Browsing by Subject "Shape Memory Alloys"
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Item Active control of underwater propulsor using shape memory alloys(Texas A&M University, 2007-04-25) Wasylyszyn, Jonathan AllenThe 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.Item Computational Thermodynamics of CoNiGa High Temperature Shape Memory Alloys(2012-10-19) Chari, ArpitaShape Memory Alloys (SMAs) are advanced materials with interesting properties such as pseudoelasticity (PE) and the shape memory effect (SME). Recently, the CoNiGa system has emerged as the basis for very promising High Temperature Shape Memory Alloys (HTSMAs), with possible applications in the aerospace and automotive industries. Although the CoNiGa system shows significant promise for its use as HTSMAs, limited studies are available on them. Hence, a more intensive investigation of these alloys is necessary to understand their phase stability over a wide range of temperature and compositions in order for further development of CoNiGabased HTSMAs and future use of the model in alloy design. This formed the basis of motivation for the present work. In this work, a thermodynamic model of the ternary system is calculated based on the CALPHAD approach, to investigate the thermodynamic properties, phase stability and shape memory properties of these alloys. The CALPHAD approach is a computational method that enables the calculations of thermodynamic properties of systems. This method uses all available experimental and theoretical data in order to calculate the Gibbs energies of the phases in the system. The software used to carry out the calculations is "ThermoCalc," which is a computational software using CALPHAD principles, based on the minimization of Gibbs energy, and is enhanced by a global minimization technique on the system. The stability of the beta phase at high temperatures was enforced accurately by remodeling the CoGa system. The binary CoGa system that makes up the ternary was remodeled, as the beta phase (which is very important as it dominates the central region of the ternary CoNiGa system where the shape memory effect is observed), re-stabilizes as the temperature increases above the liquidus in the CoGa system. Phase relations and thermodynamic properties of the CoNiGa system based on all experimental information were evaluated. Different properties like enthalpies, activities, sublattice site fraction of vacancies and phase fractions calculated in the system matched well compared to the experimental information used to model the system. Also, the phase equilibria among the gamma (fcc), beta, gamma'(Ni3Ga), delta (Ni5Ga3) and epsilon (Ni13Ga9) were determined at various temperatures.Item Developing Methods For Designing Shape Memory Alloy Actuated Morphing Aerostructures(2012-10-19) Oehler, Stephen DanielThe past twenty years have seen the successful characterization and computational modeling efforts by the smart materials community to better understand the Shape Memory Alloy (SMA). Commercially available numerical analysis tools, coupled with powerful constitutive models, have been shown to be highly accurate for predicting the response of these materials when subjected to predetermined loading conditions. This thesis acknowledges the development of such an established analysis framework and proposes an expanded design framework that is capable of accounting for the complex coupling behavior between SMA components and the surrounding assembly or system. In order to capture these effects, additional analysis tools are implemented in addition to the standard use of the non-linear finite element analysis (FEA) solver and a full, robust SMA constitutive model coded as a custom user-defined material subroutine (UMAT). These additional tools include a computational fluid dynamics (CFD) solver, a cosimulation module that allows separate FEA and CFD solvers to iteratively analyze fluid-structure interaction (FSI) and conjugate heat transfer (CHT) problems, and the addition of the latent heat term to the heat equations in the UMAT to fully account for transient thermomechanical coupling. Procedures for optimizing SMA component and assembly designs through iterative analysis are also introduced at the highest level. These techniques are implemented using commercially available simulation process management and scripting tools. The expanded framework is demonstrated on example engineering problems that are motivated by real morphing structure applications, namely the Boeing Variable Geometry Chevron (VGC) and the NASA Shape Memory Alloy Hybrid Composite (SMAHC) chevron. Three different studies are conducted on these applications, focusing on component-, assembly-, and system-level analysis, each of which may necessitate accounting for certain coupling interactions between thermal, mechanical, and fluid fields. Output analysis data from each of the three models are validated against experimental data, where available. It is shown that the expanded design framework can account for the additional coupling effects at each analysis level, while providing an efficient and accurate alternative to the cost- and time-expensive legacy design-build-test methods that are still used today to engineer SMA actuated morphing aerostructures.Item Effect of Phase Transformation on the Fracture Behavior of Shape Memory Alloys(2013-07-24) Parrinello, AntoninoOver the last few