• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 304
  • 66
  • 55
  • 30
  • 11
  • 8
  • 7
  • 5
  • 2
  • 2
  • 2
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 583
  • 583
  • 294
  • 169
  • 138
  • 134
  • 100
  • 84
  • 70
  • 62
  • 60
  • 58
  • 57
  • 56
  • 54
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Shape memory polymers : the wave of the future or a passing fad?

Sunday, Eugene Patrick 22 April 2013 (has links)
New materials always have the possibility of revolutionizing manufacturing processes and the way we live. Bronze, steel alloys, vulcanized rubber, ceramics, and fiber optic cables are just of few of the materials man has discovered which improved his quality of life. One of the more recent additions to the field of material science are materials that exhibit what is known as the shape memory effect. Both metals and synthetic polymers can acquire this property through processing and chemistry. However while shape memory polymers hold a lot of promise, it will require more research and development to make them affordable and useful in large scale applications. / text
2

Shape memory behavior of ultrafine grained NiTi and TiNiPd shape memory alloys

Kockar, Benat 15 May 2009 (has links)
The cyclic instability in shape memory characteristics of NiTi-based shape memory alloys (SMAs), such as transformation temperatures, transformation and irrecoverable strains and transformation hysteresis upon thermal and mechanical cycling limits their applications requiring high number of cycles. The main reasons for these instabilities are lattice incompatibility between transforming phases and relatively low lattice resistance against dislocation motion. The objective of this study is to increase the slip resistance and thus, to minimize the plastic accommodation upon phase transformation in NiTi and TiNiPd SMAs. The effects of grain refinement down to submicron to nanorange through Equal Channel Angular Extrusion (ECAE) on the cyclic stability were investigated as potential remedies. The influence of ECAE parameters, such as processing temperature and strain path on the microstructural refinement was explored as well as the corresponding evolution in the stress differential between the yield strength of martensite and the critical stress to induce martensite and SMA characteristics of Ni49.7Ti50.3, Ti50Ni30Pd20, and Ti50.3Ni33.7Pd16 SMAs. Severe plastic deformation via ECAE at temperatures from 300°C up to 450°C refined the grains from 50μm down to a range between 0.03μm and 0.3μm in Ni49.7Ti50.3 and 0.5μm and 1μm in TiNiPd alloys. Regardless of the material, the lower the ECAE temperature and the higher the ECAE strain path, the better the cyclic stability. ECAE led to an increase in the stress differential between the yield strength of martensite and critical stress to induce martensite due to observed microstructural refinement and this increase is responsible for the improvement in the cyclic stability of the aforementioned SMA characteristics in all investigated materials. Addition of Pd to the NiTi alloy reduced the thermal hysteresis from 36°C down to 11°C, and enhanced the cyclic stability of the SMA characteristics. In additions to positive influence of ECAE on cyclic stability, it also led to an increase in the fracture stress levels of the TiNiPd alloys due to the fragmentation or dissolution of the precipitates responsible for the premature failures. ECAE caused a slight reduction in the work output; however it was possible to obtain large stable work outputs under higher stress levels than unprocessed materials.
3

Shape memory behavior of ultrafine grained NiTi and TiNiPd shape memory alloys

Kockar, Benat 15 May 2009 (has links)
The cyclic instability in shape memory characteristics of NiTi-based shape memory alloys (SMAs), such as transformation temperatures, transformation and irrecoverable strains and transformation hysteresis upon thermal and mechanical cycling limits their applications requiring high number of cycles. The main reasons for these instabilities are lattice incompatibility between transforming phases and relatively low lattice resistance against dislocation motion. The objective of this study is to increase the slip resistance and thus, to minimize the plastic accommodation upon phase transformation in NiTi and TiNiPd SMAs. The effects of grain refinement down to submicron to nanorange through Equal Channel Angular Extrusion (ECAE) on the cyclic stability were investigated as potential remedies. The influence of ECAE parameters, such as processing temperature and strain path on the microstructural refinement was explored as well as the corresponding evolution in the stress differential between the yield strength of martensite and the critical stress to induce martensite and SMA characteristics of Ni49.7Ti50.3, Ti50Ni30Pd20, and Ti50.3Ni33.7Pd16 SMAs. Severe plastic deformation via ECAE at temperatures from 300°C up to 450°C refined the grains from 50μm down to a range between 0.03μm and 0.3μm in Ni49.7Ti50.3 and 0.5μm and 1μm in TiNiPd alloys. Regardless of the material, the lower the ECAE temperature and the higher the ECAE strain path, the better the cyclic stability. ECAE led to an increase in the stress differential between the yield strength of martensite and critical stress to induce martensite due to observed microstructural refinement and this increase is responsible for the improvement in the cyclic stability of the aforementioned SMA characteristics in all investigated materials. Addition of Pd to the NiTi alloy reduced the thermal hysteresis from 36°C down to 11°C, and enhanced the cyclic stability of the SMA characteristics. In additions to positive influence of ECAE on cyclic stability, it also led to an increase in the fracture stress levels of the TiNiPd alloys due to the fragmentation or dissolution of the precipitates responsible for the premature failures. ECAE caused a slight reduction in the work output; however it was possible to obtain large stable work outputs under higher stress levels than unprocessed materials.
4

