Soft materials have attracted much scientific and technical interest in recent years. In this thesis, attention has been placed on the underpinning relations between molecular structure and properties of one type of soft matter - main chain liquid crystalline elastomers (MCLCEs), which may have application as shape memory or as auxetic materials. In this work, a number of siloxane-based MCLCEs and their linear polymer analogues (MCLCPs) with chemical variations were synthesized and examined. Among these chemical variations, rigid p-phenylene transverse rod and flat-shaped anthraquinone (AQ) mesogenic monomers were specifically incorporated. Thermal and X-ray analysis found a smectic C phase in most of our MCLCEs, which was induced by the strong self-segregation of siloxane spacers, hydrocarbon spacers and mesogenic rods. The smectic C mesophase of the parent LCE was not grossly affected by terphenyl transverse rods. Mechanical studies of MCLCEs indicated the typical three-region stress-strain curve and a polydomain-to-monodomain transition. Strain recovery experiments of MCLCEs showed a significant dependence of strain retentions on the initial strains but not on the chemical variations, such as the crosslinker content and the lateral substituents on mesogenic rods. The MCLCE with p-phenylene transverse rod showed a highly ordered smectic A mesophase at room temperature with high stiffness. Mechanical properties of MCLCEs with AQ monomers exhibit a strong dependence on the specific combination of hydrocarbon spacer and siloxane spacer, which also strongly affect the formation of ð-ð stacking between AQ units. Poisson s ratio measurement over a wide strain range found distinct trends of Poisson s ratio as a function of the crosslinker content as well as terphenyl transverse rod loadings in its parent MCLCEs.

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.

This work focuses on the thermomechanical characterization and constitutive model calibration of shape memory polymers (SMPs). These polymers have the ability to recover seemingly permanent large deformations under the appropriate thermomechanical load path. In this work, a contribution is made to both existing experimental and modeling efforts. First, an experimental investigation is conducted which subjects SMPs to a thermomechanical load path that includes varying the value of applied deformations and temperature rates. Specifically, SMPs are deformed to tensile extensions of 10% to 100% at temperature rates varying from 1 degree C /min to 5 degree C/min, and the complete shape recovery profile is captured. The results from this experimental investigation show that the SMP in question can recover approximately 95% of the value of the applied deformation, independent of the temperature rate during the test. The data obtained in the experimental investigation are then used to calibrate, in one-dimension, two constitutive models which have been developed to describe and predict the material response of SMPs. The models include a model in terms of general deformation gradients, thus making it capable of handling large deformations. In addition, the data are used to calibrate a linearized version of the constitutive model for small deformations. The material properties required for calibrating the constitutive models are derived from portions of the experimental results, and the model is then used to predict the shape memory effect for an SMP undergoing various levels of deformation. The model predictions are shown to match well with the experimental data.

The dynamic response and bifurcations of a harmonic oscillator with a hysteretic restoring force and sinusoidal excitation are investigated. A multilinear model of hysteresis is presented. A hybrid system approach is used to formulate and study the problem. A novel method for obtaining exact transient and steady state response of the system is discussed. Simple periodic orbits of the system are analyzed using the KBM method and an analytic criterion for existence of bound and unbound resonance is derived. Results of KBM analysis are compared with those from numerical simulations. Stability and bifurcations of higher period orbits are studied using Poincar´e maps. The Poincar´e map for the system is constructed by composing the corresponding maps for the individual subsystems of the hybrid system. The novelty of this work lies in a.) the study of a multilinear model of hysteresis, and, b.) developing a methodology for obtaining the exact transient and steady state response of the system.

It is well known that the Preisach model is useful to approximate the effect of hysteresis behavior in smart materials, such as piezoactuators and Shape Memory Alloy(SMA) wire actuators. For tracking control, many researchers estimate a Preisach model and then compute its inverse model for hysteresis compensation. However, the inverse of its hysteresis behavior also shows hysteresis behavior. From this idea, the inverse model with Kransnoselskii-Pokrovskii(KP) model, a developed version of Preisach model, can be used directly for SMA position control and avoid the inverse operation. Also, we propose another method for the tracking control by approximating the inverse model using an orthogonal polynomial network. To estimate and update the weight parameters in both inverse models, a gradient-based learning algorithm is used. Finally, for the SMA position control, PID controller, adaptive controllers with KP model and adaptive nonlinear inverse model controller are compared experimentally.

