Digital image correlation shows localized deformation bands in inelastic loading of fibrolamellar bone
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Irreversible or plastic deformation in bone is associated with both permanent plastic strain as well as localized microdamage. Whereas mechanisms at the molecular and mesoscopic level have been proposed to explain aspects of irreversible deformation, a quantitative correlation of mechanical yielding, microstructural deformation, and macroscopic plastic strain does not exist. To address this issue, we developed and applied a two-dimensional image correlation technique to the tensile deformation of bovine fibrolamellar bone, to determine the spatial distribution of strain fields at the length scale of 10 mm to 1 mm in bone during irreversible tensile deformation. We find that tensile deformation is relatively homogeneous in the elastic regime and starts at the yield point, showing regions of locally higher strain. Multiple regions of high deformation can exist at the same time over a length scale of 1 to 10 mm. Macroscopic fracture always occurs at one of the locally highly deformed regions, but the selection of which region cannot be predicted. Locally, strain rates can be enhanced by a factor of 3 to 10 over global strain rates in the highly deformed zones and are lower but always positive in all other regions. Light microscopic imaging shows the onset of structural “banding” in the regions of high deformation, which is most likely correlated to microstructural damage at the inter- and intrafibrillar level.
I. INTRODUCTION
Bone is a hierarchically structured biocomposite, and, as a consequence, its response to externally applied load is expected to depend on the architecture at length scales from the supramolecular to tissue level. It has a high work of fracture,1 but the precise nature of the microand nanostructural deformation processes that occur during postyield inelastic deformation are not fully understood. These mechanisms are physiologically necessary to reduce risk of catastrophic failure. In the elastic range of deformation, inhomogeneous strain fields around osteocyte lacunae have been proposed to play a significant role in bone mechanotransduction.2 To understand the origins of bone’s toughness, microcrack toughening3 and ligament bridging (as in some ceramic composites)4 have been proposed at the microstructural level. The presence of negatively charged, highly phosphorylated proteins that form a “glue” between collagen fibrils has led to the idea of “sacrificial bonds” opening at the molecular level, mediated by calcium ions.5 In situ measurement of fibril strain using synchrotron x-ray techniques during tensile loading led us to a model of a)
These authors contributed equally to this work. Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0064 b)
J. Mater. Res., Vol. 24, No. 2, Feb 2009
http://journals.cambridge.org
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inter- and intrafibrillar shearing6,7 and, separately, to characterize the energetics of bond breaking at the molecular level to be typical of ionic bonds.8 In contrast, in compression, failure has been suggested to
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