The idea of an airplane changing shape mid-flight probably sounds farfetched to just about everyone, even fans of the Transformers movies and classic cartoons. However, new technologies that use metals called shape memory alloys (SMAs) are allowing researchers to develop components of aircraft, cars, and even medical devices that can change and then regain their original shapes to perform different functions. Science in Society spoke with Aaron Stebner, PhD fellow and SMA expert at Northwestern, to learn more.
What is a shape memory alloy (SMA)?
An alloy is a metal that is made up of two or more different elements. A pure nickel or a pure iron is just a single element (in theory – there are usually traces of other things) but to make an alloy, you’re combining substantial amounts of two or more different elements together to create properties that one of the single-element metals wouldn’t have on its own.
An SMA, then, is an alloy that has a shape memory behavior. It’s literally what it sounds like – it can “memorize” a shape. You can do all kinds of crazy things to it and then, under some kind of stimulus – usually by heating it up or removing some load from it – it will go back to its “memorized” shape. There are alloys that respond to other stimuli, such as magnetic fields, but the most abundantly used shape memory behaviors are triggered by changes in temperature or applied force. (See an SMA demo video on the SiS Blog.)
What are SMAs used for?
Today, SMAs are predominantly used in the medical field. Nearly all stents are made out of SMAs. Orthodontic wires are also made out of SMAs. Their stimulus is a change in applied load. They’re stretched out (a large load is applied to them) before they’re tightened down against your teeth. That load is removed as your teeth come back to shape, and the alloys try to go back to their original shape, much like a rubber band would, only with a lot more force since they are metallic instead of rubber. Thus, they continue to align your teeth. My sister had non-SMA braces, and she had to get them tightened every week or two as the metals they were made from stayed stretched instead of springing back. Now, I think it’s usually more like every four to six weeks because of the shape-memory behavior.
A professor here at Northwestern, David Dunand, is an expert in developing ways to make porous, or non-solid, SMAs. One of the big potential applications for this is bone replacement. During hip or knee replacement surgery, these porous SMAs could be placed in your joint, and biological cells would grow within the pores of the alloy, adhering them to your existing biological systems. In addition to providing a place for cell growth, porosity provides a huge advantage relative to a solid material, because you can tailor how big the pores are, where they are, and how many there are, making the implant properties much more nearly match those of the bone that decayed or had been removed.
This is where my PhD advisor, Cate Brinson, lends her expertise and trains many of her students, including me. She is an expert at creating mechanical and thermodynamic models to replicate as well as predict the behaviors of SMAs, with her latest specialization being these complex, porous structures. You can imagine that there is an infinite amount of choices out there when it comes to pore shape, size, and location. We develop these models and build them into computer software that allows us to focus in on a few of the more promising pore configurations in a few weeks – something that would otherwise take many, many years if we had to physically make and test thousands of configurations.
What is your area of expertise?
While I have dabbled a small amount in medical applications, my field and passion is actuation, or moving things with SMAs.
My personal research is being applied mainly in aerospace. I did my master’s research at the NASA Glenn Research Center and now collaborate a lot with Boeing and a company in California that makes SMA actuation wires called Dynalloy. We’re looking at literally morphing aircrafts, like creating Transformers, using SMAs. We’re starting by giving different parts of the aircraft the ability to change their shape for different flight segments.
You could imagine that if a vehicle is taking off, the optimal shape of its structures is a lot different than if it’s cruising at altitude – those demands are very different. The amount of air coming into the engine is very different, and the noise that it’s allowed to put out without causing too much acoustic pollution is different. Right now, without the use of SMAs, aircraft structures are designed to one compromised configuration that can meet the demands of all the different flight segments and environments the vehicle might eventually go through. We’d like to either embed or make complete structures out of SMAs that could actively or passively change shape as the environment or flight requirements change. So you could have optimal fuel efficiency, reduce carbon dioxide emissions, and reduce acoustic pollution.
I’ve heard a lot about fuel efficiency and CO2 emissions in the news, but why is acoustic pollution so important?
