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Cyclic Behavior of Superelastic Nickel-Titanium and Nickel-Titanium-Chromium Shape Memory AlloysBarbero Bernal, Laura Isabel 02 December 2004 (has links)
Shape memory alloys (SMAs) are a class of alloys that display the unique ability to undergo nonlinear deformations and return to their original shape when heat is applied or the stress causing the deformation is removed. This unique shape memory characteristic is a result of a martensitic phase-change, which can be temperature induced (shape memory effect) or stress induced (superelastic effect).
In this study, the cyclical behavior of NiTi, a binary shape memory alloy, is compared to the cyclical behavior of NiTiCr, a ternary SMA. The purpose of this study is to compare the behavior of a 0.085-in. diameter NiTiCr wire with the behavior of the same size NiTi wire to determine whether ternary SMAs are more viable ways to take advantage of the unique properties of SMAs for seismic applications. The experimental results showing the superelastic behavior of these alloys under cyclical tensile loading are summarized with attention to the effects of annealing temperature, strain rate, and cyclical training on the stress-strain hysteresis, maximum recoverable strain and equivalent viscous damping.
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A Novel Shape Memory Behavior of Single-crystalline Metal NanowiresLiang, Wuwei 31 July 2006 (has links)
This research focuses on the characterization of the structure and mechanical behavior of metal nanowires. Molecular dynamics simulations with embedded-atom method (EAM) potentials are used. A novel shape memory effect and pseudoelastic behavior of single-crystalline FCC metal (Cu, Ni, and Au) nanowires are discovered. Specifically, upon tensile loading and unloading, these wires can recover elongations of up to 50%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys. This novel behavior arises from a reversible lattice reorientation driven by the high surface-stress-induced internal stresses at the nanoscale. It exists over a wide range of temperature and is associated with response times on the order of nanoseconds, making the nanowires attractive functional components for a new generation of biosensors, transducers, and interconnects in nano-electromechanical systems.
It is found that this novel shape memory behavior only exists at the nanometer scale but not in bulk metals. The reason is that only at the nanoscale is the surface-stress-induced driving force large enough to initiate the transformation. The lattice reorientation process is also temperature-dependent because thermal energy facilitates the overcoming of the energy barrier for the transformation. Therefore, nanowires show either pseudoelasticity or shape memory effect depending on whether the transformation is induced by unloading or heating. It is also found that not all FCC nanowires show shape memory behavior. Only FCC metals with higher tendency for twinning (such as Cu, Au, Ni) show the shape memory because twinning leads to the reversible lattice reorientation. On the other hand, FCC metals with low likelihood of twinning (such as Al) do not show shape memory because these wires deforms via crystal slip, which leads to irreversible deformation.
A micromechanical continuum model is developed to characterize the shape memory behavior observed. This model treats the lattice reorientation process as a smooth transition between a series of phase-equilibrium states superimposed with a dissipative twin boundary propagation process. This model captures the major characteristics of the unique behavior due to lattice reorientation and accounts for the size and temperature effects, yielding results in excellent agreement with the results of molecular dynamics simulations.
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Influence of Inelastic Phenomena on the Actuation Characteristics of High Temperature Shape Memory AlloysKumar, Parikshith K. 2009 December 1900 (has links)
Most e orts on High Temperature Shape Memory Alloys (HTSMAs), have focused
on improving their work characteristics by thermomechanical treatment methods.
However, the in
uence of transformation induced plasticity (TRIP) and viscoplasticity
during actuation has not been studied. The objective of this dissertation
work was to study the in
uence of plasticity and viscoplasticity on the transformation
characteristics that occur during two common actuation-loading paths in TiPdNi
HTSMAs. Thermomechanical tests were conducted along di erent loading paths.
The changes in the transformation temperature, actuation strain and irrecoverable
strain during the tests were monitored. Transmission Electron Microscopy (TEM)
studies were also conducted on select test specimens to understand the underlying
microstructural changes.
The study revealed that plasticity, which occurs during certain actuation load
paths, alters the transformation temperatures and/or the actuation strain depending
on the loading path chosen. The increase in the transformation temperature and
the irrecoverable strain at the end of the loading path indicated that the rate independent
irrecoverable strain results in the generation of localized internal stresses.
The increased transformation temperatures were mapped with an equivalent stress
(which corresponds to an internal stress) using the as-received material's transformation
phase diagram. A trend for the equivalent internal stress as a function of the applied stress and accumulated plastic strain was established. Such a function can be
implemented into thermomechanical models to more accurately capture the behavior
of HTSMAs during cyclic actuation.
On the contrary, although the viscoplastic strain generated during the course of
constant stress thermal actuation could signi cantly reduce actuation strain depending
on the heating/cooling rate. Additional thermomechanical and microstructural
tests revealed no signi cant change in the transformation behavior after creep tests
on HTSMAs. Comparing the thermomechanical test results and TEM micrographs
from di erent cases, it was concluded that creep does not alter the transformation
behavior in the HTSMAs, and any change in the transformation behavior can be attributed
to the retained martensite which together with TRIP contributes to the rate
independent irrecoverable strain. As a consequence, a decrease in the volume fraction
of the martensite contributing towards the transformation must be considered in the
modeling.
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Magnetic field-induced phase transformation & power harvesting capabilities in magnetic shape memory alloysBasaran, Burak 2009 December 1900 (has links)
Magnetic Shape Memory Alloys (MSMAs) combine shape-change/deformationrecovery
abilities of heat driven conventional shape memory alloys (SMA) and magnetic
field driven magnetostrictives through martensitic transformation. They are promising
for actuator applications, and can be employed as sensors/power-harvesters due to their
capability to convert mechanical stimuli into magnetic response or vice versa.
