The Fracture Strength under Internal Pressure of the Eggshell of the Domestic Fowl

Measurements are reported of the internal fluid pressure needed to fracture the shell of the egg of the domestic fowl. The average fracture stress, calculated from the measured pressure, for a set of 38 eggs was 15.2 MPa. This is about half of the fracture stress measured by concentric loading tests previously reported (Entwistle et al. 1995). This difference can be reconciled by taking account of differences in the effective area, A_E, of the stressed surfaces. Using the statistical conclusion {σ}_f{}^m A_E is a constant, where {σ}_f is the average fracture stress and m is the Weibull index, it is predicted from the results of one of the concentric loading tests that the average fracture stress under internal pressure should be 12.6 MPa. This is sufficiently close to the measured value, 15.2 MPa, to confirm that the greater part of the difference in measured strength is accounted for by difference of stressed area. Powerful evidence of the effect of stressed area on shell behaviour was the observation that an 0.8 mm diameter hole drilled near the equator of the egg had no effect on the fracture pressure. The small effective stressed area round the hole raised the fracture strength by a factor greater than the stress concentration factor, so fracture occurred at a site away from the hole.