13,635 research outputs found
Thermo-mechanical and micro-structural characterization of shape memory polymer foams
Shape memory polymer (SMP) materials have the ability to remain in a deformed state and then recover their initial/cast shape. This property has significant potential in many different fields, including aerospace and bio-medical, in which a shape change is desirable and actuation may not be required. SMP materials have been made into nano-reinforced composites and also foamed to improve desired properties for specific applications. SMP foams offer two clear advantages over non-foam SMP materials in applications for the biomedical and aerospace fields. The key advantages are lower density and significant compressibility. The significance of this is that components made out of SMP foam are lighter than traditional SMP materials, more compressible and exhibit minimal transverse change during deformation and shape recovery. This increases the performance and efficiency of devices using SMP foam material.
The need for a set of design criteria, models, and limits for the use of shape memory polymer foams was proposed. The effect of temperature and strain on the mechanical behavior, compression, tensile, cyclic compression, constrained recovery and free strain recovery of the material was used to determine the operational limits of the material. Next, the damage mechanism and viscoelastic effects in compressive cycling were determined through further mechanical testing and with the incorporation of three dimensional structure mapping via micro-CT scanning. The influence of microstructure was determined by testing the basic thermomechanical, viscoelastic and shape recovery behavior of foams with relative densities of 20, 30 and 40 percent. A similar suite of tests was then performed on the base epoxy material to generate the material properties necessary to fit constitutive equations to enable computational modeling. This data was then combined with three dimensional microstructures generated from micro-CT scans to develop material models for shape memory foams. These models were then validated by comparing model results to the experimental results under similar conditions.Ph.D.Committee Chair: Gall, Ken; Committee Co-Chair: McDowell, David; Committee Member: Guldberg, Robert; Committee Member: Sanderson, Terry; Committee Member: Shofner, Meisha; Committee Member: Tannenbaum, Rin
Rosen-Zener Transition in a Nonlinear Two-Level System
We study Rosen-Zener transition (RZT) in a nonlinear two-level system in
which the level energies depend on the occupation of the levels, representing a
mean-field type of interaction between the particles. We find that the
nonlinearity could affect the quantum transition dramatically. At certain
nonlinearity the 100% population transfer between two levels is observed and
found to be robust over a very wide range of external parameters. On the other
hand, the quantum transition could be completely blocked by a strong
nonlinearity. In the sudden and adiabatic limits we have derived analytical
expressions for the transition probability. Numerical explorations are made for
a wide range of parameters of the general case. Possible applications of our
theory to Bose-Einstern Condensates (BECs) are discussed.Comment: 8 pages, 8 figure
Representations of environmental protest on the ground and in the cloud: The NOTAP protests in activist practice and social visual media
This article advances knowledge on activist technosocial practice by
studying the realities and representations of on-the-ground
environmental resistance and their intersections with visual
representations of protest on Twitter. It does so by focusing on the case
of resistance to the Trans Adriatic Pipeline, commonly known as TAP, in
southern Italy, and on mixed methods for data collection, including
ethnographic observations, semi-structured interviews and an AIassisted
visual ethnography of a large collection of computationally
collected and categorised images posted on Twitter. By comparing online
and offline representations of protest, the study demonstrated that only
a partial overlapping existed between them, thus adding a nuance to the
digital criminological literature premised on the existence of blurred
boundaries between online and offline experiences of injustice. Themes
overlapped in their representations of protest, with images of on-theground
visual resistance being used on Twitter to extend and amplify the
contestation of everyday spaces and to support offline and online
initiatives to stop the pipeline. Differences in the recurring themes were
instead reconnected to the inherent secrecy of some of the protest’s
strategies and to the typical ways in which Twitter tends to be used by
social movements
Melting of icosahedral gold nanoclusters from molecular dynamics simulations
Molecular dynamics simulations show that gold clusters with about 600--3000
atoms crystallize into a Mackay icosahedron upon cooling from the liquid. A
detailed surface analysis shows that the facets on the surface of the Mackay
icosahedral gold clusters soften but do not premelt below the bulk melting
temperature. This softening is found to be due to the increasing mobility of
vertex and edge atoms with temperature, which leads to inter-layer and
intra-layer diffusion, and a shrinkage of the average facet size, so that the
average shape of the cluster is nearly spherical at melting.Comment: 40 pages, 27 figure
The Growth of Black Holes and Bulges at the Cores of Cooling Flows
Central cluster galaxies (cDs) in cooling flows are growing rapidly through
gas accretion and star formation. At the same time, AGN outbursts fueled by
accretion onto supermassive black holes are generating X-ray cavity systems and
driving outflows that exceed those in powerful quasars. We show that the
resulting bulge and black hole growth follows a trend that is roughly
consistent with the slope of the local (Magorrian) relation between bulge and
black hole mass for nearby quiescent ellipticals. However, a large scatter
suggests that cD bulges and black holes do not always grow in lock-step. New
measurements made with XMM, Chandra, and FUSE of the condensation rates in
cooling flows are now approaching or are comparable to the star formation
rates, alleviating the need for an invisible sink of cold matter. We show that
the remaining radiation losses can be offset by AGN outbursts in more than half
of the systems in our sample, indicating that the level of cooling and star
formation is regulated by AGN feedback.Comment: 3 pages, 4 figures, to appear in the proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies," edited by H. Boehringer, P.
Schuecker, G. W. Pratt, and A. Finogueno
Low-mass Star Formation: Observations
I briefly review recent observations of regions forming low mass stars. The
discussion is cast in the form of seven questions that have been partially
answered, or at least illuminated, by new data. These are the following: where
do stars form in molecular clouds; what determines the IMF; how long do the
steps of the process take; how efficient is star formation; do any theories
explain the data; how are the star and disk built over time; and what chemical
changes accompany star and planet formation. I close with a summary and list of
open questions.Comment: Proceedings of Computational Star Formation Conference, Barcelon
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