The relative fracture strengths of the inner and outer surfaces of the eggshell of the domestic fowl.

A method is described for comparing the strength of the material near the inner and outer surfaces of the eggshell of the domestic fowl. Half eggs with blunt poles, obtained by cutting the egg around the equator, were subjected to concentric loading through neoprene O-rings, respectively 7 mm and 15.5 mm diameter. A finite element solution for the stress distribution showed that the point of maximum tensile stress lay on the line of the smaller ring. For downward loading on the smaller ring, the maximum stress occurred on the inner surface of the shell. For upward loading it moved to the outer surface. Upward loading gave a value for the average fracture stress of the outer surface of the shell (27.3 Nmm-2). For downward loading fracture occurred on the outer surface at the larger ring before the fracture stress was reached on the inner surface. This measurement gave a lower bound to the average fracture stress of the inner surface (30.4 Nmm-2). The fracture stress of the inner surface was measured using concentric loading along the axis of the whole egg. The average fracture stress was 36.7 Nmm-2 so the inner surface is 34% stronger than the outer surface.