Nickel-titanium, commonly referred to as Nitinol, is a shape-memory alloy with superelastic properties that make it useful in certain environments and applications. These properties include shape-memory, flexibility, and durability. Titanium, a primary material, is a material that reacts with carbon and oxygen when molten and is not easy to work with. Consequently, there are two commercially viable methods for producing Nitinol that account for titanium’s reactivity: vacuum induction melting and vacuum arc re-melting. One difficulty that engineers have encountered in the field of space exploration is creating solutions to problems that are long-term. Longevity is important because repairs to a mechanical system are often impossible post-deployment. Rovers are a well-known instance of this problem. The wheels are an important part of the rover because they are required for getting around on the planet, but they often sustain heavy damage inflicted by treacherous terrain. One way to avoid using mechanical systems is to take advantage of material systems. Therefore, space exploration is an area where Nitinol has a great deal of potential. Nitinol is being considered as the main component of a flexible spring tire that will carry future rovers across the surface of Mars. This is one of the most practical applications of Nitinol because it perfectly fits the criteria of the required material, which guarantees an improvement in the long-term viability of the design. This increased longevity means that there will be less of a need for replacement, which means that this solution is more sustainable than previous ones. However, these Goldilocks conditions are infrequent; it turns out that Nitinol is often more expensive than practical for everyday problems and certain functions that its material propertiies can perform are outperformed by mechanical systems.
Nitinol fits into a class of material called “shape-memory alloy” whose members exhibit the properties of superelasticity and shape-memory. Elasticity is the ability of a material to return to its original form after a compression or tension force is released. As an extension of that, superelasticity is a term to describe the extreme degree of elasticity these materials demonstrate. Past the point of maximum elasticity is plastic deformation, which describes what happens whenever a compression or tension force deforms a material to the point of permanence. Shape-memory is the ability of a material to return from a plastic state to a previously programmed shape.
Shape Memory by Heat Treating
Shape-memory abilities are perhaps the most impressive property Nitinol has. Nitinol is ductile, meaning that it can be drawn into wires. These wires can then be “programmed” to remember a shape by heating the material to a certain temperature, known as the threshold temperature. The shape can then be changed to something entirely different, but the Nitinol will return to the original shape once the threshold temperature is applied. There are two kinds of shape-memory:
one-way and two-way. The process of programming both oneway and two-way Nitinol falls into a broader category of molecular manipulation called heat treating.Heat treating is the general term to describe the altering of the physical properties of a material. It involves heating a material to a certain value and then allowing it to cool at different speeds. Quenching and tempering are common types of heat treating. Forged ferrous metals like steel are both quenched and tempered. The quench is done by removing the hot metal from the forge and submerging it in liquid to rapidly cool it. In steel, this facilitates the production of hard martensite, which hardens the material to an extreme; the resulting material is brittle and can shatter when struck. Tempering is the process of heating metal in the forge and letting it cool slowly. If done after a quench, the metal becomes less brittle and more flexible and will no longer shatter.
As a shape memory alloy, Nitinol is extremely flexible and, according to NASA researchers, can “undergo significant reversible strain (up to 10%), enabling the tire to withstand an order of magnitude more deformation than other nonpneumatic tires before undergoing permanent deformation.” Such other alloys include spring steel- a type of steel which has a carbon content between 0.5% and 1% and is of medium to high hardness. Spring steel is capable of deforming about .3%-. 5% before plastic deformation, which means that Nitinol outperforms spring steel when it comes to withstanding and recovering from stress. This is visually demonstrated in Figure 1 below, in which the black line is steel and the two blue lines are Nitinol. For steel, the linear increase of stress versus strain shows the range of elastic deformation of steel.
The graph begins to curve at .3%, which shows that the steel has begun to plastically deform. It continues to plastically deform until it breaks. Nitinol demonstrates a similar linear increase of the stress versus strain, but it does not experience as much stress as steel at higher strains. Following the linear increase, Nitinol also appears to show plastic deformation, which is thought to be the case because as the strain is increasing, the stress is near constant. At the elastic limit of 8% the strain on nitinol is slowly removed, but the material returns to its original shape. This means that the portion of the graph that was thought to represent plastic deformation was actually representing super elasticity. The only time that Nitinol would actually plastically deform would be once the 8% elastic limit is reached, but the material can return to its original shape if it is heated past its threshold temperature.
