Biomedical Engineering Reference
In-Depth Information
3. The displacement at each point and in each direction for which traction was
prescribed in the boundary conditions (e.g., the portion
S
u
of the boundary of
V
in Fig.
3.1
).
4. The traction at each point and in each direction for which displacement was
prescribed in the boundary conditions (e.g., the portion
S
t
of the boundary of
V
in
Fig.
3.1
).
These are obtained by solving the following sets of equations at each point
within the model:
1. Strain-displacement equations that define the strain tensor field
«
(
x
) at each
point
x
within the volume
V
T
of the tendon and
V
B
of the bone in terms of the
displacement vector field,
u
(
x
). Here and throughout the chapter, we write these
equations using index notation, in which subscripts are understood to represent
indices ranging from 1 to 3, commas represent partial differentiation, and
repeated indices within a grouping imply summation over the three values of
that index. The strain displacement relations are then:
1
e
ij
¼
2
ðu
i;j
þ u
j;i
Þ
(3.1)
in a
particular coordinate system,
u
i
represents the three components of the displace-
ment vector
u
in that coordinate system, and
u
i;j
¼ @u
i
=@x
j
.
2. Compatibility relations. Since the nominal strain tensor has six independent
components and the displacement vector only three, not every choice of a
spatially varying strain field relates to a displacement field. Strain fields that
do meet this criterion satisfy the compatibility relations:
where
e
ij
represents the 3
3 matrix of components of the strain tensor
«
e
ij;kl
þ e
kl;ij
e
jl;ik
e
ik;jl
¼
0
(3.2)
where the additional subscript following the comma represents a second deriva-
tive with respect to the components of the coordinate system, so that
e
ij;kl
¼ @
2
e
ij
=@x
k
@x
l
.
3. Constitutive relations that predict the strain tensor at a point in terms of the stress
tensor
s
. For the case of linear, isotropic elasticity, these are:
1
E
½ð
1
þ nÞs
ij
nd
ij
s
kk
e
ij
¼
(3.3)
E
ð
1
þ nÞ
e
ij
þ
E
ð
1
þ nÞð
1
2
nÞ
d
ij
e
kk
s
ij
¼
(3.4)
where
d
ij
is Kronecker's delta function that equals 1 when
i ¼ j
and 0 otherwise,
and
e
kk
¼ e
11
þ e
22
þ e
33
.
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