In order to understand the fracture toughness anisotropy of avian eggshells, we have investigated eggshells of the emu (Dromaius novaehollandiae) whereby the large size (~13 cm × 9.5 cm) enabled the fabrication of beam samples in various orientations. The emu eggshell was found to have a hierarchical microstructure similar to chicken eggshell, with the only significant difference being the absence of a continuous cuticle layer. Emu eggshell was found to have significantly lower strength when samples were tested in the outwards direction (i.e., a crack initiates on the inside of the shell and propagates towards the outer surface) as compared to the inwards testing direction. Furthermore, samples that were oriented parallel to the egg axis (i.e., the longitudinal direction) and tested inwards showed higher strength, ~24 MPa, compared to the samples that were made from the latitudinal orientation, ~20 MPa. Independent of orientation, the outwards testing direction resulted in strength values of ~15 MPa. The fracture toughness of the emu eggshell for cracking in the circumferential direction was ~0.3 MPa√m, independent of sample orientation, and this value was comparable to the fracture toughness of chicken eggshell tested in the same orientation. In the radial outwards direction, however, the fracture toughness was ~80% lower (~0.06 MPa√m) than in the circumferential direction. The low fracture toughness for this orientation was associated with the separation of the highly oriented calcite crystals in the mammillary cone layer of the eggshell structure which is easier compared to calcite crystal fracture. The large anisotropy in fracture toughness is thought to allow for easy escape of the chick while simultaneously protecting the embryo during development.

为了了解禽蛋壳的断裂韧性各向异性，我们研究了mu（Dromaius novaehollandiae）的蛋壳，其中大尺寸（〜13 cm×9.5 cm）使得能够在不同方向上制作梁样品。发现e蛋壳具有类似于鸡蛋壳的分层微观结构，唯一的显着差异是没有连续的表皮层。与向内测试方向相比，当沿向外方向测试样品时，发现a蛋壳的强度显着降低（即，裂纹在蛋壳内部引发并向外表面传播）。此外，样品的方向平行于蛋轴（即，纵向）和向内测试显示，与通过纬向取向制作的样品（〜20 MPa）相比，强度更高，约为〜24 MPa。与方向无关，向外的测试方向导致的强度值为〜15 MPa。mu蛋壳在圆周方向上的破裂韧性为〜0.3MPa√m，与样品取向无关，该值与相同方向测试的鸡蛋壳的破裂韧性相当。然而，在径向向外的方向上，断裂韧性比在圆周方向上低约80％（〜0.06MPa√m）。这种取向的低断裂韧性与蛋壳结构的乳状锥层中高度取向的方解石晶体的分离有关，这比方解石晶体断裂更容易。断裂韧性的大各向异性被认为可以使小鸡容易逃脱，同时在发育过程中保护胚胎。