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Crack Dynamics

Crack Dynamics

We see cracks every day—on the sidewalk, in a wall, or in a window. But just because they’re common doesn’t mean they’re easy to study scientifically.


Transcript

The science of cracking up. I'm Bob Hirshon and this is Science Update.

A small crack in a crucial part could crash a computer or even an airplane. Yet cracks of all kinds—from broken glass to faults in the earth—have been notoriously tough to understand. MIT engineering professor Markus Buehler says that’s because the big picture of a crack depends on the behavior of individual atoms ripping apart.

Buehler:

So there’s this challenge of coupling this large scale, which engineers are interested in, with the scale of a few atoms.

Using computer simulations, he and his colleagues found that right at the cracking point, the atomic bonds in the material radically change. Brittle materials actually soften, while flexible materials get stiffer. Understanding how that affects the whole fracture may lead to better models of everything from semiconductor failures to earthquakes. I'm Bob Hirshon, for AAAS, the Science Society.


Making Sense of the Research

First of all, you might ask: Why are cracks so hard to understand? If you’re wondering, find a couple cracks around you (in a window, a door, a desk, whatever) and take a close look at them. Are they perfectly straight? Probably not. Are they exactly the same thickness all along? Not likely. Now compare the two cracks. How are they alike? How are they different?

As you look, you’ll notice that cracks are actually pretty complicated. They don’t usually come in neat, straight lines. And predicting when they’ll happen, and what they’ll look like, is even harder. Exactly how much stress would you need to put on a plastic ruler to create a one-inch crack? Where would you have to bend it? What if you were just a little bit off? These questions may seem trivial when you’re talking about a ruler, but when you’re talking about ceramic parts in airplanes, cars, or the Space Shuttle, you’re talking about life and death. The fact is that cracks behave in mathematically complicated ways, and even top scientists with sophisticated computers have had a hard time coming up with a model that can make cracking more predictable.

Using computer simulations, Buehler is zooming in on the very beginnings of cracks: the exact point where the atoms that make up the material start to separate. Why? First of all, it’s easiest to study them at this point, since that’s when the cracking process is simplest. And second, the way the crack behaves at the starting point turns out to affect where it goes from there. Buehler’s work involves figuring out the motion of every single atom involved in this process, and analyzing how they affect each other.

Buehler’s work focused on the material’s hyperelastic properties. All materials have hyperelastic properties under the right conditions. If the material is very brittle to begin with, like glass, it becomes more flexible before it cracks. (Remember that this happens at a very tiny, microscopic point; it’s not like an entire window becomes rubbery.) In other words, the bonds between the atoms stretch a little before they rip apart.

On the other hand, if the material is more flexible, like rubber, the opposite happens: it stiffens before it breaks. You can actually observe this by pulling a rubber band tight—it gets stiffer and stronger the more you pull on it. Once it breaks, the crack can travel at faster than the speed of sound.

Buehler found that these hyperelastic properties at the cracking point are absolutely critical to understanding how cracks spread. You may not think that in order to understand an earthquake, you have to look at individual atoms, but that’s exactly what this research implies. And if mathematical models of cracking become more accurate, that could mean real results for everything from computers that rely on delicate parts to cities that sit on delicate faultlines.

Now try and answer these questions:

  1. Why are cracks hard to understand scientifically?
  2. What are “hyperelastic properties"? Why are they important in this research?
  3. Why did Buehler focus on the point where cracks begin?
  4. How could understanding how cracks form impact everyday life? Suggest possible examples not given in this story.

You may want to check out the March 10, 2006 Science Update Podcast to hear this Science Update and the other programs for that week. This podcast's topics include: what makes a song popular; spite in chimps; the up-side of parasites; the physics of cracking; fighting flesh-eating viruses; and tuberculosis.


For Educators

To explore the physics of other everyday things, see The Physics of Baseball, by ThinkQuest, The Physics of Ice Skating, created by two Penn State astronomy students, or Skateboard Science, by San Francisco’s Exploratorium.


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