In 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.

Ni-rich NiTi shape memory alloys (SMAs) are capable of attaining a wide range of transformation temperatures depending on the heat treatment conditions and superior thermo-mechanical cycling stability, which are desired for repeated solid-state actuation. High Ni-content Ni-rich SMAs have very low transformation temperatures in a solutionized condition due to the high Ni-content of the matrix. Slow cooling (furnacecooling) from solutionizing temperature and additional aging heat treatments result in the formation of Ni-rich precipitates such as Ni4Ti3, Ni3Ti2 and Ni3Ti and increase transformation temperatures above ambient by depleting excess Ni from the matrix. However, the precipitates do not undergo a martensitic phase transformation and they decrease the transformation strain by reducing the volume fraction of the material capable of transforming. Meanwhile, recent preliminary work shows that Ni3Ti precipitates dominate fatigue failure. The objectives of the present study are: (1) to eliminate Ni3Ti but still have Ni4Ti3 precipitates, which are responsible for the dimensional stability and increase transformation temperatures, (2) to investigate the effect of heat treatments on the transformation strain, and (3) to select single variant Ni4Ti3 precipitates through constrained aging for the formation of oriented internal stress and eventually obtain twoway shame memory effect (TWSME) and enhanced dimensional stability. Based on these objectives, the effect of aging heat treatment on transformation temperatures, microstructural evolution, and shape memory behavior were investigated for a Ni52Ti48 shape memory alloy (SMA) by using differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and thermo-mechanical testing, including isobaric heating-cooling experiments under various stress levels. It was observed that solutionizing at 900 degree C for 24 hours eliminated Ni3Ti type precipitates, but additional aging heat treatments are needed to form Ni4Ti3 precipitates to increase transformation temperatures. Furnace-cooling and additional aging heat treatment results in the multi-stage martensitic transformation due to chemical and stress inhomogeneities in the microstructure. Aging of the controlled furnace-cooled material at 400 degree C for 48 hours resulted in the highest transformation temperatures among all processing conditions investigated due to the combination of Ni3Ti precipitates and 27 percent volume fraction of the Ni4Ti3 precipitates, which led to the depletion of Ni from the transforming matrix. However, since overaging results in losing coherency of the precipitates, dimensional stability during isobaric thermal cycling was negatively impacted.

Composite materials are increasingly used in aerospace, underwater, and automotive structures. Their use in structural applications is dictated by the outstanding strength and stiffness while being lightweight in addition to their flexibility in tailoring the desired performance in the design of structures. The present study focuses on the failure analysis and shape control of smart composite laminates under coupled hygrothermal, electric and mechanical stimuli. A linear thermo-electro-elastic constitutive model for transversely isotropic materials is used for each ply in the composite laminates. The first-ply failure and ultimate laminate failure criteria of composite laminates are used to predict the failure stress and mode of the composite laminate where we incorporate various commonly known macroscopic failure criteria including Tsai-Hill, Tsai Wu, maximum stress and maximum strain for each lamina. We study the use of piezoelectric materials such as lead zirconate titanate (PZT) and piezoelectric fiber composites as actuators for controlling deformation in composite laminates; this study focuses on bending deformation. The purpose is to minimize unwanted deformation, such as the one due to hygrothermal effect, by applying counter deformation to avoid failure in such composite laminates. In addition, analysis based on the Classical Laminate Theory (CLT) is performed for Carbon/Epoxy (AS4/3501-6) thin laminate with stacking sequence [90/45/-45/0]s under uniaxial and biaxial in-plane loading. One of the major types of failure in smart structures is caused by debonding of the actuator from the host structure which is caused by the high stress discontinuity between the interface of the host structure and the active part. By using embedded actuators, such that the active part is incorporated into one of the layers of the composite beam during the manufacturing process, the stress concentration effect can be reduced while obtaining similar actuation values. Moreover, a control algorithm is proposed that enables the composite laminate to overcome the failure load by using piezoelectric materials where a counter electric voltage could be applied which prevents failure from occurring. Furthermore, computer software called “Hyper Composite” was developed using Action Script® and Adobe Flash® in order to perform stress and failure analysis for general composite laminates. Several carpet plots were also generated to show the interacting behavior of two independent variables such as Young’s modulus, Poisson’s ratio, shear modulus and the coefficient of thermal and moisture expansion at different percentile constitutions for the laminate different plies. This computer software is useful for estimating overall properties of smart composite laminates in designing smart composite structures.

