|M.Sc Student||Orlova Yevgeniya|
|Subject||Crack Evolution in Chicken Eggshell during Pressure Drop|
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Raphael Linker|
|Dr. Boris Spektor|
|Full Thesis text|
In this work eggshell crack expansion during rapid pressure drop was studied. The initial crack was induced to the eggshell by two weak mechanical impacts close enough to induce a macro-crack that was mostly oriented along the meridian direction. During the subsequent pressure drop, crack expansion was imaged in the middle of the crack in the near-equator region, in which the crack was roughly straight. Three phases of expansion vs. time were observed: a linear mode, an exponential mode and a transition zone between them. Exposing the egg to repeated pressure drops showed that the linear mode and transition zone correspond to irreversible deformation of the shell while the exponential mode corresponds to a reversible deformation of the eggshell. A model that associates the phases of expansion with the stretching and tearing of the outer membrane was proposed and confirmed experimentally. The important role of the outer membrane to the overall resistance of a cracked eggshell to an additional stress was demonstrated.
The egg contains an air cell at its blunt end which is enclosed between the inner and outer membranes. When a cracked egg is exposed to a sudden pressure drop, the effective pressure inside the egg depends on the airflow through the crack to the outside. Modeling of this outward airflow showed that the airflow from the blunt end to the crack, rather than the airflow through the crack, is the limiting factor.
Finally, different parameters of crack evolution were calculated from the expansion curves: the slope of the linear mode, the durations of the different phases, the initial crack width, the crack width at the end of linear expansion and at the beginning of exponential expansion, and the characteristic time of the exponential mode. Several shell parameters were investigated: dynamic stiffness, shell thickness, shape index, egg mass and egg volume. We investigated the correlations between these parameters and the eggshell parameters. The slope of the linear mode was negatively correlated with the thickness of the shell and positively correlated with the egg volume, although the correlations were weak. A weak negative correlation was found between the values of the initial crack width and the values of the shell thickness. A moderate positive correlation was found between crack expansion and propagation, although further research is needed.