Biomedical Engineering Reference
In-Depth Information
Fig. 6 A schematic representation of microdamage morphologies and their relationship to an
artificially introduced notch on the a compressive and b tensile sides of specimens subjected to
bending fatigue to the primary and tertiary phases of modulus loss. ''x'' and ''-'' represent
diffuse damage and linear microcracks, respectively. Reprinted from permission with Elsevier
[
10
]
mostly from their resistance to crack propagation rather than initiation only
[
62
-
64
].
Thus, in contrast to focusing on microdamage that initiates and accumulates
with age, recent studies have focused more on the ability of bone to resist prop-
agation. In particular, several crack propagation studies have been conducted in
which propagation toughness has been measured using a fracture mechanics
approach [
7
,
45
,
62
,
65
]. Results show that discrete microcrack formation occurs
behind the tip of a propagating fracture crack (frontal process zone) that dissipates
energy and decelerates the advancing fracture [
7
,
63
]. The frontal process zone
develops into a region of microcracks surrounding the propagating fracture
(process zone wake), and both regions absorb energy during loading and lead to
increased crack growth resistance [
29
,
45
,
62
,
63
]. Energy dissipation through
microcracking may breed into other toughening mechanisms such as uncracked
ligament formation and crack deflection [
65
]. It is likely that the formation of
uncracked ligaments involves microdamage that arrests a propagating crack and
initiates a new crack [
66
]. Hence, microdamage forms during crack propagation
(de novo microdamage) and plays a significant role in determining bone's
toughness [
63
]. Consequently, increased bone fragility with age may also be due in
part to bone's decreased ability to form de novo microdamage. Additional studies
are needed to examine this possibility.
Consistent with the above concepts, it has been shown that under fatigue
loading, cortical bone forms and compartmentalizes microdamage in order to
dissipate energy (Fig.
6
)[
10
]. During the primary phase, diffuse damage formed
on the tensile side while few linear microcracks formed on the compressive side.
Furthermore, specimens notched on the compressive side accumulated more
microdamage near the notch and had high toughness. In contrast, specimens
notched on the tensile side had low toughness since the region was already filled
with diffuse damage. Continued loading of specimens into tertiary phase caused
significant accumulation of linear microcracks on the compressive side. More
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