On a structure using the material which demonstrates its mechanical function to the limit under given conditions, the most effective design of the function can be expected because the design of material and structure matches pertinent. One of the actual structures is natural bone. Design concept is based on the assumption that bones have optimum structures to mechanical conditions. Therefore, development of the new material that considers modelling of mechanical composite structure of the bone to the optimum design of the new material, that is biomimetic design, is very rational thought. Cortical bone is the load support structure of the bone. The structural unit, osteon, has a multilayered cylinder of collagen fibers and is oriented to the loading axis. Static compression test with AE monitoring and SEM observation were curried out to estimate the load support function of cortical bone. Pillar specimens which were obtained from presumed domain of largest compressive stress in cross section of bovine femurs showed a ductile material like stress-strain curve. The deformation expanding in cross section of osteons under the axial compression can be restricted by collagen fibers with the orientation inclining from the loading axis. As a result, a large breaking strain due to a reinforcing mechanism the fibers was measured.
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