Expansion of cracks in chicken eggs exposed to sub-atmospheric pressure

The expansion of an existing crack in chicken eggs exposed to an abrupt pressure drop was investigated. The initial crack was induced in the eggshells by two weak mechanical impacts close enough to induce a macro-crack oriented mainly along the direction of the meridian. The cracked eggs were placed in a custom-made pressure chamber in which the pressure was abruptly decreased to −25 kPa. During this pressure drop, crack expansion was measured using an imaging system and three phases of expansion (evolution of equivalent crack width vs. time) were observed: a linear mode, an exponential mode and a transition zone between them. A model that associates these phases of expansion with the stretching and tearing of the outer membrane is proposed. According to this model, the linear mode corresponds to reversible membrane stretching, which ends with membrane tearing during the transition zone. This model was supported by visual observations that revealed that membrane fragments became visible in the transition zone, and by the crack expansion curves obtained during weaker pressure drops (−10 kPa), which showed that only linear (reversible) crack expansion occurred. It was also shown that the air cell that is initially enclosed between the inner and outer membranes at the egg blunt-end plays in important role in the crack expansion. A simple model of the airflow that escapes from the egg through the crack shows that the airflow from the blunt-end to the crack, rather than the airflow through the crack itself, is the limiting factor. [Copyright &y& Elsevier]