The aim of this work is to present a model for the superelastic response of Shape Memory Alloys (SMAs) by developing a Preisach Model with thermodynamics basis. The special features of SMA superelastic response is useful in a variety of applications (eg. seismic dampers and arterial stents). For example, under seismic loads the SMA dampers undergo rapid loading{unloading cycles, thus going through a number of internal hysteresis loops, which are responsible for dissipating the vibration energy. Therefore the design for such applications requires the ability to predict the response, particularly internal loops. It is thus intended to develop a model for the superelastic response which is simple, computationally fast and can predict internal loops. The key idea here is to separate the elastic response of SMAs from the dissipative response and apply a Preisach Model to the dissipative response as opposed to the popular notion of applying the Preisach Model to the stress{strain response directly. Such a separation allows for the better prediction of internal hysteresis, avoids issues due to at/negative slopes in the stress{strain plot, and shows good match with experimental data, even when minimal input is given to the model. The model is developed from a Gibbs Potential, which allows us to compute a driving force for the underlying phase transformation in the superelastic response. The hysteresis between the driving force for transformation and the extent of transformation (volume fraction of martensite) is then used with a Preisach model. The Preisach model parameters are identi ed using a least squares approach. ASTM Standards for the testing of NiTi wires (F2516-07^sigma 2), are used for the identi cation of the parameters in the Gibbs Potential. The simulations are run using MATLAB R . Results under di erent input conditions are discussed. It is shown that the predicted response shows good agreement with the experimental data. A couple of attempts at extending the model to bending and more complex response of SMAs is also discussed.

The effect of moisture absorption on the glass transition temperature (Tg) and stress/strain behavior of network polyurethane shape memory polymer (SMP) foams has been investigated. With our ultimate goal of engineering polyurethane SMP foams for use in blood contacting environments, we have investigated the effects of moisture exposure on the physical properties of polyurethane foams. To our best knowledge, this study is the first to investigate the effects of moisture absorption at varying humidity levels (non-immersion and immersion) on the physical properties of polyurethane SMP foams. The SMP foams were exposed to differing humidity levels for varying lengths of time, and they exhibited a maximum water uptake of 8.0 percent (by mass) after exposure to 100 percent relative humidity for 96 h. Differential scanning calorimetry results demonstrated that water absorption significantly decreased the Tg of the foam, with a maximum water uptake shifting the Tg from 67 °C to 5 °C. Samples that were immersed in water for 96 h and immediately subjected to tensile testing exhibited 100 percent increases in failure strains and 500 percent decreases in failure stresses; however, in all cases of time and humidity exposure, the plasticization effect was reversible upon placing moisture-saturated samples in 40 percent humidity environments for 24 h.

Shape memory alloys are new, functional materials used in actuator applications with their high power to weight ratio. The high strength or displacement usage of shape memory alloys makes them suitable for direct drive applications, which eliminate use of power transmission elements. The aim of this research is to develop the methodology and the necessary tools to design and produce linear shape memory alloy actuators to be used in missile systems, space applications, and test equipments. In this study, the test apparatus designed and built to characterize shape memory alloy thin wires is described, and then the characterization tests, modeling and control studies performed on a wire are explained. In the control studies, displacement control through strain, resistance and power feedback is investigated and different control strategies (proportional-integral, proportional-integral with feedforward loop, and neural network) are employed. The results of the characterization tests, simulations and experiments are all presented in graphical and tabular form. From the results it is concluded that through careful characterization, the behavior of SMA wire can be closely approximated through models which can be used effectively to test various control strategies in simulations. Also, satisfactory position control of SMA wires can be achieved through both classical and NN control strategies by using appropriate feedback variables and power is found to be a viable feedback variable. Lastly, a linear SMA wire actuator is designed as a case study. The actuator prototype is produced, suitable control strategies are applied and actuator is experimented to validate the theoretical assumptions. The actuator developed through this work is a technology demonstration and shows that shape memory alloy elements can be utilized in several defense and space applications contracted to T&Uuml / BiTAK-SAGE as well as certification test equipments. The development of shape memory alloy actuators that can be used in defense and later in aeronautical/space applications is a critical research and development project for national defense industry.

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Stone matrix asphalt (SMA) mixtures rely on stone-on-stone contacts among particles to resist applied forces and permanent deformation. Aggregates in SMA should resist degradation (fracture and abrasion) under high stresses at the contact points. This study utilizes conventional techniques as well as advanced imaging techniques to evaluate aggregate characteristics and their resistance to degradation. Aggregates from different sources and types with various shape characteristics were used in this study. The Micro-Deval test was used to measure aggregate resistance to abrasion. The aggregate imaging system (AIMS) was then used to examine the changes in aggregate characteristics caused by abrasion forces in the Micro-Deval. The resistance of aggregates to degradation in SMA was evaluated through the analysis of aggregate gradation before and after compaction using conventional mechanical sieve analysis and nondestructive X-ray computed tomography (CT). The findings of this study led to the development of an approach for the evaluation of aggregate resistance to degradation in SMA. This approach measures aggregate degradation in terms of abrasion, breakage, and loss of texture.