Expert: Jeanette A - 12/14/2008

Question
I'm wondering why exactly an egg shell is so strong - and then why from the inside, it is presumably less strong, so that a baby chick can get out.  I've studied 'structures' and have been told that a dome distributes the load yada yada, but can you tell me anything more in depth?
Thanks.

Answer
Hello Andrea,

The structure of an eggshell is weak in tension and strong in compression. For example, young chicks are not strong, but by exerting tensional forces on the inside of the shell by poking with their beaks, they are able to break out of their shells.

A dome of an eggshell is actually an arch rotated about its vertical axis. Domes are stiffer than arches, because of their three-dimensional structure. Arches and domes are both statically stable. The eggshell is strong under compression because a dome has both a horizontal and a vertical component, and so compression applied to any one point is evenly distributed across the dome's surface.

Such dome structures are strong because they can exert horizontal as well as vertical forces to resist compression. This means that great pressure can be applied to an egg's two domes before the shell fractures. As weight is placed on top of the egg, the outer portion of the shell will be subject to compression, while the inner wall of the shell will experience tension. The shell will fracture initially at the inner surface under the point of contact with the weight.
However, the strength of the dome structure of an eggshell is dependent on its geometry, in particular, the radius of curvature. Pointed arches require lower stabilizing forces when subjected to compression than a simple semicircle arch. This means that a highly vaulted dome (low radius of curvature) is stronger than a flatter dome (high radius of curvature).

Below are a couple of websites I thought will be of some use, they visually represent the concept and also there are some experiments which you can perform to gain a better understanding.

http://www.4to40.com/QA/index.asp?id=2820&category=science

http://www.csun.edu/~mk411573/discrepant/discrepant_event.html

Goodluck, I hope this has helped.


Warm Regards,
Jeanette