Science Explains Why CDs Shatter Upon Spinning too Fast

cd-shattering
The CD shattering at more than 76,000 frames per second. The Slow Mo Guys / YouTube

You might not expect this, since CDs and DVDs are made to be spun around, but if you rotate one of them very quickly it will shatter, appearing to immediately become a bunch of chunks of sharp plastic.

But this process is not, in fact, an instant one. If you film it happening in super, super slo-mo, you can see cracks spreading throughout the disc in an intricate, beautiful pattern.

It looks like this:

But what exactly is going on? Can science help explain why it cracks in this way, and so quickly?

“When the disc rotates, the centrifugal force causes it to move outward and expand,” says Min Zhou, an engineer and expert on the mechanics of materials at the Georgia Institute of Technology. Centrifugal force is that which causes your arms to fling out when you spin around. “The faster the rotation, the larger the expansion,” he continues. This causes the disc to stretch, with the energy of the motion being converted into elastic energy, like that stored in a rubber band.

But because the disc’s polycarbonate material is rather brittle, it can’t stretch much, and so the elastic energy leads stress to build up. At high speeds, it also becomes slightly uneven, like an askew car tire, and begins to wobble or vibrate, says Huseyin Sehitoglu, a mechanical engineer at the University of Illinois. This adds to to the growing stress.  

Once the amount of strain exceeds the “tensile stress” of the material, or how much it can take before breaking, “the failure can be unexpected and very sudden,” Sehitoglu says. Zhou likens it to a rubber band snapping.

“This happens normally at one location, the weakest point, first where there is microscopic defect such as a very small void,” Zhou adds.

The crack spreads very quickly, at speeds over 3,000 miles per hour, he adds.

But the crack is also unstable; instead of going straight along one path, it rapidly branches as quickly as possible. This is “because two cracks can consume more energy than one crack, [so] developing multiple crack fronts is the fastest way to release energy,” he says.

It’s basically like a DVD-shaped plastic band snapping into a hundred pieces of glimmering polycarbonate.