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The irregular fracture surface of cortical bone, which is caused by complex multilevel micro-nanostructure, reflects the mechanical properties and fracture mechanisms. It is of great significance to characterize some characteristic parameters from the fracture surfaces of bone. In this research, anisotropic fracture mechanical properties of bovine femoral cortical bone along transverse, longitudinal and radial direction are firstly obtained by three-point bend experiment. Then the fracture routes and fracture surfaces are observed by scanning electron microscope. The observation shows that the formed fracture surfaces, which are caused by different crack routes, are extremely rough and have complex textures. Lastly, the combined method of fractal and gray level co-occurrence matrix are adopted to describe the morphology of fracture surface of cortical bone objectively and quantitatively. It is shown that the fracture surface of cortical bone has obvious fractal characteristics and four statistical texture feature parameters (contrast,angular second moment, correlation and entropy) of GLCM of fracture surfaces can describe a certain fracture texture character. The relationship between the characteristic parameters and macroscopic mechanical properties are established. The quantitative analysis and automatic class identification for the fracture surfaces of cortical bone can be achieved.

The toughening mechanism of cortical bone is closely related to its hierarchical microstructure. Osteon is the most important microstructure of cortical bone. Therefore, it is very important to study the toughening mechanism of the microstructure of osteon. There are three main kinds of cracks in cortical bone: external crack of osteon, internal radial cracks of osteon and microporous damage cracks. Numerical models for these three kinds of cracks are established by XFEM and the progressive damage approach, respectively. The multi-toughening mechanisms of microstructure of osteon are found. The cement line on the outside of osteon is its first toughening mechanism, which can make the crack deflection and improve the fracture resistance of osteon. The resistance of cement line to fracture increases with the decrease of the strength and the increase of the thickness. The second toughening mechanism is elliptical osteocyte lacunae, which can attract the crack into the elliptical lacunae and cause stress redistribution to prevent the crack propagation. The annularly elliptical lacuna structure is an optimized arrangement and shape of microstructure, which is the third toughening mechanism of osteon. This microstructure can determine the location of the crack initiation and make the microcracks propagate along the annular direction rather than penetrating into the haversian cannal to protect the integrity of the osteon. The study of these toughening mechanisms provides new ideas for the research and design of synthetic composite structures.

Scanning electron microscope (SEM) observation shows that the cuticle of Dorcus titanus is a kind of natural sandwich structure consisting of upper and lower panels and middle pillars. The observation also shows that the material of the sandwich structure is a biocomposite consisting of chitin-fiber layers and sclerous-protein matrix. More careful observation shows that the fiber layers in the sandwich structure continuously join the panels and the pillars to form a fiber-continuous panel–pillar sandwich structure. The strength of the fiber-continuous panel–pillar structure is investigated and compared with that of the non-fiber-continuous panel–pillar structure based on their representative models. It is shown that the fiber-continuous panel–pillar structure has higher ultimate strength compared to that of the non-fiber-continuous panel–pillar structure. Based on the observations and analyses, the fiber-continuous panel–pillar structure is biomimetically fabricated with a special mould and process. The ultimate strength of the structure is tested and compared with that of the non-fiber-continuous panel–pillar structure. It is indicated that the ultimate strength of the fiber-continuous panel–pillar structure is distinctly larger than that of the non-fiber-continuous panel–pillar structure. Keywords: Dorcus titanus cuticle, Fiber-continuous panel–pillar structure, Model analysis, Biomimetic fabrication

The mechanical properties of the cortical bone of fresh bovine femora along three different directions are tested through four-point bending experiments. It is indicated that the fracture energy along the transversal direction of the bone is distinctly larger than those of the longitudinal and radial directions. The fracture surfaces of the three different directions are observed by scanning electron microscope (SEM). It is shown that the roughness of the fracture surface of the transversal direction is obviously larger than those of the fracture surfaces of the longitudinal and radial directions. It is also revealed that the osteons in the bone are perpendicular to the fracture surface of the transversal direction and parallel to the fracture surfaces of the longitudinal and radial directions. Based on these experimental results, the fractal dimensions of the fracture surfaces of different directions are calculated by box-counting method in MATLAB. The calculated results show that the fractal dimension of the fracture surface of the transversal direction is remarkably larger than those of the fracture surfaces of the longitudinal and radial directions. The fracture energies of different directions are also calculated based on their fractal models. It is denoted that the fracture energy of the transversal direction is remarkably larger than those of the longitudinal and radial directions. The calculated results are in good agreement with the tested results.

Animal bones bear varied impact loadings during in the movements of animals. The impact resistance and micro-damage of bones are influenced by their various microstructures at different length scales. In this paper, according to the microstructure of osteon, three 2-D microstructure models (circumferential ellipse lacunae (Model A), radial elliptical lacunae (Model B) and circular lacunae (Model C) were constructed for investigating the influences of the arranged direction and shape of osteocyte lacunae on resisting impact and micro-damage. Impact analytical results show that the maximal stress of the Model A is the minimum and that of the Model B is the maximal under same boundary conditions, which indicates that the circumferentially elliptical lacunae, whose minor axis is along the radial direction of the osteon (Model A), can enhance impact resistance of osteons effectively. The investigated results of the progressive damage show that the circumferentially ellipse lacunae (Model A) are more benefit to resist micro-damage and that the micro-cracks in the model are mainly along the circumferential direction of the osteon. These investigated results for the novel microstructures found in osteon can serve engineers as guidance in the designs of biomimetic and bioinspired tubular structures or materials for engineering applications.