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November 28, 2017

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What Makes Nitinol So Special?

November 28, 2017

 

 

Nitinol is an alloy of nickel and titanium, discovered by Buehler and Wang at the Naval Ordnance Laboratory in the late 50s. At the time, they were working to develop a better missile nose cone material to withstand heat, fatigue and impact.  As the team was evaluating the crumpled samples made at the lab, one of the lab members applied heat from his pipe lighter to the sample, and almost magically, the sample reverted back to its original straight shape.  As exciting a material as nitinol was, it was very difficult to manufacture it until the 80s, where better manufacturing processes became available. This allowed for shape memory alloys, such as nitinol to find their place and applications as novel and exotic alloys. Nitinol goes through a transformation (martensitic transformation), where it changes its crystal structure when subjected to mechanical or thermal stress. At high temperatures, it has a simple cubic structure (austenitic or parent phase), whereas in low temperatures it assumes a monoclinic crystal structure (martensitic or daughter stage). Its biocompatible nature and its shape memory and superplastic properties, make nitinol an excellent material for biomedical devices, such as orthopedic implants, catheters, stents, and orthodontics. A nitinol-based device can be inserted into the body in a collapsed or compactly folded stage at low temperatures, and upon warming to body temperature, it resumes it originally designed shape, to restore a collapsed artery for instance. Nitinol orthopedic devices such as staples or screws are made to be inserted in one shape and upon changing shape, they can be used to generate stress across a fracture to induce healing.

 

If you take a sheet of nitinol that is meant to transition to a stiffer state at a temperature just a few degrees above freezing, you can put it on ice (like a ski on snow) and it will be very soft in its martensitic metallic state.  If you then heat it up just a few degrees above its transition point, it will almost instantly stiffen up as it enters its austenitic metallic phase.  The light bulb immediately went on and it occurred to us, what if you could put some nitinol inside a ski between the usual layers of wood or fiberglass. You could harness these properties to create a ski that changes stiffness on the go as needed!  Instead of using the usual layer of titanal or stainless steel, why don’t we put in a layer or nitinol with a very thin heating element hooked up to a small lithium ion battery somewhere on the ski or on the boot. We started tinkering and ordered some nitinol sheets from Germany and approached a custom ski maker here in the Boston area to make us our first prototype, after running extensive finite element modeling on the computer to convince ourselves that the idea had legs to stand on.  We also instrumented a standard ski with thermistors (digital thermometers) and skied for a whole season at different mountains in New England, in order to understand how the core temperature of a ski changes during use and with various conditions. We then modeled different temperature and snow conditions from cold hard snow all the way to slushy snow to test the stiffness of the skis subjected to carving skiing loads at different angles with the nitinol before and after a phase change.

 

Nitinol related youtube videos:

https://www.youtube.com/watch?v=2YVwpBAiA1A (by MIT)

https://www.youtube.com/watch?v=FBaIdvgbBAM

https://www.youtube.com/watch?v=gvb77eV7_XM (in French)

https://www.youtube.com/watch?v=JKBM9my5eOA

https://www.youtube.com/watch?v=UEaM-GjsV2o

 

 

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