Durability
Durability is an important quality for any material to have, but it does not always mean the same thing for every material because material durability is implicitly dependent on how well the material performs its function, which can vary significantly. A comparison of Nitinol and steel applications is useful in explaining this; steel is good for bridge-building because it is strong and rigid to a useful extent, but it was found to be not durable in its applications in the tires that will be discussed in later sections. On the other hand, Nitinol would make a poor bridge because it is flexible, which is why it performs well as a tire. As the wheel traverses over rocks and other sharp objects, it is able to deform greatly rather than sustain permanent damage. Ultimately, this allows the wheel to last longer, which is crucial in an environment where repairs of damaged components are virtually impossible. Additionally, this lack of a need for repairs will reduce the amount of Nitinol waste and will limit the amount of resources used. The manufacturing of the material contributes greatly to the durability of Nitinol because work hardening can reduce deformation stress. Work hardening also increases strength and gives it two-way shape memory abilities. The manufacturing process is an important step in the production of Nitinol because it provides an opportunity to adjust the properties of the material by way of composition. This is another one of the main characteristics of Nitinol.
Nitinol Tires
A History of Rover Tires and Introduction of a Problem In the 1960s and ‘70s, before Nitinol tires or even the Spring Tire model were conceived, there were three designs that NASA and other space agencies experimented with. The Russian Lunokhod lunar rover used rim wire mesh with protruding ridges for grip, stabilized by what were essentially bicycle spokes The Modularized Equipment Transporter (MET) Rover was a two-wheeled cart featuring rubber tires that were inflated with nitrogen. They were designed to make it easier to pull the cart through the soft and forgiving lunar soil. The NASA Lunar Roving Vehicle used mesh tires with inner frames to compensate for inner deflection. None of these models were very durable and they would plastically deform. Finally, the Spring Tire was designed in the early 2000s. The original Spring Tire was a construct of springs made from spring steel woven together into a flexible mesh sheet, which was arranged around a central axis in the shape of a tire. This tire design was better than previous designs because it was made of a more durable material and generated good traction on sand and rocks, but the spring steel tire was still susceptible to deformation because of the hollow nature of the structure. These wheels would handle well on the moon, but the entire surface of Mars is treacherous terrain and a wheel that is permanently deformed is not useful. Damping capabilities, which are not provided by the tire, are essential for maintaining the performance of instruments and other components that cannot be repaired easily after deployment. Curiosity used a rocker-bogie wheel system, which mechanically helps provide the soft ride that NASA was looking for. The attached wheels are the bogie and the single wheel is the rocker. Each set of these systems is attached in the middle to the other side via a differential. This system allows the rover to maintain a relatively constant weight on each wheel. It also provides stability and minimizes rover tilt. but this system limits the rover to only being able to drive over rocks as tall as 31 inches. This system also limited the travel speed to an average of 30 meters an hour, which is incredibly slow . In 2013 when the wheels on Curiosity began to experience significant wheel damage, engineers were concerned that the rover would not be able to travel far enough to complete its mission. Engineers began to examine the Spring Tire as a potential solution to the unforgiving Martian terrain and began to work on improving the prototype Spring Tire models for future rovers.
Nitinol Spring Tires
Nitinol’s ability to overcome plastic deformation made the use of Nitinol a significant improvement over the use of spring steel in tire models. With the same construction, the Nitinol tires could deform up to thirty times more than what could be achieved with steel without having permanent deformation occur. According to Santo Padula, a Materials Scientist who was an integral part in the development of the Nitinol Spring Tire, the tire could “deform all the way down to the axle and [still] return to shape”. Even if the wheel did have to ability to plastically deform, the wheel can be heated past the transformation temperature and the shape memory ability will kick in. The extreme deformation is valuable because the surface of the tire can deform to match the contours of the surface it is on, create a powerful grip on the ground, then return to the original round tire shape as the wheel turns. With the increased flexibility, the wheel is able to grip the terrain better than an ordinary wheel as it is able to conform to the exact geometry of the terrain. Additionally, the flexible potential of Nitinol can be changed by adjusting the shape and composition of the material. This allows the wheels to deflect a greater amount, resulting in more traction on various terrains. Ultimately, this means that there are more design options with differing levels of Nitinol dependence and mechanical system dependence. Other changes include limiting the energy transferred to the vehicle and increasing load carrying potential. As a direct result of this flexibility and increased grip, thespeeds of the vehicle can be increased, and the load carrying potential is greaterDefinite values for the top speed and load carrying potential are currently not available because more testing needs to be done before they can be determined.
