Introduction to SMAs
Shape memory alloys (SMAs) are metallic alloys that undergo solid-to-solid phase transformations induced by appropriate temperature and/or stress changes and during which they can recover seemingly permanent strains. Such alloys include NiTi, NiTiCu, CuAlNi, and many other metallic alloy systems . The phase transformation of an SMA is unique because such transformation is accompanied by large recoverable strains that have the potential to result in significant stresses when the material element is sufficiently constrained. Such strains are referred to as transformation strains and are in addition to standard thermoelastic strains. Because of their ability to recover strain in the presence of stress, SMAs are included in the class of materials known as active materials, which also includes piezoelectrics, magnetorestrictive materials, and shape memory polymers, among others. SMAs provide high actuation forces and displacements compared to other activematerials, though at relatively low frequencies.
Although they have been around for over half a century, new applications continue to be developed for SMAs. Many of these applications are intended to serve the needs of the biomedical industry while others are intended for use in consumer products.However, the aerospace industry is actively pursuing the development of new SMA technologies as well as assimilation of SMAs into existing systems.An SMA component, being both structural and active, can effectively reduce the complexity of a system when compared to the same system utilizing standard technology (i.e. an electromechanical or hydraulicactuator). This increased simplicity gained by trading multiple moving parts for a single active element can lead to higher overall reliability, especially at low cycles. Such an integration of structure and actuator can also be accomplished in a compact arrangement.This compact integration is possible due to the high actuation stresses and strains generated, leading to high energy density. These beneficial attributes make SMAs an attractive active material candidate as the aerospace industry continues to push for so-called‘smart’ structures and ‘intelligent’ systems. Thisis a natural evolution within the aerospace field as these systems are often the only viable solution to very complex engineering problems. Furthermore, the technological requirements of the industry, especially in the area of defence, often reduce the importance of cost as a design driver. However, as more SMA applications are designed, produced, and used, the affordability of SMAs will continue to increase.
Properties of SMA behaviour
As previously mentioned, phase transformation plays the key role in the SMA’s unique behaviour. The martensitic transformation converts the material between two particular phases, namely austenite and martensite. Austenite is the high temperature or ‘parent’ phase and exhibits a cubic crystalline structure whereas martensite is the low temperature phase that exhibits a tetragonal or monoclinic crystalline structure. The martensitic transformation is a shear-dominant, diffusionless transformation that occurs via the nucleation and growth of the marten sitic phase from the parent austenitic phase. The transformation from austenite to martensite may lead to twinned martensite in the absence of internal and external stresses or detwinned martensite if such
stresses exist at a sufficient level. Because the transformation from austenite to twinned martensite resultsin negligible macroscopic shape change, twinned martensite is often referred to as self-accommodated martensite. The reorientation of twinned martensite into detwinned martensite can take place under the application of sufficient stress.
Aerospace applications of SMAs
Many fields have been developing ways to convert thermal energy into mechanical work via the crystallographic phase change of SMAs,which have now been used in real-world applications for several decades. One of the most well-known of
these early applications was the hydraulic tubing coupling used on the F-14 in 1971 . Since that time,designers have continued to utilize both the shape memory and pseudoelastic effects of SMAs in solving engineering problems in the aerospace industry. Such implementations of SMA technology have spanned the areas of fixed wing aircraft, rotorcraft, and space craft; work continues in all three of these areas. The following section describes some of the more recently explored aerospace applications of SMAs and then briefly summarizes the challenges facing the designers of such systems.
Fixed-wing aircraft and rotorcraft applications
Applications which apply specifically to the propulsion systems and structural configurations of fixedwing aircraft will first be considered. Perhaps two of the most well-known fixed-wing projects of the past are the Smart Wing program and the Smart Aircraft and Marine Propulsion System demonstration. The Smart Wing program was intended to develop and demonstrate the use of active materials, including SMAs, to optimize the performance of lifting bodies. The project was split into two
phases with the first being the most SMA-intensive.Here, SMA wire tendons were used to actuate hingeless ailerons while an SMA torque tube was used to initiate spanwise wing twisting of a scaled-down F-18. In each of these applications, the SME is used to provide actuation via shape recovery, and the recovery occurs at a non-zero stress as described in section 1.2. Unlike the previous discussion, however, the stress state during actuation is variable and is a function of the elastic response of the actuated structure, in this case the wing. Although the SMA was able to provide satisfactory actuation at 16 per cent scale, it was found that the SMA torque tube in particular was not of sufficient strength to actuate a full-scale wing. As SMA material providers continue to increase their output, however,fabrication of larger SMA components for stronger actuation is now practical.