decades, Shape Memory Alloys (SMAs) have been increasingly explored in order to take advantage of their unique properties (i.e., pseudoelasticity and shape memory effect), in various actuation, sensing and absorption applications. In order to achieve an effective design of SMA-based devices a thorough investigation of their behavior in the presence of cracks is needed. In particular, it is important to understand the effect of phase transformation on their fracture response. The aim of the present work is to study the effect of stress-induced as well as thermo-mechanically-induced phase transformation on several characteristics of the fracture response of SMAs. The SMA thermomechanical response is modeled through an existing constitutive phenomenological model, developed within the framework of continuum thermodynamics, which has been implemented in a finite element frame-work. The effect of stress-induced phase transformation on the mechanical fields in the vicinity of a stationary crack and on the toughness enhancement associated with crack advance in an SMA subjected to in-plane mode I loading conditions is examined. The small scale transformation assumption is employed in the analysis according to which the size of the region occupied by the transformed material forming close to the crack tip is small compared to any characteristic length of the problem (i.e. the size of the transformation zone is thirty times smaller than the size of the cracked ligament). Given this assumption, displacement boundary conditions, corresponding to the Irwin?s solution for linear elastic fracture mechanics, are applied on a circular region in the austenitic phase that encloses the stress-induced phase transformation zone. The quasi-static stable crack growth is studied by assuming that the crackpropagates at a certain critical level of the crack-tip energy release rate. The Virtual Crack Closure Technique (VCCT) is employed to calculate the energy release rate. Fracture toughness enhancement associated with transformation dissipation is observed and its sensitivity on the variation of key characteristic non-dimensional parameters related to the constitutive response is investigated. Moreover, the effect of the dissipation due plastic deformation on the fracture resistance is analyzed by using a Cohesive Zone Model (CZM). The effect of thermo-mechanically-induced transformation on the driving force for crack growth is analyzed in an infinite center-cracked SMA plate subjected to thermal actuation under isobaric mode I loading. The crack-tip energy release rate is identified as the driving force for crack growth and is measured over the entire thermal cycle by means of the VCCT. A substantial increase of the crack-tip energy release rate ? an order of magnitude for some material systems ? is observed during actuation as a result of phase transformation, i.e., martensitic transformation occurring during actuation causes anti-shielding that might cause the energy release rate to reach the critical value for crack growth. A strong dependence of the crack-tip energy release rate on the variation of the thermomechanical parameters characterizing the material response is examined. Therefore, it is implied that the actual shape of the strain- temperature curve is important for the quantitative determination of the change of the crack-tip energy release rate during actuation.Item Influence of Inelastic Phenomena on the Actuation Characteristics of High Temperature Shape Memory Alloys(2010-07-14) Kumar, Parikshith K.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.Item Microstructural Characterization and Shape Memory Response of Ni-Rich NiTiHf and NiTiZr High Temperature Shape Memory Alloys(2014-08-14) Evirgen, AlperNiTiHf and NiTiZr high temperature shape memory alloys (HTSMAs) have drawn a great deal of attention as cheaper alternatives to Pt, Pd and Au alloyed NiTi-based HTSMAs while NiTiZr alloys also providing at least 20% weight reduction then its NiTiHf counterparts with the same stoichiometry. (Ti + Hf/Zr)-rich compositions were already reported to have high thermal hysteresis, poor dimensional and thermal stability due to their low matrix strength hampering their practical applications. However, Ni-rich compositions of NiTiHf alloys were shown to have very promising shape memory responses recently due to generation of fine Ni-rich particles after proper heat treatments not only strengthening the matrix but also leading to relatively high transformation temperatures. Comparable studies have not been performed on Ni-rich NiTiZr compositions. Furthermore, very few published work are present on these new Ni-rich NiTiHf and NiTiZr systems. Hence many critical characteristics still remains unknown and further investigation is necessary to reveal the effect of precipitation on the microstructures and its subsequent effect on the transformation characteristics and shape memory responses. The present study focuses on the extensive microstructural and thermo-mechanical property characterizations of the Ni-rich NiTiHf and NiTiZr HTSMAs in order to develop the fundamental knowledge necessary for the optimization and development of reliable, cheap, lightweight HTSMAs operating up to 300 ?C with improved thermal and dimensional stability. Several different compositions of Ni-rich NiTiHf and NiTiZr HTSMAs are systematically precipitation heat treated for the microstructural control