Boiling and brittle

I had to finish breaking some plastic yesterday. A client had sent me some polycarbonate parts that were cracking in service and wanted to know why. This is the sort of thing I do when I’m not writing shell scripts—it pays better.

Normally, when you have a broken part, you can look at the broken surface—what we in the business call a fracture surface—to learn about how and why it broke. There are times, though, when the part has cracked but it hasn’t separated into two pieces. In those cases, you need to finish breaking it to get a look at the fracture surfaces.

Which leads to the question, How do I break it? Here are the criteria:

  1. You want make sure the original fracture doesn’t get changed by your fracture. This can happen if the material is ductile and bends all to hell while you’re breaking it.
  2. You need to be able to distinguish between your part of the fracture and the original fracture you’re supposed to be analyzing. Generally, this means the texture of the new fracture should be distinct.
  3. You want it to be easy. There’s no point in expending more energy than necessary.

All these criteria can usually be met by dunking the piece in liquid nitrogen before making the final break. Polymers that are ductile at room temperature tend to get brittle when they’re really cold. That satisfies Criterion 1. Unless the device was exceptionally cold while in service, the original fracture won’t be a purely brittle fracture the way your fracture will, and there’ll be a well-defined boundary between the two. That satisfies Criterion 2. Finally, brittle fractures are, by definition, low-energy fractures. That satisfies Criterion 3.

I wish I could show you some examples, but all the examples I have are proprietary. I can, however, show you something I always enjoy: the boiling that occurs when you drop a room temperature object into liquid nitrogen. You’ve probably seen demonstrations with liquid nitrogen—hammering a nail with a banana, shattering rose petals—but unless you’ve been right next to the person doing the demonstration, you’ve only seen fog coming out of the container.

The boiling slows down as the plastic approaches the temperature of the liquid nitrogen. What’s going on is qualitatively the same as quenching a red-hot piece of iron into a bucket of water, but with everything several hundred degrees colder.

You’ll note that I wasn’t wearing gloves. Don’t turn me in to the science fair safety police.