Nitinol wire (shape memory alloy) - experiments and devices

Nitinol is an alloy that will remember a shape you select, and whenever you heat it to a certain temperature called the transistion temperature, it will automatically go back to the remembered shape. Below are things I've made using nitinol wire along with some tips I've learned along the way.

Simple experiment - the letter "R" for Rimstar.org

A simple first experiment was to bend nitinol wire around some nails into the shape of a letter R. I then heated it using a candle flame to above 500C (950F) while still held in the shape of an R. I next quickly dipped it cool water. After that the shape is remembered by the wire.

Bent in the shape of an "R".
The nitinol wire bent in the shape of an
Heating above 500C (950F).
Heating the nitionl wire above 500C (950F) to program its shape.
Quickly cooling in water.
Quickly cooling the nitinol wire in water.

I next straightened out the wire and then quickly put it in the candle flame again in order to get it to the transistion temperature. It went back to the remembered shape of the letter R.

Straightening the wire.
Straightening the nitinol wire before restoring its shape.
Restoring the shape.
Restoring the remembered shape by heating the nitinol wire to 
      its transistion temperature.
The shape restored.
The nitinol wire's remembered shape restored.

Measuring the force of a nitinol spring and using electricity to heat it

I also made a spring by winding some nitinol wire around a thick, rigid clothes hanger wire.

Making a spring out of nitinol wire.
How to make a spring using nitinol wire.

I wanted to test how much force it could produce when heated back to its remembered shape. To heat it to its transition temperature I ran electric current through it with the setup shown below. The nitinol spring was hung, stretched out, from a spring balance and the current was measured on a meter. A potentimeter was connected in series with the circuit to help limit the current.

Setup for testing the force.
Setup for testing the force when the nitinol wire is going
      back to its remembered shape.

As the current was turned up and reached 0.7 to 0.8 amps, the nitinol wire began to return to its remembered spring shape. In doing so, it pulled down on the spring balance as much as it could. The balance showed 27 grams. 27 grams x 0.0098 Newtons/gram gives a force of 0.26 Newtons.

Measuring the equivalent mass.
Measuring the equivalent mass of the nitinol spring when
      returning to its remembered shape.
The equivalent mass - 28 grams.
The equivalent mass, 28 grams.

Nitinol wire/shape memory alloy inchworm

As a fun project I made an inchworm using a nitinol wire coil to make it walk across a table.

Nitiniol wire inchworm.
The equivalent mass, 28 grams.

Tips for working with nitinol wire

Here are some things I learned from working with the nitinol wire.

Accidentally increasing the transition temperature

I purchased the wire that I demonstrate above by shaping into an R and that I shape into a spring below from Kellogg's Research Labs. According to their forum, to set the wire's shape into it's memory you heat it to 500C (950F). Also, according the the specs for the one I bought, the transistion temperature at which it restores itself back to its remembered shape is 70C (160F). But I couldn't get it to go back to its remembered shape even by putting it in hot water up to 100C (212F). Instead I had to put it in a much hotter candle flame or run electrical current through it, producing a higher temperature.

My guess is that at some point I had unknowingly raised the transistion temperature. Again, according to Kellogg's forum, heating this wire to 350C (660F) will raise the transistion temperature. My guess is I'd done that by leaving it in the candle flame sometimes for too short a time when trying to set the shape. It's also possible I'd sufficiently heated part of the wire to set the shape but since I hadn't cut that part from the rest of my wire, the section of wire next to it got heated to a lower temperature around 350C (660F), raising the transistion temperature for that section. When I later went to use that section, it had a higher than expected transistion temperature.

So be careful of the temperatures you expose your wire to.

Accidentally reprogramming it

Since I didn't know the new transistion temperature for my wire, I sometimes used a candle flame to get it to go back to its programmed shape. If I left it too long in the flame then its temperature sometimes went as high at the required 500C (950F) to set a new shape. The remembered shape changed to whatever shape it happened to be in at the time.

So again, good temperature control helps. For my spring experiments, at least by using electrical current to get it to go back to its programmed shape, and since I had control of the current using my power supply and potentiometer, I had some control of the temperature that way.

Video - Nitinol wire - How to Use it

In the following video I start with the demonstration of making the letter R and then follow that with making the spring and measuring the force as the spring restores its shape using electric current to produce the heat.

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