The thermal induced martensitic phase transition in TiNiHf was exploited for bi-directional actuation with TiNiHf/SiO 2 /Si composites. When compared to free-standing films of similar thickness, films on a substrate exhibit a reduced fatigue effect upon thermal cycling and a smaller hysteresis width. Differential scanning calorimetry (DSC) and cantilever deflection measurements (CDM) results showed that the transition temperatures of fabricated TiNiHf films and TiNiHf/SiO 2 /Si bimorph composites decrease with thermal cycling. The change in transition temperatures after 40 thermal cycles is significantly reduced for TiNiHf films bound to a SiO 2 /Si substrate compared to the functional fatigue DSC results reported for freestanding films. The thermal hysteresis width is also reduced for TiNiHf films constrained by SiO 2 /Si and Si substrates compared to freestanding films of similar thicknesses. With proper composition selection and microstructural control, TiNiHf films can be promising SMA films for bistable actuators with PMMA/TiNiHf/Si composites.

Aerodynamic devices, such as vortex generators, are often used to reenergize flow and improve aerodynamic performance of aircraft control surfaces. Often the static, non-moving surfaces are designed for specific flight conditions and decrease performance, such as increasing drag and fuel consumption, at other conditions. One example is vortex generators (VGs), small vanes located throughout the aircraft surfaces. VGs are typically not required for the entire flight profile but are essential for conditions such as low speeds during take-off and landing. The static nature of standard VGs stems from the inability to adapt conventional actuators due to mass, complexity, or footprint constraints given their small size and placement on outer surfaces of the aircraft. Shape memory alloys (SMAs) present an opportunity to enable actuation of such devices with a minimal mass and dimension, while still providing high energy densities. Additionally, SMAs can be passively used as sensors if carefully "tuned" to respond to the altitude temperature differential and passively actuate without the need for heaters, active controls, or additional sensors and instrumentation. In this work, the authors report on the development of low temperature SMAs for passively actuating VGs based on temperature changes from ground to cruise altitudes.

Conventional vortex generators (VG) in aeronautical applications are static vanes mounted on aircraft surfaces used to improve aircraft efficiency during low speed operations. However, during the cruise phase of flight, these static devices are always deployed and produce drag penalties. With the goal of improving aircraft efficiency, Boeing in collaboration with NASA Glen Research Center have developed and successfully flight tested environmentally activated SMART-VGs that repeatedly and accurately retract during cruise and deploy during take- off and landing. This application is distinctively enabled by the ability of shape memory alloy (SMA) actuation to produce large work outputs in compact volumes and operate as both a sensor and actuator. The SMART-VG project highlighted here was built upon recent advancements in SMA rotary actuation technology that included improved alloy systems, design tools, best practices, published standards and high-level wind tunnel and flight test demonstrations. This program successfully matured and validated the targeted alloy development and associated design processes in a unique way by demonstrating shape memory alloy reconfigurable technology (SMART) in-flight. The data from this flight test is being used to optimize a next generation design of the SMART-VGs that will be tested in 2022.

One of the effective methods to retrofit seismically vulnerable building structures is the use of supplemental energy dissipation devices. Such devices may decrease the seismic displacement and acceleration demands of the retrofitted structures, thereby mitigating damage to both structural and non-structural components. Due to the unique mechanical properties of superelastic (SE) Nitinol, such as high strength, significant elasticity, substantial energy absorption, and excellent fatigue resistance, various forms/shapes of SE Nitinol have been used to develop self-centering damping devices. SE Nitinol rings are particularly effective because they offer large ductility, can resist compression without buckling, allow multi-directional loading, and are cost-effective. Recently, an innovative class of self-centering damping devices incorporating SE Nitinol rings, termed SMA-based multi-ring (SBMR) devices, has been developed and numerically evaluated by the authors. Each SBMR damping device consist of at least one SE Nitinol ring and at least one supplemental energy dissipating (ED) ring. The rings are concentrically and tightly positioned inside one another such that they deform together. The ED rings are made of metals with high hysteretic damping capacity, such as mild steel or shape memory (SM) Nitinol. Under diametric deformation, both the SE and ED rings absorb energy, whereas the SE ring(s) are primarily intended to provide self-centering. Due to their shape, the SBMR devices may be installed in building frames through a variety of approaches, among which cross bracing is particularly efficient. This presentation evaluates the performance of SBMR devices through an extensive experimental study. This presentation discusses an extensive experimental study on four SBMR damping devices with different ring configurations. The initial test results for two single SM and SE Nitinol rings along with a double-ring device demonstrated the stability of the hysteretic responses of the proposed devices and their effectiveness in providing a balanced combination of damping and self-centering capabilities.

Auxetic shape memory alloy (SMA) materials are candidates for ballistics, deployable antennas, actuators, stretchable electronics, and biomedical devices. Auxetic materials are periodic structures characterized by a negative Poisson's ratio, meaning they expand laterally when stretched longitudinally. The work presented here shows the mechanical properties of auxetic structures are significantly improved by using sputtered superelastic shape memory alloy materials compared to traditional materials (e.g., Cu, Si). In particular, sputtered freestanding TiNiCuCo SMAs offer advantages as substrates for stretchable electronics. Two novel auxetic structures with enhanced expandability and compressibility are presented, fabricated from sputtered TiNiCuCo. The novel geometries presented in this work are based on the combination of the auxetic rotating rectangle structure where the rotating hinges are replaced by two common stretchable interconnects (e.g., serpentines and Archimedean spirals). The influence of functional fatigue on the electrical properties, thermal-induced and stress-induced phase transformations of the novel stretchable auxetic TiNiCuCo thin-films are presented.