Eggshells -- nature's ceramic material

UC San Diego engineers investigate why eggshells are so strong

Think breaking an egg is easy? Try holding it sideways between your hands and pressing it, you'll find that it's almost impossible to break. That's what Eric Nicholas Hahn, a UC San Diego Ph.D. alumnus from the Department of Materials Science and Engineering, demonstrates in the video below:

Hahn was part of a team of researchers led by mechanical engineering professor Marc Meyers that investigated what makes eggshells so strong. Their findings were published last week in Journal of the Royal Society Interface.

The function of the eggshell is to protect the embryo from the environment, but it cannot be too strong otherwise the chick would not be able to break out and hatch. It is made of calcium carbonate, an important biomineral, which is different from hydroxyapatite, the mineral component of bone.

In the study, eggs of different sizes, from quail to ostrich, were tested on their strength using an electromechanical system that compressed the eggs between two pieces of rubber. When an egg is compressed in this way, tensile stresses develop radially in the shell. It is only when this radial tensile stress reaches a critical level, equal to the tensile strength of calcium carbonate, that the egg breaks.

Eric Nicholas Hahn, the first author of the study.

Chicken eggs were found to have a compressive strength of 100 lbs, whereas ostrich eggs gave values of more than 1000 lbs. Size and shell thickness were the most important factors in determining shell strength. The strength of eggshells decreases with increasing size and thus thickness, but the force required to break the egg increases because the stress (force/area) is less.

"This paper revealed, for the first time, the mechanism by which the eggs break when subjected to axial compression. It is not the compression by my hands that breaks the egg, but the tension generated radially,” Meyers said.

Meyers was interested in this topic since he was a child growing up in Brazil. He added, "We would like to check the universality of our equations by testing eggs of all kinds of birds. There are a variety of interesting birds that we did not test because their eggs are difficult to come by: penguins, eagles, dinosaur..."

Source: The Royal Society Publishing Blog

Full paper: "Nature's technical ceramic: the avian eggshell."