Abstract Shape memory alloys (SMA) provide a compact, robust, light-weight and scalable rotary actuation technology suitable for many aerospace applications that require precise control and high torque over a discrete output range. With their compact form factor and high energy density, SMA rotary actuators can often be directly integrated into or near a hinge line reducing the need for complex kinematics (e.g., linkages and gears) that are typically required to transfer the work from a rotary actuator’s output to an effector such as a flap or aileron. Boeing has executed high technology readiness level (TRL) demonstrations and production use of scalable SMA rotary actuation technology across several size scales and aeronautical platforms. Applications range from wind tunnel models to full-scale laboratory demonstrators and flight tests. Successful demonstrations include reconfigurable vortex generators, actuated wind tunnel models, adaptive trailing edges, power door opening systems, spanwise adaptive wings, variable area nozzles and variable geometry structures.

Abstract Shape memory alloys (SMA) provide a compact, robust, light-weight and scalable rotary actuation technology suitable for many aerospace applications that require precise control and high torque. Over the last 25 years Boeing has fabricated, processed and characterized several hundred NiTi-based tubes with the objective of optimizing performance for aerospace applications. The effects of supplier, material composition, processing, heat treatment, training parameters and component size were characterized and mapped in NiTi and NiTiHf systems. The effects of lower and upper cycle temperature (LCT and UCT, respectively), applied torsional loading (including nominal, minimum, maximum, reversed and varying), rotational limits (blocking) and repeated thermal cycling (to over 100,000 cycles) were systematically investigated. Based on those results, torsional SMA components were fabricated for optimal performance and evaluated under repeated thermal cycling under load to assess their ability to meet actuator requirements for an applications’ required life cycle. Additionally, isothermal torque-deflection and constant torque thermal cycling test (similar to E3414) have been performed across all relevant actuator parameters. This paper reviews and summarizes test outcomes, providing additional insight into the design and characterization of SMA rotary actuator components in context of the SMA material, component processing, operational parameters and their intended use in applications.

In this paper a series of low boom adaptive structure hardware demonstrators are described.The designs are enabled by recent advances in SMA technology including improved high temperature materials, better design and modeling tools, and industry approved test methods.The test hardware replicates the centerline keel of a representative supersonic aircraft.An array of SMA based actuators are used to modify the keel structure resulting in a change to the geometry of the Outer Mold Line (OML).The shape changes are intended to maintain a low boom signature in response to changes in Mach, angle of attack, flight path, or atmospheric profile.A description of the hardware design, SMA control system, and integration into a supersonic flight simulator are shown.Test results showing real-time geometry changes minimizing the predicted boom are presented.The design, build, and test of the adaptive structure demonstrators was led by a team of undergraduate students,

A method is presented to approximate perturbations in sonic boom loudness due to off-design flight conditions and non-standard atmospheres for a low-boom aircraft originally designed for idealized flight and atmospheric conditions. This approach leverages the ideal reference conditions to decouple the various perturbation sources in the near and far-field that affect loudness variability. By doing so two parallel studies are possible to evaluate the overall loudness variability instead of using a monolithic approach that is computationally more expensive. One study focuses on the variability of the aircraft near-field signature due to uncertain Mach and angle of attack while idealizing the boom propagation to the ground with a standard atmosphere definition. The other study focuses on the influence of temporal and spatial variability in atmospheric profiles on the ground signature while using the ideal near-field signature generated from ideal flight conditions. The loudness perturbations estimated in the near- and far-field are then aggregated with low error to estimate a net deviation from the ideal designed loudness of the low-boom aircraft. The method is assessed with an Euler CFD tool for near-field simulations and a non-linear wave propagation tool for far-field simulations using atmospheric profiles from a large weather database representing the temporal and spatial variation seen along a hypothetical transcontinental flight path.

Shape Memory Alloy (SMA) actuation is often cited as significantly lighter weight and more compact than conventional actuation methods such as electromechanical (EM) or hydraulic motors. Yet there few studies directly comparing and demonstrating a SMA actuation design to a conventional design for a specific application. In this paper a complete system level design, build, and test using both SMA and EM actuation are described. Adaptive spars for a medium sized UAV wing twist application are demonstrated and a detailed comparison of size, weight, power, and performance clearly demonstrates the advantages of SMA actuation.

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Near-equiatomic NiTi shape memory alloy (SMA) torsional tube actuators were trained for two-way shape memory effect (TWSME) by repeated thermal cycling under an isobaric load. Performance of the trained actuators was assessed by thermally cycling through a complete phase transformation under a range of isobaric loads that varied from negative to positive and included loads near zero. To assess the actuation performance of the trained SMA components, extended isobaric thermal cycling and cycling under varying loads to constant strain limits was performed. Additionally, isothermal loading was applied in the fully martensitic state prior to and following training. Results show stable TWSME when cycling under significant isobaric loading in the trained direction, however at low or negative loads (loads applied opposite to the training direction) a degradation of TWSME occurred during thermal cycling. Isothermal loading showed that martensite variant reorientation and detwinning was redistributed in the trained actuator when compared to an untrained actuator. Thermal cycling against constant strain limits was shown to have a negligible effect on the stability of the TWSME and overall performance of the trained actuator. Various combinations of isothermal and isobaric loading were shown to expand the operating range at low and negative loads. Additionally, load paths were identified which limit the degradation of TWSME over extended cycling. The aforementioned results are discussed in the context of the correlation between uniaxial isobaric and isothermal loading and texture measurements obtained by in-situ neutron diffraction at stress and temperature. These results show various thermomechanical combinations of heating and loading sequences that yield the same final martensite texture in SMA, which highlights the ability to take different paths yet still obtain the desired actuator response while minimizing irrecoverable deformation mechanisms. The implications of extending these uniaxial results to the design and fabrication and ultimately improving the performance of torsional SMA actuators are discussed.