and then subjected to multi-scale microstructural and thermo-mechanical characterizations to achieve this goal. Differential scanning calorimetry measurements are conducted on the aged samples to reveal the transformation characteristics and furthermore generate the time-temperature-transformation temperature (TTT) diagrams of the individual alloy systems. The shape memory response and characteristics of the alloys are investigated through load-biased thermal cycling and superelasticity tests. The microstructures of the aged samples are extensively characterized using transmission electron microscopy (TEM) to build up microstructure-property relationships as well as providing deeper understanding of precipitate crystal structure, composition and morphology. Such an experimental approach is crucial for the development of new ternary alloy compositions and for the careful control of the microstructure to obtain desired properties. The outcomes of the present study is expected to help to reveal the potential of these alloys to be utilized in a wide range of applications at elevated temperatures in aerospace, automotive and oil-gas industries.Item Reinforcement Learning for Active Length Control and Hysteresis Characterization of Shape Memory Alloys(2010-01-16) Kirkpatrick, Kenton C.Shape Memory Alloy actuators can be used for morphing, or shape change, by controlling their temperature, which is effectively done by applying a voltage difference across their length. Control of these actuators requires determination of the relationship between voltage and strain so that an input-output map can be developed. In this research, a computer simulation uses a hyperbolic tangent curve to simulate the hysteresis behavior of a virtual Shape Memory Alloy wire in temperature-strain space, and uses a Reinforcement Learning algorithm called Sarsa to learn a near-optimal control policy and map the hysteretic region. The algorithm developed in simulation is then applied to an experimental apparatus where a Shape Memory Alloy wire is characterized in temperature-strain space. This algorithm is then modified so that the learning is done in voltage-strain space. This allows for the learning of a control policy that can provide a direct input-output mapping of voltage to position for a real wire. This research was successful in achieving its objectives. In the simulation phase, the Reinforcement Learning algorithm proved to be capable of controlling a virtual Shape Memory Alloy wire by determining an accurate input-output map of temperature to strain. The virtual model used was also shown to be accurate for characterizing Shape Memory Alloy hysteresis by validating it through comparison to the commonly used modified Preisach model. The validated algorithm was successfully applied to an experimental apparatus, in which both major and minor hysteresis loops were learned in temperature-strain space. Finally, the modified algorithm was able to learn the control policy in voltage-strain space with the capability of achieving all learned goal states within a tolerance of +-0.5% strain, or +-0.65mm. This policy provides the capability of achieving any learned goal when starting from any initial strain state. This research has validated that Reinforcement Learning is capable of determining a control policy for Shape Memory Alloy crystal phase transformations, and will open the door for research into the development of length controllable Shape Memory Alloy actuators.Item Transformation Induced Fatigue of Ni-Rich NiTi Shape Memory Alloy Actuators(2011-02-22) Schick, Justin RyanIn this work the transformation induced fatigue of Ni-rich NiTi shape memory alloys (SMAs) was investigated. The aerospace industry is currently considering implementing SMA actuators into new applications. However, before any new applications can be put into production they must first be certified by the FAA. Part of this certification process includes the actuator fatigue life. In this study, as-received and polished at dogbone SMA specimens underwent transformation induced fatigue testing at constant loading. The constant applied loading ranged from 100 MPa to 200 MPa. Specimens were thermally cycled through complete actuation (above Af to below Mf ) by Joule heating and environmental cooling. There were three cooling environments studied: liquid, gaseous nitrogen and vortex cooled air. It was shown that polished specimens had fatigue lives that were two to four times longer than those of as-received specimens. Test environment was also found to have an effect on fatigue life. Liquid cooling was observed to be corrosive, while the gaseous nitrogen and vortex air cooling were observed to be non-corrosive. The two non-corrosive cooling environments performed similarly with specimen fatigue lives that were twice that of specimens fatigue tested in the corrosive cooling environment. Transformation induced fatigue testing of polished specimens in a non-corrosive environment at 200 MPa had an average fatigue life of 14400 actuation cycles; at 150 MPa the average fatigue life was 20800 cycles and at 100 MPa it was 111000 cycles. For all specimens constant actuation from the beginning of testing until failure was observed, without the need for training. Finally, a microstructural study showed that the Ni3Ti precipitates in the material were one of the causes of crack initiation and propagation in the actuators.