Bone as a Structural Material
Tóm tắt
As one of the most important natural materials, cortical bone is a composite material comprising assemblies of tropocollagen molecules and nanoscale hydroxyapatite mineral crystals, forming an extremely tough, yet lightweight, adaptive and multi‐functional material. Bone has evolved to provide structural support to organisms, and therefore its mechanical properties are vital physiologically. Like many mineralized tissues, bone can resist deformation and fracture from the nature of its hierarchical structure, which spans molecular to macroscopic length‐scales. In fact, bone derives its fracture resistance with a multitude of deformation and toughening mechanisms that are active at most of these dimensions. It is shown that bone's strength and ductility originate primarily at the scale of the nano to submicrometer structure of its mineralized collagen fibrils and fibers, whereas bone toughness is additionally generated at much larger, micro‐ to near‐millimeter, scales from crack‐tip shielding associated with interactions between the crack path and the microstructure. It is further shown how the effectiveness with which bone's structural features can resist fracture at small to large length‐scales can become degraded by biological factors such as aging and disease, which affect such features as the collagen cross‐linking environment, the homogeneity of mineralization, and the density of the osteonal structures.
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Tài liệu tham khảo
Currey J. D., 2006, Bones: Structure and Mechanics
DHLNL and HLNL are respectively dehydro‐dihydroxylysinonorleucine and hydroxylysinonorleucine.
Hodge A. J., 1963, Aspects of Protein Structure, 289
Odetti P., 2005, Maillard Reaction: Chemistry at the Interface of Nutrition, Aging, and Disease, 710
Martin R. B., 1989, Structure, Function, and Adaptation of Compact Bone
The fracture toughness KIc is defined as the critical stress intensity at fracture instability; if measured correctly it strictly characterizes the crack‐initiation toughness.
The fracture toughness can be expressed as the critical value of the stress intensityKfor unstable fracture in the presence of a pre‐existing crack i.e. in mode I whenK=Yσapp(πa)½=KIc whereσappis the applied stress ais the crack length andYis a function (of order unity) of crack size and geometry. Here the stress intensityKis a measure of the amplitude of the elastic stress and displacement fields at the crack tip. Alternatively the toughness can be measured as a critical value of the strain‐energy release rate Gc defined as the change in potential energy per unit increase in crack area in an elastic solid. In the presence of local plasticity that is no longer small enough to be ignored both approaches can also be expressed in terms of theJ‐integral defined as the amplitude of the nonlinear elastic stress and displacement fields at the crack tip and/or as the change in potential energy per unit increase in crack area in a nonlinear elastic solid. Where nominally elastic conditions prevail J=G.
Stress‐intensity factors can be defined for three modes of crack displacement specifically KIfor mode I under tensile opening KIIfor mode II in shear KIIIfor mode III in anti‐plane shear as described in greater in Figure.
Comparable effects can occur in bone with aging irradiation damage and disease where abnormal mineralization and/or cross‐linking profiles within the matrix can reduce the relative inhomogeneity between the bone matrix and the cement lines again contributing to less deflected crack paths.
Burr D. B., 2004, J. Musculoskelet. Neuronal Interact., 4, 184
Cole W. G., 1997, Clin. Orthop. Relat. Res., 235
Woven bone is type of bone structure produced during fast bone growth. It is often present in young individuals or during fracture healing. The overall structure is characterized by a random orientation of collagen and mineral high mineralization as well as high porosity.
Mirra J. M., 1995, Skeletal Radiol., 24, 163