Shape Memory Alloy Mechanisms for CubeSats

Glenn's revolutionary SMARS device is fabricated from nickel-titanium-halfnium (NiTiHf), nickel-titanium-zirconium compositions, or a combination. These compositions contain a secondary, nanometer-sized precipitate phase, which is produced through processes of compositional control and ageing heat treatments. Glenn's novel materials and processes have yielded a SMA composition that produces much higher stresses than other SMAs on the commercial market. The SMARS device is composed of 1) SMA material as the actuating member; 2) a casing heater placed around the SMA member; 3) a DC or AC power source to provide current through the heater; 4) pointed tips for acute penetration into rock formations; and 5) a hand-press to reset the SMA element after each use. In the rock-splitting process, a hole equal to the diameter of the SMA element is drilled in the portion of the rock where the fracture is desired. Next, the pre-compressed SMA is inserted into the hole, and AC or DC current is applied to energize the devices heaters. Once the heater achieves the critical transformation temperature, the SMA will begin to expand within seconds. Since its expansion is constrained by the rock walls, the SMA will eventually exert up to 1500 MPa of stress, splitting the rock apart. When the current is removed and the heater cools, the SMA material returns to its pre-compressed state. At this point, the material can be recovered, so the process is repeatable after reshaping. The SMA actuating members were also designed to achieve displacement greater than the materials strain output. Glenns SMARS device provides high-powered rock fracturing that is controllable, reliable, and comparatively simple without the use of explosives, hydraulics, or chemicals.

Glenn researchers have optimized how shape memory alloys (SMAs) are trained by reconceptualizing the entire stabilization process. Whereas prior techniques stabilize SMAs during thermal cycling, under conditions of fixed stress (known as the isobaric response), what Glenn's innovators have done instead is to use mechanical cycling under conditions of fixed temperature (the isothermal response) to achieve stabilization rapidly and efficiently. This novel method uses the isobaric response to establish the stabilization point under conditions identical to those that will be used during service. Once the stabilization point is known, a set of isothermal mechanical cycling experiments is then performed using different levels of applied stress. Each of these mechanical cycling experiments is left to run until the strain response has stabilized. When the stress levels required to achieve stabilization under isothermal conditions are known, they can be used to train the material in a fraction of the time that would be required to train the material using only thermal cycling. As the strain state has been achieved isothermally, the material can be switched back under isobaric conditions, and will remain stabilized during service. In short, Glenn's method of training can be completed in a matter of minutes rather than in days or even weeks, and so SMAs become much more practical to use in a wide range of applications.

Actuators typically have large footprints and mass to meet the power output needed for operation, leading to design hurdles for aircraft and space applications. Innovators at NASA Glenn developed two novel actuators with different configurations of tubes of SMA to provide rotary output. The SMA tubes are deformed in their martensitic condition and when exposed to a thermal stimulus, the tubes will revert to their original state while providing rotary motion. One variation of the innovation nests the SMA tubes within a rotary actuator imparting several technical benefits. Nested SMA tubes can decrease the length of the actuator while achieving the same twist angle. For the same actuator length, a nested configuration of SMA tubes can multiply the twist angle and improve the power output. A second variation utilizes SMA components as transmission elements in a ring drive gear to enable continuous rotation in one direction. Previous similar SMA actuators rotate in one direction while heating and the other while cooling, which can limit the output of the rotary actuator. The innovation developed by NASA allows for continuous rotation in ANY direction, thereby allowing the rotational output capability to be independent from the amount of cyclic angular twist provided by the SMA tubes.

Shape memory alloys (SMAs) are metals that can return to their original shape following thermal input. They are commonly used as functional materials in sensors, actuators, clamping fixtures and release mechanisms across industries. SMAs can suffer from dimensional/thermal instability, creep, and/or low hardness, resulting in alloys with little to no work output in the long term. To combat these deficiencies, NASA has developed a process of incorporating nanoparticles of refractory materials (i.e., carbide, oxide, and nitride materials with high temperature resistance) into the alloys. Using various processing methods, the nanoparticles can be effectively mixed and dispersed into the metal alloys as shown in the figure below. In these processes the SMA and refractory material powder is mixed and the refractory nanoparticles incorporated through extrusions, melting, or directly used in additive manufacturing to create parts for applications across the aerospace, automotive, marine, or biomedical sectors. The nanoparticle dispersion is a controllable method to strengthen the SMAs, increasing the hardness of the alloys, reducing the impact of creep, and improving the overall dimensional and thermal stability of the alloys. The related patent is now available to license. Please note that NASA does not manufacture products itself for commercial sale.

The Glenn technology utilizes SMA structural elements (wires and springs) interlocked via a unique layering pattern, allowing the structure to take on tubular geometries while exhibiting the same ride performance as traditional tires. Though previous tires have used SMA elements as load-carrying members, this new design offers an improved structural pattern - consisting of two layers of SMA elements. The primary layer is a single wire shape set into a coil and wrapped circumferentially around a wheel, which sets the overall tire geometry and provides added strength in the radial and axial directions. The secondary layer consists of smaller SMA springs interlocked with each other as well as the primary coil, acting as a sheath that sets the coil spacing and provides the necessary shear stiffness. SMAs are superelastic in nature and can take up to 8% effective reversible strain without yielding. The SMAs can handle up to 30x more strain, allowing the tire structure to undergo high levels of deformation without permanent damage. Because these tires do not rely on air, the risks associated with a flat tire are eliminated, and tire stiffness never varies (the tires never run 'under-inflated'). Furthermore, this airless tire design may enable the redesigning of wheel and braking systems. The first bicycle tire prototype was estimated to have a tire stiffness comparable to a road bike tire inflated to 75 psi, but with improved lateral and shear stiffness. By varying the SMA wire geometry, a wide range of tire sizes and stiffnesses is achievable. Rubber tread surfaces may be attached to the outside of the SMA tire for sufficient traction on a variety of terrain.