Spacecraft applications
Space applications are those which seek to address the unique problems of release, actuation, and vibration mitigation during either the launch of a spacecraft or its subsequent operation in a microgravity and zeroatmosphere environment. Although actuated structures in space are subject to low gravitational forces which reduce required actuator power, heat transfer can quickly become problematic because of the lack of a convective medium. It should be noted that for most designs described below, little or no modellingof the SMA behaviour was performed. Systems were designed through careful experimentation. Perhaps the most prolific use of SMAs in space is in solving the problem of low-shock release. These devices are quite popular in the design of spacecraft, and have been in development for some time . It has been estimated that, up to 1984, 14 per cent of space missions experienced some type of shock failures, half of these causing the mission to be aborted. Pyrotechnic release mechanisms were often found to be the root cause. Because they can be actuated slowly by gradual heating, SMA components are suited for use in low-shock release mechanisms and have been introduced for use on both average sized and smaller ‘micro’-sized satellites. The advent of these smaller satellites has created a need for more compact release devices which are an order of magnitude smaller than their off-the-shelf counterparts. Investigation into this unique problem has led to devices which are currently available, including the popular Qwknut. Other, much smaller devices which use SMA elements for actuation have also been proposed, such as the Micro Sep-Nut. In both of these devices, the simple SME is used. The active componentis deformed and detwinned beforeinstallation. In orbit, the element is then heated, shape is recovered, and release occurs. Repeated use mechanisms such as the rotary latch have also been introduced. Even smaller rotary actuators are being developed through microfabrication methods such as shape deposition manufacturing and electroplating. Using these methods, it was demonstrated that rotary actuators could be constructed with a maximum dimension of 5 mm, yet provide an actuation angle of 90◦. Each of these small release devices demonstrates the scalability of designing with SMA components. To provide the same compact actuation with conventional methods (e.g. electric motors) would require that very small moving parts be fabricated. Active SMA components, on the other hand, are on the same size scale as the actuator housing itself.
Design advantages, challenges, and the future of SMAs
Advantages and challenges of SMA design
SMAs are capable of providing unique and useful behaviours. The SME, especially when utilized under applied stress, provides actuation.The pseudoelastic effect provides two very useful advantages to the aerospace designer: a non-linearity which allows vibration isolation and large recoverable deformations as well as an accompanying hysteresis which can dissipate energy and therefore dampen vibration. Because of these, SMAs can provide a highly innovative method of addressing a given design problem and are often the only viable option.When considering actuation, a single SMA component represents a significantly more simplified solution than a standard electromechanical or hydraulic actuator. Compared to other classes of active materials, SMAs are able to provide substantial actuation stress over large strains. The subsequent high energy density leads to compact designs. Finally, SMAs are capable of actuating in a fully three-dimensional manner, allowing the fabrication of actuation components which extend, bend, twist, or provide a combination of these and other deformations. Each of the actuation application examples listed above exploit one or more of these positive attributes of SMA behaviour. Some require simplicity and resulting reliability. Others require compact actuation (active skin, microspace actuation), and still others impose geometric challenges (active chevrons , rotor blade actuation). Because of their unique properties, SMAs are able to provide solutions to each of these sets of problems.
The future of SMAs
The enabling advantages of SMA utilization often outweigh the challenges, and because of this, the future of this field is promising. As more applications across all industry sectors are designed and put into use, the SMA market will continue to grow and the cost of the material will continue to fall. The medical industry
seems to be a key driver of this trend. At the same time, the quality of material produced will increase while advances in SMA research will lead to new alloys and much improved design and analysis tools.The ‘smart materials’ market worldwide is growing at a strong pace, and will continue to grow into the foreseeable future .
Although the market for conventional SMAs continues to grow, new alloys are also being developed.As described above, conventional SMAs are capableof providing motion and force as a result of manipulating a single field, namely temperature, over a reasonable range. However, new alloy systems are being designed which increase the utility of SMAs, and research on these classes of materials is currently very active. One class can be used to provide actuation as a result of applied magnetic fields, and these materials are known as magnetic shape memory alloys (MSMAs) .Because they convert the energy of magnetic fields into actuation, MSMAs are not hindered by the relatively slow mechanisms of heat transfer. Therefore, high frequency actuation is possible. Although several such alloys have been discovered (NiMnGa, FePd, NiMnAl) and actuators based on these alloys are already commercially available, fundamental research will continue as the mechanics community seeks to understand the constitutive behaviour of these novel alloys. Another new alloy type can actuate at high temperatures, and this class is therefore known as high temperature shape memory alloys (HTSMAs). HTSMAs, include NiTiPd, NiTiPt, and TiPd, and are being widely studied. These alloys can actuate at temperatures ranging from 100 to 800 ◦C, and potential applications include oil drilling support and actuation of internal jet engine components. Basic research on these materials will include experimental observation and theoretical modelling of any viscous behaviour exhibited due to sometimes lengthy exposure times at elevated temperatures.