Formation and development of induced mechanical strains in the presence of residual strains: A in-situ X-ray indentation on avian eggshells

The eggshell evolved with the paradox of being strong enough to preserve the embryo from mechanical stresses but brittle enough to allow the chick to hatch. The shell is a biomineralized tissue composed of distinct calcitic layers incorporated with organic molecules at both structural (intercrystalline) and atomic (intracrystalline) scales. These inclusions and microarchitectural arrangements of the crystalline layers result in the combination of high Young’s modulus and direction-dependent fracture toughness in the bioceramics. On the other hand, impurities come with consequence, the formation of interfaces with mismatches that can evolve and integrate internal forces within materials. The presence of these internal forces, which are the sources of residual strains/stresses, has been reported. However, to this day , how these residual strains/stresses modulate the mechanical performance and functionality of the eggshell remains unclear.