The Effect of Crystallographic Orientation and Thermo-mechanical Loading Conditions on the Phase Transformation Characteristics of Ferromagnetic Shape Memory Alloys

Zhu, Ruixian 2009 December 1900 (has links)
The effects of crystallographic orientation, temperature and heat treatment on superelastic response of Ni45Mn36.5Co5In13.5 single crystals were investigated. Superelastic experiments with and without various magnetic field were conducted under compression on a custom built magneto-thermo-mechanical test setup. Magnetostress, which is the difference in critical stress levels for the martensitic transformation with and without magnetic field, was determined as a function of crystallographic orientation, heat treatment and temperature parameters. Magnetostress of [111] crystals was observed to be much higher than that of [001] crystals with same heat treatment. Water quenched samples have the highest magnetostress among other samples with the same orientation that were oil quenched and furnace cooled. Crystal structure and atomic ordering of the samples were examined using Synchrotron High-Energy X-Ray Diffraction to rationalize observed differences. Magnetostress levels were also traced at various temperatures. A Quantum Design superconducting quantum interference device (SQUID) was utilized to examine the magnetic properties of the material. The difference in saturation magnetization at various temperatures was analyzed to explain the temperature effect on magnetostress. Calculations based on the energy conversion from available magnetic energy to mechanical work output were used to predict the magnetic field dependence of magnetostress, which provides a guideline in material selection for the reversible magnetic field induced martensitic phase transformation. Isothermal superelastic response and load-biased shape memory response of Co48Ni33Al29 single crystals were determined as a function of temperature and stress, respectively. The aim of the work is to provide a new direction to understand the anomaly of transformation strain and hysteresis for ferromagnetic shape memory alloys. Thermo-mechanical behavior of Co48Ni33Al29 single crystal was determined by a custom built thermo-mechanical compression setup based on an electromechanical test frame made by MTS. Transformation strain was observed to decrease with increasing applied stress in isothermal tests or increasing temperature in superelastic experiments. The variation in the lattice constant in martensite and austenite was verified to account for such a trend. It was also discovered that both thermal and stress hysteresis decreased with increasing applied stress and temperature, respectively. Multiple factors may be responsible for the phenomenon, including the increase of dislocation, the compatibility between martensite and austenite phase.
5

Fabrication and Characterization of Nanowires

Phillips, Francis Randall 2010 August 1900 (has links)
The use of nanostructures has become very common throughout high-tech industries. In order to enhance the applicability of Shape Memory Alloys (SMAs) in systems such as Nano-Electromechanical Systems, the phase transformation behavior of SMA nanostructures should be explored. The primary focus of this work is on the fabrication of metallic nanowires and the characterization of the phase transformation of SMA nanowires. Various metallic nanowires are fabricated through the use of the mechanical pressure injection method. The mechanical pressure injection method is a template assisted nanowire fabrication method in which an anodized aluminum oxide (AAO) template is impregnated with liquid metal. The fabrication procedure of the AAO templates is analyzed in order to determine the effect of the various fabrication steps. Furthermore, metallic nanowires are embedded into polymeric nano bers as a means to incorporate nanowires within other nanostructures. The knowledge obtained through the analysis of the AAO template fabrication guides the fabrication of SMA nanowires of various diameters. The fabrication of SMA nanowires with di fferent diameters is accomplished through the fabrication of AAO templates of varying diameters. The phase transformation behavior of the fabricated SMA nanowires is characterized through transmission electron microscopy. By analyzing the fabricated SMA nanowires, it is found that none of the fabricated SMA nanowires exhibit a size eff ect on the phase transformation. The lack of a size e ffect on the phase transition of SMA nanowires is contrary to the results for SMA nanograins, nanocrystals, and thin films, which all exhibit a size eff ect on the phase transformation. The lack of a size eff ect is further studied through molecular dynamic simulations. These simulations show that free-standing metallic nanowires will exhibit a phase transformation when their diameters are sufficiently small. Furthermore, the application of a constraint on metallic nanowires will inhibit the phase transformation shown for unconstrained metallic nanowires. Therefore, it is concluded that free-standing SMA nanowires will exhibit a phase transformation throughout the nanoscale, but constrained SMA nanowires will reach a critical size below which the phase transformation is inhibited.
6

