In the last twenty years Reversible Shape-Memory (RSM) alloys have become the source of considerable technological interest as a result of their ability to generate spontaneous and reversible changes of shape on thermal cycling. This has led to the development of a range of reversible shape-memory devices for thermostatic sensing applications. In these devices the alloy is subjected to several thousand shape-memory cycles and the stability of the reversible shape-memory is therefore an important alloy property. Data on the effect of shape-memory cycling on the long-term stability of the reversible shape-memory, however, is extremely limited. The present work, conducted to fill this gap, has shown that there is an inherent instability in the reversible shape-memory, with changes in the operating temperatures and cumulative reductions in the maximum shape-strain output of actuators on long-term thermal cycling under conditions simulating real devices. Extensive investigation has shown that these instabilities result from a number of sources, ageing of the shape-memory martensites and most importantly from morphological disruptions in the "trained" martensites caused by two-stage stress-induced transformation and due to the build-up of transformation-induced dislocation debris. This shape-strain degradation has also been successfully modelled by means of a simple two-stage stress-induced martensitic transformation model.

Thesis (MTech (Technology : Mechanical Engineering))--Cape Peninsula University of Technology, 2006 / Recent years have witnessed a tremendous growth and significant advances in "smart" composites and "smart" composite structures. These smart composites integrate active elements such as sensors and actuators into a host structure to create improved or new functionalities through a clever choice of the active elements and/or a proper design of the structure. Such composites are able to sense a change in the environment and make a useful response by using an external feedback control system. Depending on their applications. smart composites usually make use of either the joint properties of the structure or the properties of the individual elements within the composites. The accumulation in the understanding of materials science and the rapid developments in computational capabilities have provided an even wider framework for the implementation of multi-functionality in composites and make "smart" composites "intelligent". This thesis is a contribution towards the global endeavour to innovate using smart structures to enhance our everyday lives. One of the phenomena of shape memory alloys. the shape memory effect was put to use in the development of an active surface. Here the pre-stressed shape memory alloy (in its de-twinned martensitic state) is surrounded or embedded in a non-SMA matrix material. This active surface can be used in a variety of applications that requires active shape control to change the shape of a flexible structure member such as a submarine stem, aerospace control surfaces and aircraft wings. An experimental protocol was developed to treat or stabilize shape memory alloys that are used as actuators within composite structures. Shape memory alloys exhibit complex behaviour during their quasi-plastic material response. The complex behaviour includes variability in yield values and the transformation region/range.

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2005 / The working characteristics of a shape memory alloy element providing either sensor or actuator capability is specified by the beginning and/or completion temperatures of the actuation (Le. austenitic start and finish temperatures and martensitic start and finish temperatures), the working actuator stroke (i.e. load induced twinned=:>de-twinned martensitic phase transformation also known as quasi-plastic deformation), and the working actuator force (provided by the temperature induced de-twinned martensite

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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.

Shape memory alloys and polymers have been extensively researched recently because of their unique ability to recover large deformations. Shape memory polymers (SMPs) are able to recover large deformations compared to shape memory alloys (SMAs), although SMAs have higher strength and are able to generate more stress during recovery. This project focuses on procedure for fabrication and Finite Element Modeling (FEM) of a shape memory composite actuator. First, SMP was characterized to reveal its mechanical properties. Specifically, glass transition temperature, the effects of temperature and strain rate on compressive response and recovery properties of shape memory polymer were studied. Then, shape memory properties of a NiTi wire, including transformation temperatures and stress generation, were investigated. SMC actuator was fabricated by using epoxy based SMP and NiTi SMA wire. Experimental tests confirmed the reversible behavior of fabricated shape memory composites. The Finite Element Method was used to model the shape memory composite by using a pre-written subroutine for SMA and defining the linear elastic and plastic properties of SMP. ABQUS software was used to simulate shape memory behavior. Beside the animated model in ABAQUS, constitutive models for SMA and SMP were also developed in MATLAB® by using the material properties obtained from experiments. The results of FEM simulation of SMC were found to be in good agreement with experimental results.

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.

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.