Biomedical Engineering Reference
In-Depth Information
strength is much weaker in transverse direction compared to longitudinal.
In addition, the longitudinal ultimate strain is between 2.2 and 4.0% in tension,
*1.7% in compression, and *5.2% in torsion, whereas the transverse ultimate
strain in tension is only *0.7%.
3.3.4 Toughness
Toughness is usually evaluated by the total energy (work) required to produce
failure during a mechanical test, which is equivalent to energy dissipated by the
specimen until failure. Bone toughness depends on the level of micro damage
induced during deformation and the energy required to generate this micro damage
[
85
]. There are two general types of energy dissipation in bone: surface energy
dissipated by the newly formed surfaces, and plastic strain energy due to the
permanent deformation of the tissue. Since mineral and collagen phases each have
limited capacities to be plastically deformed, the plastic (residual) strain during the
post-yield deformation of bone is most likely due to the relative displacement
between the two phases [
86
].
Vashishth et al. [
87
] investigated the question of whether micro cracking during
loading is a toughening mechanism in bone, and reported that damage accumu-
lation in bone increases with crack extension thus leading to an increase in fracture
toughness of the tissue. Based on observations from ceramics and bone fracture
specimens, it is speculated that micro crack formation occurs in two stages
[
87
,
88
]: the formation of a frontal process zone (Stage I) and the formation of a
process zone wake (Stage II). The formation of the microcracks absorbs energy,
thereby decelerating the crack propagation. This has been verified using laser
confocal microscopy in other studies [
89
], in which diffuse damage was found to
consistently form and accumulate in the bone matrix between canaliculi and in the
vicinity of lacunae. Besides crack-tip shielding, there are other possible tough-
ening mechanisms by damage accumulation. At the microstructure level, cement
lines are thought to deflect the crack path [
90
]. At the ultrastructure level, collagen
fibers can bridge the crack, thus retarding its growth [
91
].
3.4 Viscoelastic Properties of Cortical Bone
Cortical bone is viscoelastic. Two possible causes of bone viscoelasticity are: (1)
fluid flow within the bone porosity, and (2) viscous response of solid bone tissue.
The first mechanism is supported by the fact that bone viscosity depends on
hydration condition [
92
]. Wet bone exhibits a larger viscoelastic damping (tand)
than dry bone over a broad range of frequency (5 mHz-5 kHz in bending). The
viscoelastic relaxation due to fluid flow in bone usually occurs at fairly high
frequencies, perhaps above 10 kHz [
93
]. The value of tand in a frequency range of
1-100 Hz is relatively minimal, suggesting that fluid flow in bone might be
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