The armadillo has a unique protective bony armor, called the osteoderm, which confers to its shell-like skin distinctive mechanical properties. The top layer of the shell is made out of a dark-brown keratin layer with bimodal size scales. Beneath the keratin layer, the osteoderm consists of hexagonal or triangular tiles having a composition that is the same as bone. The tiles are connected by non-mineralized collagen fibers, called Sharpey's fibers. The tough and highly mineralized tiles have a tensile strength of approximately 20 MPa and toughness of around 1.1 MJ/m3. In comparison, the hydrated osteoderm has a lower tensile strength of approximately 16 MPa and a toughness of 0.5 MJ/m3. The tensile failure occurs by the stretching and rupture of the Sharpey's fibers. In a specially designed punch test in which an individual tile is pushed out, the shear strength is approximately 18 MPa, close to the tensile strength of the osteoderm. This surprising result is interpreted in terms of deformation in the Sharpey's fibers in the hydrated condition. The armadillo shell and a turtle shell are compared, with their corresponding similarities and differences.
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