A variably twisting wing has many beneficial qualities that can improve aircraft performance for a variety of flight conditions. Beneficial variable wing twist features include a means to reduce induced drag in cruise conditions, to increase lift performance, and to increase roll control. However, the actuation hardware usually required to twist large scale wings, especially given their structural stiffness, presents a substantial limitation. Through the use of shape memory alloy (SMA) torque tubes and other associated active spars, such structural deformations can be enabled. In this work, we consider the development of a benchtop and wind tunnel testing platform for potentially assessing the response of a variety of SMA-based wing twisting concepts. Using additive manufacturing (3D printing), we have designed, built, and tested a small scale prototype. The SMA actuated twisting wing developed herein consists of a rapid prototype shell that was specifically designed using a finite element approach to maintain stiffness in bending while reducing torsional rigidity. Using LabView and a simple PID controller, we have shown that an SMA torque tube was able to drive and steadily maintain the spanwise linear twist in the wing under both benchtop and wind tunnel conditions. This prototype will allow future assessment of new control schemes, new SMA actuator materials, and new structural configurations toward the development of flight-capable self-twisting wings.

The response of shape memory alloy (SMA) components employed as actuators has enabled a number of adaptable aero-structural solutions. However, there are currently no industry or government-accepted standardized test methods for SMA materials when used as actuators and their transition to commercialization and production has been hindered. This brief fast track communication introduces to the community a recently initiated collaborative and pre-competitive SMA specification and standardization effort that is expected to deliver the first ever regulatory agency-accepted material specification and test standards for SMA as employed as actuators for commercial and military aviation applications. In the first phase of this effort, described herein, the team is working to review past efforts and deliver a set of agreed-upon properties to be included in future material certification specifications as well as the associated experiments needed to obtain them in a consistent manner. Essential for the success of this project is the participation and input from a number of organizations and individuals, including engineers and designers working in materials and processing development, application design, SMA component fabrication, and testing at the material, component, and system level. Going forward, strong consensus among this diverse body of participants and the SMA research community at large is needed to advance standardization concepts for universal adoption by the greater aerospace community and especially regulatory bodies. It is expected that the development and release of public standards will be done in collaboration with an established standards development organization.

Shape memory alloy (SMA) actuators have recently been developed in the form of torsional tubes that can undergo large twisting deformations. Wing twisting has been investigated as a means to reduce induced drag in cruise conditions in small aircraft, but the actuation hardware required to generate wing twist at larger scales is prohibitively cumbersome. Replacing conventional actuators with SMA torque tubes provides a way to minimize weight of the twisting system but wing structural design then becomes more challenging. This analysis-driven design study examines an SMA torque tube as applied to the twisting wing design problem. A composite skin is considered to maximize wing performance under combined twist and aerodynamic loads. The SMA has been analyzed using a 3-D thermo-mechanical constitutive model while a preliminary study was performed to determine a composite lamina with appropriate unidirectional properties. An optimization was then completed to find an ideal composite layup. This optimization also included the design of a passive torque tube used to properly balance the twist generated by the SMA against that required in the wing. Localized buckling in the twisted wing was also considered and avoided. The product of this optimization was a composite wing that twisted while considering constraints of stress on the SMA. To validate the controllable use of SMA actuators, testing was completed on a scaled wing model fitted with a rapid prototype shell.

This chapter contains sections titled: Introduction Experimental setup and test matrix Fatigue tests results Conclusions Acknowledgments

While bulk shape memory alloys (SMAs) have proven a successful means for creating adaptive aerospace structures in many demonstrations, including live flight tests, the time required to cool such actuators has been identified as a property that could inhibit their commercial implementation in some circumstances. To determine best practices for improving cooling times, several approaches to increase the surface area and reduce the mass of existing bulk actuator technologies have been examined. Specifically, geometries created using traditional milling and EDM techniques were compared with micro-channel geometries made possible by a new electrochemical milling process developed at Northwestern. The latter technique involves imbedding steel space-holders in a matrix of NiTi powders, hot isostatic pressing the preform into a dense composite, and then electro-chemically dissolving the steel. Thus, in a two-step process, it is possible to create an actuation structure with numerous micro-channels with excellent control of geometry, shape, size and placement, to reduce weight and increase surface area (and thus decrease response time) without compromising actuator performance.In this paper, the new, lighter-weight, faster cycling shape-memory alloy actuation structures resulting from each technique are reviewed. Their performances are compared and contrasted through the results of a numerical study conducted with a 3D SMA constitutive law developed specifically to handle the complex, non-proportional loadings that arise in porous structures. It is shown that using micro-channel technology, cooling times are significantly reduced relative to traditional machining techniques for the same amount of mass reduction.