dr. Daniel H. Kalantar
Lawrence Livermore National Laboratory
"Solid-State Experiments at High Pressure and Strain Rates"
We are developing experiments on intense laser facilities to study shock compressed metal foils in the solid state. At high pressure, Rayleigh-Taylor induced perturbation growth can be reduced by the strength of the material. [1] We use this to characterize the strength of the metal foils accelerated at high pressure in the solid state. In our experiments, Al and Cu foils are compressed and accelerated with staged shocks using a temporally shaped x-ray drive that is generated in a Nova laser hohlraum target. [2] The peak pressures exceed 1 Mbar (100 GPa), and strain rates are very high, le7-le0/s. The instability growth is observed by x-ray radiography.
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Further Information: Cheryle Wills at (858) 534-3980