Eggshell strength: A relationship between the mechanism of failure and the ultrastructural organisation of the mammillary layer

Stress analysis using modern computer modelling techniques indicated that the shell should fail in tension as a result of the accumulation of stress at its inner most surface with the characteristic notched appearance of the mammillary layer providing the ideal crack initiation site. 2. Practical observations however indicated that crack initiation does not inevitably lead to catastrophic failure. This partly explains why previous attempts to predict shell failure from classical engineering theory underestimated the actual force required to break an egg. 3. Crack initiation should now be denned as localised trauma, characterised by a series of radial cracks which propagate outwards from the load point at first in a stable manner. As the interaction between the shell and its loading environment continues, one or more of these minor cracks become unstable, thus giving rise to catastrophic failure. 4. The fracture toughness of the eggshell is a measure of its resistance to unstable crack growth and in this respect there appears to be a link between the latter and the structural organisation of the mammillary layer.