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October 2, 2012

Slate: The physics of cracking an egg

(Continued)

If you take an egg, hold it between your thumb and middle finger by either pole (the tip and bottom of the egg), and press down hard, it is unlikely to crack. Your thumb and middle finger can't exert sufficient force to bring the shell to a breaking point. Even if you press against each pole with both hands, as hard as you can, chances are you won't be able to crack the egg. It has very high rigidity, or stiffness, in that direction.

That's because the most crack-resistant part of the eggshell is its narrow tip. The more sharply the egg curves, the more rigid the shell becomes. However, if you turn the egg 90 degrees and press down right in the very center, it will be far easier to crack — and the MIT model accounts for that. "We can't tell you precisely when the egg is going to break," says Reis, "but we can tell you why it makes sense to break it along that direction."

Reis and Lazarus aren't the only scientists who are intrigued by the unusual properties of eggshells. There is a surprisingly long history of scientific studies of eggshells, not only in the agricultural sciences but also in the aerospace industry during the 1950s and 1960s, when the focus was on failure analysis of the metal shells used to build airplanes. Failure analysis is the study of how things break, exactly how and why brittle fracturing occurs when one drops a glass, or crashes an airplane — and eggshells' resistance to shattering made them interesting to airplane engineers.

Eggshells aren't quite as brittle as glass or the metal shell of an airplane, because they are laced with organic material. Any given material gets its properties, like resistance to cracking, from its crystalline structure. Eggshell is similar to tooth enamel or seashells; all of these materials are made up primarily of calcium carbonate (calcite) crystals embedded within a protein matrix. It's the latter that gives the shell its remarkable toughness, which is bolstered by a thin inner membrane made of collagen. Rather than splitting an eggshell in one clean break, cracks in an eggshell spread bit by bit, millimeter by millimeter. "Cracks propagate with great difficulty and the eggshell doesn't shatter," says Michelle Oyen, a materials scientist at Cambridge University. "The damage is very localized." In that respect, an eggshell is nature's perfect packaging.

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