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A shape memory polymer (SMP) is stimuli-responsive with the fantastic capacity to “memorize” a temporary shape under certain conditions and to recover to its permanent shape upon exposure to certain external stimulus (e.g. heat, light, electromagnetic field). In the past few decades, various SMPs have been investigated and applied in the area of aerospace, biomedicine, and textiles, etc. Recently, a special type of SMP called a ‘two-way reversible shape memory polymer’ or ‘reversible shape memory polymer’ (RSMP) capable of transitioning between two temporary shapes without the need for reprogramming after each change has attracted the attention of many researchers. In this class of polymer, the semicrystalline RSMP was studied considerably due to the various chain structures produced by relatively simple synthesis routes. The crystallization-induced elongation (CIE) and melting-induced contraction (MIC) of the oriented crystal domains has been theorized as the main mechanism of semicrystalline RSMP. However, most RSMPs are predominantly thermosets, which implies significant drawbacks regarding reprocessing and recycling. This thesis focuses on the development of RSMP based on polyolefin materials, especially novel high-performance polyolefin elastomers, due to the advantages of high crystallizability, varying chain structures, tunable and broad melting transitions, and low cost. The thesis starts off by demonstrating the reversible shape memory effect (RSME) of the thermoplastic ethylene/1-octene diblock copolymer, which contains the ethylene-rich hard segments and the 1-octene-rich soft segments. The delicately designed chain structure exhibited a broad melting transition and strong physical crosslinks, which contributed to the resulting RSME and the CIE/MIC effect at load-free conditions. Furthermore, the commercially available polyolefin elastomer blends demonstrated the RSME. The utilization of commercial products and simple processing method to achieve a thermoplastic RSMP offers easy production in large scale and low costs. The second part of the thesis developed a polyolefin-based RSMP with reconfigurable network by introducing a transesterification catalyst into a crosslinked poly(ethylene-co-vinyl acetate). The network reconfiguration achieved a dynamic covalent polymer network by breaking the ester bonds and reconnecting. The third part of the thesis explored a new RSMP foam material developed by utilizing polyolefins. The polyolefin elastomers of differing compositions were blended and foamed to fabricate the porous structure. The RSME in a load-free condition was then demonstrated successfully. This thesis represents significant progress in the development of polyolefin-based RSMPs, outlining new structural design, processability improvements, and potential applications. / Thesis / Doctor of Philosophy (PhD) / Shape memory polymer (SMP) is stimuli-responsive capable of “memorizing” a temporary shape and yet recovering to its permanent shape upon a certain external trigger. SMPs are widely studied and applied in the areas of aerospace, biomedicine, textile, etc. On the other hand, a reversible shape memory polymer (RSMP) is a new type of SMP that can shift back and forth between two different temporary shapes without the need of reprogramming between transitions, and has been applied in soft actuators, microrobotics, and artificial muscles. In this thesis, unique polyolefin-based RSMP were developed with good reprocessability and shown in new application scenarios. Firstly, a thermoplastic semicrystalline polymer was demonstrated to exhibit the reversible shape memory effect (RSME) by using a lab-designed ethylene/1-octene diblock copolymer and commercial polyolefin elastomer blends. Subsequently, the reprocessability of a crosslinked poly(ethylene-co-vinyl acetate) (PEVA) RSMP was improved by introducing a dynamic covalent polymer network. Finally, transitional changes between shapes was amplified by developing a RSMP foam by utilizing polyolefin elastomer blends. This thesis represents significant progress in the study of polyolefin-based RSMPs.

Nitinol shape memory alloys have revolutionized many traditional engineering designs with the unique properties of pseudoelasticity and shape memory effect. At the present moment, primary fabrication processes for Nitinol-based devices include laser cutting and manual techniques. As the interest in incorporating Nitinol in different micro applications and devices increases, the development of effective technology for micro-welding of Nitinol becomes necessary. In general, welding processes may induce significant changes to the processed area rendering the component incompatible or unusable. Strength reduction, inclusions of intermetallic compounds, and changes in pseudoelastic and shape memory effects are all examples of how Nitinol can be affected by welding. The current study has examined the effects of two welding techniques on Nitinol: micro-resistance spot welding (MRSW) and laser micro-welding (LMW). Ni-rich Nitinol wires were welded in a crossed-wire configuration at different energy inputs by varying welding currents for MRSW and peak powers for LMW. The characterization of weld properties focused on the mechanical properties and bonding mechanisms, weld microstructure and formation, and phase transformation temperatures. Additionally, the effects of surface oxide on joint performance were also examined. During MRSW, the primary bonding mechanism was solid state, which consisted of 6 main stages: cold collapse, dynamic recrystallization, interfacial melting, squeeze out, excessive flash, and surface melting. Attempt was made to correlate the joining mechanism with the contact resistances. Joint strength and fracture mechanism were closely linked to the metallurgical properties of the welds. Finally, differential scanning calorimetry (DSC) tests showed that weld metal underwent phase transformation at lower temperatures compared to base material. The second part of this study investigated the effects of Nd:YAG laser micro welding have on Nitinol wires. The fracture strength, weld microstructure, and phase transformation temperatures resulting from the use of varying peak power inputs were studied and compared to both base metal and welds produced using the MRSW process. Results showed good retention of base metal strength and pseudoelastic properties. Moreover, the fusion zone underwent phase transformation at higher temperatures compared to base metal, which substantially altered the active functional properties of Nitinol at room temperature.

Nitinol shape memory alloys have revolutionized many traditional engineering designs with the unique properties of pseudoelasticity and shape memory effect. At the present moment, primary fabrication processes for Nitinol-based devices include laser cutting and manual techniques. As the interest in incorporating Nitinol in different micro applications and devices increases, the development of effective technology for micro-welding of Nitinol becomes necessary. In general, welding processes may induce significant changes to the processed area rendering the component incompatible or unusable. Strength reduction, inclusions of intermetallic compounds, and changes in pseudoelastic and shape memory effects are all examples of how Nitinol can be affected by welding. The current study has examined the effects of two welding techniques on Nitinol: micro-resistance spot welding (MRSW) and laser micro-welding (LMW). Ni-rich Nitinol wires were welded in a crossed-wire configuration at different energy inputs by varying welding currents for MRSW and peak powers for LMW. The characterization of weld properties focused on the mechanical properties and bonding mechanisms, weld microstructure and formation, and phase transformation temperatures. Additionally, the effects of surface oxide on joint performance were also examined. During MRSW, the primary bonding mechanism was solid state, which consisted of 6 main stages: cold collapse, dynamic recrystallization, interfacial melting, squeeze out, excessive flash, and surface melting. Attempt was made to correlate the joining mechanism with the contact resistances. Joint strength and fracture mechanism were closely linked to the metallurgical properties of the welds. Finally, differential scanning calorimetry (DSC) tests showed that weld metal underwent phase transformation at lower temperatures compared to base material. The second part of this study investigated the effects of Nd:YAG laser micro welding have on Nitinol wires. The fracture strength, weld microstructure, and phase transformation temperatures resulting from the use of varying peak power inputs were studied and compared to both base metal and welds produced using the MRSW process. Results showed good retention of base metal strength and pseudoelastic properties. Moreover, the fusion zone underwent phase transformation at higher temperatures compared to base metal, which substantially altered the active functional properties of Nitinol at room temperature.

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.

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.

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.

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.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 130-134).

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.