Strength-fracture toughness synergy strategy in ostrich tibia's compact bone: Hierarchical and gradient

Ostriches are the fastest bipeds in the world, but their tibias are very thin. How the thin tibia can withstand the huge momentum impacts of the heavy body during running? The present work revealed that the combination of hierarchical and gradient design strategies was the main reason for their high strength and fracture toughness. The microstructure of ostrich's tibias compact bone was self-assembled into the 6-level hierarchical structure from the hydroxyapatite (HAP) crystals, collagen fiber (sub-nano), mineralized collagen fiber (nano-), mineralized collagen fiber bundle (sub-micro), lamellae (micro-) and osteon (macro-scales). The most distinctive design in the ostrich compact bone was that the HAP crystals were embedded in collagen fibers as well as wrapped in the outer layer of mineral collagen fibers (MCFs) in the form of HAP nanocrystals, thus achieving a high degree of soft and hard combination from the nanoscale. The bending strength was gradient-structure dependent and up to 787.2 ± 40.5 MPa, 4 times that of a human's compact bone. The fracture toughness (K_Jc) is 5.8 ± 0.1 MPa m^1/2. Several toughening mechanisms, such as crack deflection/twist, bridging, HAP fibers pulling-out, and fracture of the MCF bundles were found in the compact bone.