Strange Effects When Nitinol is Annealed at 300 C electrically

[scott_alexsk]

Hello,

I was curious about any thoughts any of you might have concerning an interesting result I found today. While I was annealing a 32 cm wire of nitinol at about 300 C, which for its diameter is about 1 amp, I noticed that when the wire was made less tense, it moved up and down systematically. I noticed if I blew on the wire, making it cold, it tensed up and the effect temporarily disappeared. The effect also went away when I decreased the amperage a couple hundreths or increased the tension.

When my teacher and I looked at the wire more closely, besides moving up and down systematically about 4 millimeters, for 32 cm of wire, the wire seemed to bunch up forming a continuous standing wave. The best we could think to explain it is that since 300 C is the minimum temperature at which nitinol can be annealed, changing air currents or changing the amperage, decreased it below that certain threshold. The waves we believe were caused by the fact that we had an AC generator right below the sample on the counter, generating the one amp.

Even with this explanation though, certain descrepancies remain. The wire for one moved systematically up and down, and to a lesser extent side to side, besides having the apparent standing wave. I am going to go back tomorrow and test it at a location further away from the AC generator to see if there is a change, but I would appreciate any comments on possible reasons for this effect.

Thanks,
-scott

 

[scott_alexsk]

Could a mentor please change the thread title to "Strange Effects When Nitinol is Annealed at 300 C electrically"? The current thread title is misleading.

Thanks,
-scott

 

[Gokul43201]

The setup and behavior is not entirely clear. Can you include a picture?

More questions:
1. What's the TT for this particular wire?
2. What kind of power supply were you using? What was the frequency of the AC?
3. What was the frequency of the motion of the wire?

 

[scott_alexsk]

Gokul,

I recently looked at the setup again. I tried annealing another long wire at a distance significantly farther away from the powersource. I was using DC current. Anyways I found that the continual motion was not so systematic with the second experiment. I think now that since I was annealing nitinol on the edge of its annealing range, slight changes in air temperature resulted in changes in grain response to stress. Essentially internal stress was relieved at 300 C, and the wire moved easily in the direction of applied stress, in this case with gravity, but when slightly cooled the grains stopped migrating and turned back to a stiffer position. I am not sure what caused the small standing waves on the wire. It could have been an actual magnetic field effecting it, or it could have been that I was just straining my eyes too hard. Attached is a diagram of the experiment setup. I was orginally doing this to reset wires for Two-Way Effect tests.

Thanks,
-Scott