Why Is This Important
The idea to use Nitinol wheels on future Mars rovers is one that still requires a lot of research and time until it is seen in application. Traditionally aluminum wheels have been acceptable at traversing the surface of Mars because of howlightweight and strong the material is. So there is still a lot of testing that needs to be done before the traditional aluminum wheels are retired. From the research that has been performed, it can be concluded that Nitinol wheels currently outperform the traditional wheel. The application of Nitinol wheels will affect the future of planetary exploration, and the human race. Ultimately, outer space research has led innovators to the development of countless technologies that are now used widely by the public. The continuation of this research is required in order to further the technological capabilities of the modern world. Nitinol will help accelerate this process, and will lead to new discoveries, making it a viable engineering innovation.
This technology is not only applicable to Mars rovers, it can actually be applicable to many aspects of everyday life. Glasses can be made out of a nitinol alloy, enabling them to be extremely flexible and not break when bent. Airplanes can have more efficient wing designs as the winglets on the endscan easily be manipulated to change the drag on the wing. Nitinol wire can even be used in orthodontics as orthodontic wire so that the wires do not need to be adjusted as often. These are all ways the technology can be implemented today, but there are probably many more undiscovered capabilities of Nitinol technology.
Sustainability
Whenever creating a new product or solution, engineers have to consider the sustainability of that innovation. The questions of sustainability need to be asked. When answering that question for the case of Nitinol the overall answer is that Nitinol is sustainable. A contributing factor to this is the durability of Nitinol. This is because the increased durability of Nitinol reduces the amount of waste due to rocket launches and repairs needed to supply or fix rovers. This is because there will be a lower demand for the replacement of old rovers and the wheels on them. Additionally, the increased durability will lower the amount of waste on Mars due to old and damaged wheels. Any actual waste due to Nitinol can be re-melted and then reformed into replacement Nitinol wheels. The only problem with this is that expensive systems are required to do this and so either expensive machinery would need shipping to Mars or the Nitinol would need to be shipped back to Earth.
This is not as great of a concern as Nitinol is actually quite environmentally friendly. Nitinol has been used inside of humans as stents and to function as bone casts. Another factor that contributes to the sustainability of this technology is the flexibility of Nitinol. As colonies are established on Mars there will be a need to have a central landing location for food and other necessities, and this means that these goods will need transported. With current rovers that move at a rate of 30 meters per hour, this would take an extremely long time. This could lead to the possibility of colonies not receiving shipments in times and thus have to come up with a way to solve that problem. Since Nitinol wheels will be able to increase the speed of the rovers, this will not be as much of a problem because the rovers will be able to move quicker, getting the goods to their destinations on time. This increased rate of movement also allows colonies to be established further away, possibly allowing for different research opportunities at different locations. These increased research opportunities will facilitate the development of new technologies and will be beneficial in the long term. Overall the sustainability of Nitinol when applied to rover wheels is very high. The features of Nitinol that make this application sustainable are the high durability, flexibility, research opportunities, and increased range. The problem when considering the sustainability of Nitinol is the cost, and overall the benefits outweigh the high initial cost of Nitinol wheels on rovers.
Nitinol is a fantastically unique material that has the ability to do things that other materials are simply incapable of. Sometimes it seems like a supermaterial. But if it is this amazing, why can you not find it everywhere? Frankly, Nitinol is expensive, it can be impractical, and there are sometimes better alternatives. For example, research was done investigating whether or not shape-memory alloys would perform well as winglets on airplane wings. As it turns out, it is very difficult to control the temperature of a material that isflying several hundred miles per hour on a planet that has seasons. It can be done but with difficulty. Maintaining a shape with a mechanical system made far more sense than by doing it with material properties. Another example is that Nitinol can be used very effectively as an orthodontic wire, but it is not practical when much less expensive materials can be used almost as effectively. And while the Nitinol spring wheelwill likely perform well on Mars, on Earth they would be expensive, have poor traction on flat asphalt where most driving is done, and be expensive to repair. This would make Nitinol not sustainable on Earth applications, but when considering the applications on Mars the material is very sustainable.
In short, Nitinol is useful, but it has a niche, and outside that nichel it is not always practical or better than competing materials. At this point in time Nitinol rover wheels should be considered as they are very sustainable and will lead to advancements in society. Ultimately, in the case of rover wheels for Mars missions, there are no other designs that are challenging the re-invented wheel.
