Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: breaking bimetallic beads

Learn more physics!

Most recent answer: 12/19/2013
Q:
Hi, I have a question regarding thermal expansion or thermal shock. I know very little about this but I'm trying to figure it out. I have silver plated brass crimp beads that break when taken on long trips on an airplane. I know temperature affects metal and I figure that it can be due to a bimetallic strip formed by these two metals fused together and if one expands/contracts at a different rate than the other, it can crack. I've googled all types of bimetallic strips but no one can tell me what happens when there's a bimetallic ROUND object. Would it cause it to break when exposed to extreme temperatures? How extreme would it have to be? Thanks
- esther (age 23)
Brooklyn,ny USA
A:

Your general idea makes sense, but I think there must be some other complication in this case. The reason is that silver and brass have very nearly the same thermal expansion coefficients: 18.7x10-6/K for brass, 
19.5x10-6/K for silver.

It's possible that the thermal strain is caused by the outside cooling down quickly while the inside is still near room temperature. That could cause the outside plating to shrink and crack.

 Another possibility is that the brass undergoes a phase transition from one crystal form to another, which can happen a bit below room temperature for certain compositions of brass. I can't say exactly what temperature because it depends on the copper/zinc ratio of your brass. Also, the phase diagrams I've found don't go below room temperature, although from the part I could find it looks like these phase transitions would be present below room temperature. There's a story (nobody seems to be sure if it's true) that Napoleon's retreat from Moscow turned into a catastrophe largely because the tin buttons on the French winter uniforms disintegrated when they underwent a phase transition caused by the cold.

It's also conceivable that there are some fumes in the luggagae compartment that get into the brass and cause the "season cracking" described in the Wikipedia article on brass ().

How could you test these ideas?

If it's the first (thermal strain due to a temperature difference between the outside and the inside), then you could get the beads very cold (maybe to dry ice temperatures) without cracking them, so long as the cooling was very slow. Even modest cooling (say to ice temperature) might crack them if it's fast enough, say by immersion in ice water. 

If ithe cracking is caused by a brass phase transition, then the rate of cooling shouldn't matter much, only how cold they get.

If it's a chemical effect, you could probably get it to happen by exposing the beads to ammonia fumes at room temperature. Don't breath the fumes, ammonia is bad for you.

Mike W.


(published on 12/19/2013)

Follow-up on this answer.