Ellis Davies reports on a temperature triggered shape memory alloy that could make aircraft more efficient.
When debating the viability of commercial supersonic flight, the question of efficiency is one that plays a large role. A new shape memory alloy (SMA) could help boost the efficiency of sub- and supersonic flight by allowing wing tips to fold down or up during cruising, without the use of heavy hydraulics. A folded wing tip can improve aerodynamic properties and increase controllability – the static nature of conventional wing tips induces drag and inefficiencies in the flight. Most commercial aircraft have static wing tips, meaning that they stay in one position at all times. However, they are only really used during take off and landing.
Developed collaboratively by Armstrong, NASA’s Glenn Research Centre, Langley Research Centre, Boeing Research & Technology, and Area-I Inc, USA, as part of NASA’s Spanwise Adaptive Wing project (SAW), the alloy is nickel-titanium-hafnium, engineered with a microstructure containing fine nanometre sized precipitants to give it increased strength under loading.
Off the shelf, the alloy is not suitable for use in aeronautical applications, as it has limited resistance to high temperatures, meaning it will overheat, or never cool, just by sitting in an airport or somewhere hot.
Dr Othmane Benafan, Materials Research Engineer at NASA, told Materials World about the changes made to the alloy. ‘We have taken this alloy and added another element to it – hafnium. This gives us multiple capabilities, one being raising the transformation temperature, so that the activation point will never be reached accidentally.’ By doing this, the team is able to activate the SMA using a control signal that activates heaters, which bring the material to the correct temperature for actuation to begin. Once the temperature is reached, the wing tips fold to a predesigned angle, which can increase the efficiency of the aircraft in flight.
Using hafnium to raise the transformation temperature of an alloy is not a new discovery, and has been carried out multiple times at lab scale. ‘What we’ve done is not only create the new chemistry,’ said Benafan. ‘But we have taken it all the way to producing hundreds of pounds of material, showing how you can scale to a production level that a vendor can use commercially.’
In order for the SMA to perform as needed, the researchers have to carry out what they call training. This process involves thermo- mechanically cycling the alloy over a period of days to train it to respond, and actuate, in a certain way, to a given temperature. This can take weeks with a standard SMA, but the microstructure of the hafnium infused alloy allows researchers to drastically cut this time. ‘We introduced a secondary phase, which involves precipitants inserted into the microstructure of the metal,’ Benafan said. ‘They aid, block, and insert deformation modes, so plasticity doesn’t occur early, which can ruin the metal. Now, we can train in a day or two. This reduces the time significantly, but also lessens any damage that may occur. This is the key to these alloys.’
Why foldable wings?
‘The biggest benefit of foldable wing tips, in our opinion, is in supersonic, especially for future commercial flights,’ explained Benafan. ‘If we can move the tips down once the transition from subsonic to supersonic speed is complete, we can reduce drag significantly and increase the compression lift. That’s a huge benefit given that the aircraft is going at a much higher speed and consuming more fuel.’ Benafan also mentioned storage benefits, with longer wingspan aircraft being able to decrease size to fit into storage hangers.
The team carried out a successful test flight using a drone in December 2017, but the SMAs have other uses outside of aeronautics – chiefly, says Benafan, in space. ‘We send a lot of satellites into space, and you can imagine the temperature extremes they face,’ he said. ‘We are using some of these alloys in folding solar panels and gates for thermal control. They are being used right now on a satellite on which we wanted to deploy panels after launch, during which it needed to be as compact as possible. Once it reaches space, the solar panels are deployed when activated by the heat of the sun.’
The SMAs could also find their way into the oil and gas industry, in the form of valves. ‘If you have a valve in the system, and something gets very hot, a SMA will sense the heat passively and will close/open the valve to rectify the situation,’ said Benafan.
This passive activation is also something that the team will look at going forward. However, these alloys will be activated by the decrease in air temperature from the ground to cruising altitude, so will need to be re-trained to be cold temperature alloys, which Benafan called a significant challenge.