Biomedical Engineering Reference
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Fig. 9 Released elastic
energy by damage
accumulation with respect to
the applied strain for human
cortical bone in both tension
and compression modes
dependent on the loading mode [
76
]. Cross-hatch linear cracks are often observed
in compression, whereas so-called diffuse damage most likely occur in tension
[
77
,
78
]. Differences in micro damage accumulation between tension and com-
pression are also reflected in the distinct way of energy dissipation. The released
elastic strain energy (U
er
) by formation of newly formed damage surfaces is less in
compression than in tension (Fig.
9
). From the damage mechanics point of view,
the energy dissipation by micro damage accumulation is realized through the
formation of new crack surfaces (i.e., surface energy), thereby resulting in
decreased elastic modulus. It is speculated that crosshatched damage formation
causes minimal energy dissipation through creation of new crack surfaces
(i.e., released elastic strain energy) compared with the presumably less hazardous
diffuse damages in tension. Although the surface energy dissipation by micro crack
accumulation serves as one of toughening mechanisms of bone, its contribution to
the total deformation of bone is limited. The major mechanism for energy dissi-
pation still ties with the plastic deformation (residual strain) during the post-yield
behavior of bone.
3.3 Post-Yield Properties of Cortical Bone
3.3.1 Yielding
Yield stress and strain are usually determined using the conventional 0.2% strain
offset method. However, using the progressive diagnostic test, the yield strain can
be estimated more accurately [
79
]. In general, the values obtained by the 0.2%
strain offset method are usually greater than those determined using the progres-
sive diagnostic technique. The longitudinal yield strain of human femur estimated
using the 0.2% strain offset method is *0.6-0.7% in tension [
79
] and *0.6-1.1%
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