The purpose of the present work was to investigate magneto-thermo-mechanical
(MTM) response of various MSMAs, under simultaneously applied magnetic field, heat
and stress. To accomplish this, two novel testing systems which allowed absolute control
on magnetic field and stress/strain in a wide and stable range of temperature were
designed and manufactured.
MTM characterization of MSMAs enabled us to determine the effects of main
parameters on reversible magnetic field-induced phase transformation (FIPT), such as
magnetocrystalline anisotropy energy, Zeeman energy, stress hysteresis, thermal
hysteresis, critical stress to start stress induced phase transformation and crystal
orientation. Conventional SMA characteristics of single crystalline Ni2MnGa and
NiMnCoIn and polycrystalline NiMnCoAl and NiMnCoSn MSMAs were investigated
using the macroscopic MTM testing system to reveal how these conventional properties
were linked to magnetic-field-induced actuation. An actuation stress of 5 MPa and a
work output of 157 kJm?3 were obtained by the field-induced martensite variant reorientation (VR) in NiMnGa alloys. FIPT was investigated both in Ni2MnGa MSMA
and in NiMnCoIn metamagnetic SMA. It proved as an alternative governing mechanism
of field-induced shape change to VR in Ni2MnGa single crystals: one-way and reversible
(0.5% cyclic magnetic field induced strain (MFIS) under 22 MPa) stress-assisted FIPTs
were realized under low field magnitudes (< 0.7 Tesla) resulting in at least an order of
magnitude higher actuation stress levels than those in shape memory alloys literature.
The possibility of harvesting waste mechanical work as electrical power by
means of VR in NiMnGa MSMAs was explored: without enhanced pickup coil
parameters or optimized power conditioning circuitry, 280 mV was harvested at 10 Hz
frequency within a strain range of 4.9%.
For the first time in magnetic shape memory alloys literature, a fully recoverable
MFIS of 3% under 125 MPa was attained on single crystalline metamagnetic SMA
NiMnCoIn by means of our microscopic MTM testing system to understand the
evolution of FIPT under simultaneously applied magnetic field and stress.
Conventional SMA characteristics of polycrystalline bulk NiMnCoAl and
sintered compacted-powder NiMnCoSn metamagnetic SMAs were also investigated,
with and without applied field.
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Constitutive modelling of shape memory alloys and upscaling of deformable porous mediaPopov, Petar Angelov 29 August 2005 (has links)
Shape Memory Alloys (SMAs) are metal alloys which are capable of changing
their crystallographic structure as a result of externally applied mechanical or thermal
loading. This work is a systematic effort to develop a robust, thermodynamics based,
3-D constitutive model for SMAs with special features, dictated by new experimental
observations. The new rate independent model accounts in a unified manner for the
stress/thermally induced austenite to oriented martensite phase transformation, the
thermally induced austenite to self-accommodated martensite phase transformation
as well as the reorientation of self-accommodated martensite under applied stress. The
model is implemented numerically in 3-D with the help of return-mapping algorithms.
Numerical examples, demonstrating the capabilities of the model are also presented.
Further, the stationary Fluid-Structure Interaction (FSI) problem is formulated
in terms of incompressible Newtonian fluid and a deformable solid. A numerical
method is presented for its solution and a numerical implementation is developed.
It is used to verify an existing asymptotic solution to the FSI problem in a simple
channel geometry. The SMA model is also used in conjunction with the fluid-structure
solver to simulate the behavior of SMA based filtering and flow regulating devices.
The work also includes a numerical study of wave propagation in SMA rods.
An SMA body subjected to external dynamic loading will experience large inelastic
deformations that will propagate through the body as phase transformation and/or
detwinning shock waves. The wave propagation problem in a cylindrical SMA is
studied numerically by an adaptive Finite Element Method. The energy dissipation
capabilities of SMA rods are estimated based on the numerical simulations. Comparisons
with experimental data are also performed.
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Active control of underwater propulsor using shape memory alloysWasylyszyn, Jonathan Allen 25 April 2007 (has links)
The development of a leading edge propeller blade reconfiguration system using Shape Memory Allow (SMA) muscles is presented. This work describes the design and testing of a leading edge flap, which is used to alter the local camber of a propeller blade. The leading edge flap is deflected by SMA wires housed in the blade and maintained in a fixed position with a shaft locking and releasing mechanism. A locking and releasing mechanism is utilized so that constant actuation of the SMAs is not required to maintain leading edge deflection. The profile at 70% span of the propeller blade was used to create a two-dimensional blade for leading edge flap design implementation and load testing. Deflection of up to five degrees was obtained with the final design of the leading edge flap and locking and releasing mechanism. The SMA muscles used to deflect the leading edge were actuated electronically through resistive heating and were controlled by a proportional/integral gain control algorithm with closed-loop feedback from a linear displacement sensor within the blade. With the final design of the leading edge flap and locking and releasing mechanism, a preliminary design for a three-dimensional propeller was created.
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Design principle of actuators based on ferromagnetic shape memory alloy /Liang, Yuanchang. January 2002 (has links)
Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 130-134).
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Processing of NITI reinforced adaptive solder for electronic packaging /Wright, William L. January 2004 (has links) (PDF)
Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, March 2004. / Thesis advisor(s): Indranath Dutta. Includes bibliographical references (p. 45-47). Also available online.
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Ferromagnetic shape memory alloysUnderhill, Daniel Martin Lennard January 2013 (has links)
No description available.
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Degradation of Ni-Ti alloy in cyclic loadingLim, Tzi-shing Jesse 12 1900 (has links)
No description available.
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