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R. Jungmann, M.E. Szabo, G. Schitter, R. Tang, D. Vashishth, P.K. Hansma, P.J. Thurner:
"Local strain and damage mapping in single trabeculae during three-point bending tests";
Journal of the Mechanical Behavior of Biomedical Materials, 4 (2011), 4; 11 pages.

The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals
is of limited value. However, there is growing evidence that information on trabecular
microarchitecture can improve the assessment of fracture risk. One current strategy is to
exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular
bone structures obtained from non-invasive imaging modalities for the prediction of
apparent mechanical properties. However, there is a lack of FE damage models, based on
solid experimental facts, which are needed to validate such approaches and to provide criteria
marking elastic-plastic deformation transitions as well as microdamage initiation and
accumulation. In this communication, we present a strategy that could elegantly lead to future
damage models for FEA: direct measurements of local strains involved in microdamage
initiation and plastic deformation in single trabeculae. We use digital image correlation to
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stress whitening in bone, reported to be correlated to microdamage, to quantitative local
strain values. Our results show that the whitening zones, i.e. damage formation, in the
presented loading case of a three-point bending test correlate best with areas of elevated
tensile strains oriented parallel to the long axis of the samples. The average local strains
along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior
to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric
in tension and compression, with failure originating and propagating over a large range of
tensile strains.

Trabecular bone, Local strain detection, Damage model, Whitening Microdamage