Commonly used thermoplastic biodegradable polymers are generally brittle and lack appreciable elasticity at physiological temperature and thereby fail to mimic the elastic nature of many human soft tissues such as blood vessels. Thus, there is a need for biomaterials which exhibit elasticity. Biodegradable elastomers are promising candidates whose elasticity more closely parallels that of soft tissues. In this research, we developed hybrid biodegradable elastomers comprised of organic and inorganic polymer components in a block copolymer system: poly(e-caprolactone) (PCL) and poly(dimethylsiloxane) (PDMS), respectively. A block structure maintains the distinct properties of the PCL and PDMS components. These elastomers may be useful for the tissue engineering of soft tissues as well as for shape memory polymer (SMP) devices. Tri-block macromers of the form PCLn-block-PDMSm-block-PCLn were developed to permit systematic variations to key features including: PDMS block length, PCL block length, PDMS:PCL ratio, and crosslink density. The macromer was capped with acrylating groups (AcO) to permit their photochemical cure to form elastomers. Thus, a series of biodegradable elastomers were prepared by photocrosslinking a series of macromers in which the PCL blocks varied (n = 5, 10, 20, 30, and 40) and the PDMS block was maintained (m = 37). All elastomers displayed hydrophobic surface properties and high thermal stability. These elastomers demonstrated systematic tuning of mechanical properties as a function of PCL block length or crosslink density. Notable was strains at break as high as 814% making them suitable for elastomeric bioapplications. Elastomers with a critical PCL block length (n = 30 or 40) exhibited shape memory properties. Shape memory polymers based on an organic-inorganic, photocurable silicon-containing polymer system is a first of its kind. This SMP demonstrated strain fixity of 100% and strain recovery near 100% after the third thermomechanical cycle. Transition from temporary to permanent shape was quite rapid (2 sec) and at temperatures near body temperature (60 degrees C). Lastly, porous analogues of the biodegradable elastomers were created using a novel porogen - salt leaching technique. Resulting porous elastomers were designed for tissue engineering scaffolds or shape memory foams.

Shape memory alloys (SMAs) are an important class of smart materials that have the ability to remember a shape. Current practical uses of SMAs are limited to below 100 degrees C which is the limit for the transformation temperatures of most commercially successful SMAs such as NiTi and Cu-based alloys. In recent years, the CoNiGa system has emerged as a new ferromagnetic shape memory alloy with some compositions exhibiting high martensitic transformation temperatures which makes CoNiGa a potential high temperature shape memory alloy (HTSMA). In this study, the microstructural evolution and martensitic transformation characteristics of CoNiGa (mainly Co46Ni27Ga27 and Co44Ni26Ga30 in at.percent) HTSMAs were investigated in as-cast and hot-rolled conditions as a function of different heat treatments. Heat treatment conditions were selected to introduce single, two, and three phase structures, where two precipitate phases (ductile Y and hard Y') do not martensitically transform. Calorimetry, X-ray analysis, scanning and transmission electron microscopy, thermo-mechanical process and cycling techniques are applied to understand the structural and chemical factors influencing the thermal stability and transformation characteristics. The main findings include improvement of ductility, most cyclically stable compositions with narrow transformation hysteresis (<40 degrees C) and transformation temperatures in the range of 100 degrees C to 250 degrees C, formation of new phases and their effects, and associated compositional changes in the matrix, on the transformation temperatures and on the microstructural evolution. In addition, Ms temperature depends linearly on the valence electron concentration (e/a) of the matrix, only if the Ga content is constant, and the samples with narrow transformation hysteresis demonstrate reversible martensitic transformation in constant-stress thermal cycling experiments.

The damaging effects of strong ground motions on highway bridges have revealed the limitations of conventional design methods and emphasized the need for innovative design concepts. Although seismic isolation systems have been proven to be an effective method of improving the response of bridges during earthquakes, the performance of base-isolated structures during near-field earthquakes has been questioned in recent years. Near-field earthquakes are characterized by long period and large- velocity pulses. They amplify seismic response of the isolation system since the period of these pulses usually coincides with the period of the isolated structures. This study explores the feasibility and effectiveness of shape memory alloy (SMA)-based isolation systems in order to mitigate the response of bridge structures against near-field ground motions. SMAs have several unique properties that can be exploited in seismic control applications. In this work, uniaxial tensile tests are conducted first to evaluate the degree to which the behavior of SMAs is affected by variations in loading rate and temperature. Then, a neuro-fuzzy model is developed to simulate the superelastic behavior of SMAs. The model is capable of capturing rate- and temperature-dependent material response while it remains simple enough to carry out numerical simulations. Next, parametric studies are conducted to investigate the effectiveness of two SMA-based isolation systems, namely superelastic-friction base isolator (S-FBI) system and SMA/rubber-based (SRB) isolation system. The S-FBI system combines superelastic SMAs with a flat steel-Teflon bearing, whereas the SRB isolation system combines SMAs with a laminated rubber bearing rather than a sliding bearing. Upon evaluating the optimum design parameters for both SMA-based isolation systems, nonlinear time history analyzes with energy balance assessment are conducted to compare their performances. The results show that the S-FBI system has more favorable properties than the SRB isolation system. Next, the performance of the S-FBI systems is compared with that of traditional isolation systems used in practice. In addition, the effect of outside temperature on the seismic response of the S-FBI system is assessed. It is revealed that the S-FBI system can successfully reduce the response of bridges against near-field earthquakes and has excellent re-centering ability.