Transformation surfaces and normality for random and textured pseudoelastic shape memory alloys

Aleong, Douglas Kent 05 1900 (has links)
No description available.
7

Electrical Resistance and Natural Convection Heat Transfer Modeling of Shape Memory Alloy Wires

Eisakhani, Anita January 2012 (has links)
Shape memory alloy (SMA) wires are becoming increasingly popular as actuators in automotive applications due to properties such as large recovery strain, low weight, and silent actuation. The length change and thus actuation in SMA wires occur when the wire is heated, usually by passing a direct current through them. One of the difficulties in controlling electrically-heated SMAs occurs in monitoring their temperature, which is done to control the transformation and hence, actuation and avoid possibly permanent damage due to overheating. The temperature of a SMA wire is usually calculated theoretically based on the wire???s natural convection heat transfer coefficient(h).First-order convective heating models are typically used to calculate the natural convection heat transfer coefficient for SMA wires, but there is often significant uncertainty in these calculations due to a lack of existing correlations for thin cylinders, where curvature effects are significant. The purpose of this investigation is to develop models for SMA wires that may be used to predict the temperature of a current-carrying SMA wire without using direct temperature measurement methods. The models were developed based on experimental results for 0.5 mm diameter NiTi SMA wire. First the effect of various parameters such as wire inclination angle, wire length, ambient pressure, phase transformation time rate and applied external stress were investigated on the SMA wire???s electrical resistance. The electrical resistance of the SMA wire was monitored during one complete heating and cooling cycle. Later, based on the experimental results, a resistance model was developed for the current-carrying SMA wires that can be used to predict the wires??? temperature based on electrical resistance. Second, a natural convection heat transfer correlation was developed for NiTi SMA wire, in the range 2.6E-8??? RaD ??? 6E-1, which is appropriate for modeling natural convection in most practical applications at ambient conditions. A pressure variation method was used to obtain a range of Rayleigh number for a heated SMA wire. The ambient pressure was controlled within a vacuum chamber, from 1 atm to 2E-4 atm (0.1 MPa to 2E-5 MPa). Data were collected for the wire at various angles under both 100 MPa and stress-free conditions between horizontal to vertical at each set pressure. The new correlation can be used to determine the convective heat transfer coefficient of an SMA wire of known diameter and inclination angle. The convection coefficient (h) is determined using the correlation along with the Prandtl number (Pr), air dynamic viscosity (??), air compressibility factor (Z), air thermal conductivity (k), and gas constant (Rc). The wire temperature can then be determined by substituting this coefficient into the convective heat transfer equation.
8

The constitutive modeling of shape memory alloys /

Liang, Chen, January 1990 (has links)
Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 186-197). Also available via the Internet.
9

Distributed control of a segmented and shape memory alloy actuated biologically inspired robot

Schubert, Oliver John. January 2005 (has links) (PDF)
Thesis (M.S.)--Montana State University--Bozeman, 2005. / Typescript. Chairperson, Graduate Committee: Hongwei Gao. Includes bibliographical references (leaves 63-66).
10

An experimental study on cyclic behavior of extended end-plate connections equipped with shape memory alloy bolts

Xie, Lang Kun January 2017 (has links)
University of Macau / Faculty of Science and Technology / Department of Civil and Environmental Engineering

Page generated in 0.0435 seconds