The ability to reduce it allows larger airplanes to take off and land at regional airports, which are often located close to residential zones where there are strict FAA regulations as to how much noise can be made. I don’t know exact numbers off the top of my head, but the ability to use a single aircraft for a wide range of both national and international flights translates to a great deal of savings for the airline. When an airline only needs to order one aircraft to service all of its flights, they often receive a discount for ordering so many of them. The savings pile on as the planes stay in service: they only have to train their technicians to service a single aircraft, order replacement parts for a single aircraft, etc.
Where is this technology currently?
The new Boeing 747-8 and 787 engines were designed and tested using SMAs, work that was led by Tad Calkins and James Mabe of Boeing. If you look at the rear of one of the nacelles (engine cases), you’ll see that it’s scalloped like the edge of a seashell. These scalloped structures, called chevrons, create a disturbance in the airflow coming out of the engine, which in turn reduces the noise that the aircraft is emitting.
SMA beams were embedded inside of those chevrons in a test plane and used to change how far or nearly immersed in and out of the airflow each chevron was during a test flight. The goal was to find the configuration that would best optimize fuel consumption during cruise while still meeting FAA acoustic pollution requirements for landing at regional airports.
Using SMAs, eleven different configurations of chevron immersions were tested in a couple of hours. To acquire the same amount of in-flight data without the SMA-activated chevrons, Boeing/GE would have needed to build 11 different nacelles, land the plane after testing each one, change the rear engine casing, and then take off and fly again. Doing this eleven different times would take weeks if not months – not a very practical idea.
As a result of this optimization testing, the new 787s can do both regional and overseas flights, long and short distances, and land at small and large airports alike.
Ultimately, we’d like to have production (passenger) aircrafts with these SMA actuators embedded in the chevrons, so that they [can be put] into the airflow and really reduce the noise when [the plane is] landing, and then pulled out completely when [the plane is] flying. This technology will redefine “optimal performance.”
What research still needs to be done before this can happen?
We still have a lot of unknowns regarding the lifetime of these SMAs as they undergo dramatic environmental changes. The reason that most of the market [for SMAs] is in the medical field right now is because the temperatures and loads inside of our bodies are almost always constant. If you get a five-degree increase in body temperature, that’s a really bad fever. Relative to the SMA’s world, that’s a pretty steady environment. However, with an aircraft, you’ll have really large changes in environmental temperature as well as conditions – high winds, low winds, rear winds, head winds – that put all kinds of non-steady demands on the alloys.
I know you incorporate SMAs into your undergraduate design classes. Can you give me an example of their work?
Last spring, two groups of freshman and sophomore students (Jake VanderPloeg, Frank Cummins, and Ben Woldenberg; Michael Chen, Evan Hunt, Lyndon Sapozhnik, and Gregory Budd) redesigned the mechanism that releases the oxygen masks in case of emergency in a 747.
Currently, in production 747s, there’s a pneumatic, or air-driven, cylinder that deploys the oxygen masks. The students, who had never seen SMAs before my class, were able to design in a ten-week quarter a new latch-and-release mechanism that reduced the weight of the air cylinder by 70%. The original air cylinder weighed between 35 and 40 grams, and theirs weighed somewhere between six and eight grams. (See a video demo of the design on the SiS Blog.) You might think, in the weight of an aircraft, what would a few grams matter? But if you add these up over a few hundred oxygen mask compartments per vehicle, and then look at how valuable a pound is to an airline – it is a big deal to them. According to Northwest Airlines, every 25 pounds they remove from an airplane saves $445,000 per year (NY Times, 2008).
When did you decide that you wanted to be an engineer?
My grandfather, who was a motor salesman and then a self-taught engineer, started his own engineering company in the mid-70s. I didn’t make the decision at this age, but he used to take me to work with him from the time I was five or six. He would sit me down at a drafting table with a T-square and slide rule or put me at a workbench with a box of alligator clips and I’d build and draw things. I would stay completely fascinated. So he was really my inspiration and my role model as far as going into engineering.
I made the decision in high school and then doubted the decision shortly after high school. I actually dropped out of college a year before finishing my bachelor’s degree and worked at a nightclub and was a DJ for a few years. But then I felt a strong attraction to teaching, and subsequently realized that I needed to be in engineering again, so now I’m doing the teaching and engineering together. I love them both equally.