There are more than 150,000 anterior cruciate ligament reconstructions each year with the goal of recovering the balance between knee stability and mobility. As many as 25 percent of these procedures will end in joint instability that can cause further damage. The risk of developing degenerative joint disease (DJD) increases in patients with previous knee injury, resulting in a higher instance of total knee arthroplasty (TKA). There are more than 400,000 TKA procedures each year, but the waiting lists for this surgery shows that many more patients are hoping to undergo this procedure. TKA provides improved knee function and pain relief for patients suffering from DJD. Although this procedure is considered successful, as younger patients undergo this treatment, the long-term performance must be improved. Major mechanisms of failure include component loosening from stress-shielding, poor integration of the implant with native tissue, and ion release from the implant. TiNb alloys are more biocompatible than currently used alloys, such as NiTi, and have mechanical properties closer to bone, so they would reduce the instance of stress shielding. TiNb can be made porous for better integration with the native bone and has superior corrosion resistance than NiTi. Engineered ligaments have generally failed to achieve mechanical properties sufficiently similar to their native counterparts, but also lack the osteochondral interface critical to the transfer of load between ligament and bone. The osteochondral interface could be incorporated through a gradient of inorganic content toward the bony insertion ends of the ligament graft, as we showed that in increase of inorganic content resulted in the transdifferentiation of osteoblasts toward chondrocyte-like cells (bone to cartilage-like). A composite scaffold composed of an electrospun mesh with either a hydrogel component or extracellular matrix (ECM) produced by the cells may be a suitable tissue engineered ligament graft. The non-linear stress-strain behavior seen in native ligament is exhibited by both of these systems, and the ECM produced by these systems is consistent with ligament tissue. The ECM-electrospun mesh composite exhibited higher elastic modulus than the fibrin-electrospun mesh composite, but required extensive pre culture while the fibrin-electrospun mesh composite could be fabricated in situ.

Titanium-nickel is a unique class of material known as Shape Memory Alloy (SMA). A thermoelastic martensitic phase transformation is responsible for its extraordinary properties such as shape memory effect and superelasticity. The near equiatomic Ti-Ni alloys are the commercially most exploited SMAs because of the unique combination of these properties and superior ductility, strength, fatigue resistance and corrosion resistance. The properties of Ti-Ni SMAs are very sensitive to composition and the processing parameters. The properties of Ti-Ni SMAs can be modified to a great extent by choice of composition, mechanical working and heat treatment. Thermo-mechanical treatments are required to strengthen the matrix and improve the shape memory characteristics. Plastic deformation and subsequent annealing is the common way to improve shape memory properties. In the present study, Ti- 50 at% Ni wire specimens are produced and used for the investigation of the effect of different heat treatment and cold working processes on shape memory characteristics. To investigate the thermomechanical behavior of differently processed wire specimens, a fully computerized servo hydraulic thermomechanical testing machine was designed and constructed. Testing machine was capable to perform different types of tests that are selected by the user. It can both heat and cool the specimen automatically according to the testing sequence by applying DC current directly through the SMA wire or by sending liquid nitrogen into the cooling chamber. Temperature is measured by a K-type thermocouple directly mounted on the wire specimen with a glass tape. Force that is applied to the specimen is produced by hydraulic power unit with a double action cyclinder and it is controlled by a controller which takes the feedback from the loadcell and LVDT (Linear Variable Distance Transducer). During performig thermomechanical-tests all the data of loadcell, LVDT and thermocouple are collected by a data acqusition system integrated with a host computer that operates the program XPC Target. Ti-Ni alloy with equiatomic composition is prepared in vacum induction furnace. Specimen cast in the form of rod was then hot swaged. Subsequent to swaging, cold wire drawing, intermediate annealing at 500 &amp / #61616 / C and water quenching was applied to obtain SMA wire with a diameter of 1.52 mm. Ti-Ni wires produced were subjected to four different processes. All the samples were initially solution heat treated at 925 &amp / #61616 / C for 30 minutes prior to water quenching. Some of the samples were further treated by an intermediate anneal at 500 &amp / #61616 / C. To see the effect of cold working / prior to intermediate annealing, 20 % or 40 % cold work was applied to another group of specimens. To study the shape memory characteristics of specimens subjected to the above mentioned processes, four types of test, namely constant stress free recovery test, constant strain free recovery test, constant stress constrained recovery test and constant strain constrained recovery test, were designed and applied cyclically. The tests have shown that the stress plateau observed in the first cycle of the tests disappear upon cycling and the shape memory characteristics improve and stabilize with cycling. Once trained by cycling, fractional free recovery was observed to reach to 100 % and recovery stress to reach 120% of the applied stress if shape recovery is prevented.