The fracture toughness of eggshell

Abstract

a b s t r a c tThe shells of avian eggs are very brittle, but how brittle? Fracture toughness, K c is a standard measure used widely to characterise engineering materials. We devised a novel way to measure K c and applied it to commercial hens' eggs, obtaining a value of 0.3 MPa p m. This value is much lower than previous published values, which we argue are incorrect and misleading. We discuss how this exceptionally low toughness value (in comparison to that of other natural materials made from calcium carbonate) has been achieved by prevention of toughening mechanisms. Eggshell has an unusual combination of mechanical properties (low fracture toughness combined with high Young's modulus), making it ideally suited as a container for the developing chick, which must be stiff and rigid but also brittle enough to be broken when required. Further testing and analysis using the Theory of Critical Distances and Weibull probability theory allowed us to describe the effects of defects of various types: cracks, holes and notches, on the strength of whole eggs. These results are of commercial importance because many eggs break prematurely as a result of microscopic defects. Statement of SignificanceThis paper presents the first accurate measurements of the fracture toughness of eggshell. These results are important because eggshell is a brittle material which fails from microscopic defects, so knowledge of the fracture toughness is essential to understand its mechanical performance. The toughness value obtained is discussed in the context of other mechanical properties, of eggshell and other natural materials. This is useful for understanding how eggshell's stiffness and toughness make it ideally suited to its purpose, and the mechanisms by which toughness is achieved. The paper also contains analysis of the effect of defect type, including cracks, notches and holes, to provide a fuller picture of defect tolerance which will be useful in the egg producing industry.

Self‐healing materials: what can nature teach us?

“…mussel, limpet and conch) is almost a pure ceramic, calcium carbonate, with just a few per cent of organic material and water. The K c value of this material in bulk mineral form is about 0.3 MPa√m, and a similar value can be measured in eggshell, which has a very simple microstructure. The more complex microstructure of nacre, which is made up of tile‐like units at the micron scale with specially developed interfaces, confers additional toughness through crack deflection and several other toughening mechanisms, which are currently the subject of much research.…”

On the relationship between indentation hardness and modulus, and the damage resistance of biological materials

“…Hydrated materials are less likely to crack, because of their increased toughness, and decreased indentation hardness and modulus. Data are from (21,122,123 with the length of the crack introduced by brittle fracture, c, given by eq. 4b (including another geometrical constant, A 4 , in the nominator).…”

“…Hydrated materials are less likely to crack, because of their increased toughness, and decreased indentation hardness and modulus. Data are from(21,122,123, own data) (biological materials), (121) (mild steel, soda lime glass, and diamond), (62, 124) (aragonite), (125) (apatite, calcite) and (126, 127) (hydroxyapatite).…”

Nature's technical ceramic: the avian eggshell

“…For the geometry closely resembling the one in eggs, one can approximate Y 1.12. A recent study by Taylor et al [40] reports K c 0:3 MPa ffiffiffiffi m p for chicken eggs. By setting a c ¼ ft, with t being the eggshell thickness we can estimate a portion of it, f, that is pre-cracked.…”