Biomedical Engineering Reference
In-Depth Information
S
(MPa)
220
270
230
400
1
450 MPa/s
4.5 MPa/s
0.9
1
0.9
Tris
-1
Liquid N
2
F
0.2
-2
0.1
0.05
-3
Bioglass-coated
alumina
-4
5.4
5.6
5.8
6.0
Ln
S
(MPa)
FIGURE 2.18
Plot of Ln Ln[1/(1 −
F
)] versus Ln
S
for Bioglass-coated alumina in a Tris-hydroxyaminomethane
buffer and liquid nitrogen.
F
is the probability of failure and
S
is strength. (From Ritter J.E. Jr et al. 1979.
J. Biomed.
Mater. Res.
13:260. With permission.)
Both
m
and
s
o
are constants in the equation. Figure 2.18 shows a good fit for Bioglass-coated alumina
(Park and Lakes, 1992).
A minimum service life (
t
min
) of a specimen can be predicted by means of a proof test wherein it is
subjected to stresses that are greater than those expected in service. Proof tests also eliminate the weaker
pieces. This minimum life can be predicted from the following equation:
t
min
=
−−
B
σσ
p
N
2
N
(2.2)
a
Al
2
O
3
14
13
50 Ye ars at 69 MPa
12
Air
11
Tris
10
Ringer's
solution
9
1.5
1.7
1.9
2.1
2.3
2.5
σ
P
/σ
a
(log scale)
FIGURE 2.19
Plot of Equation 2.3 for alumina after proof testing.
N
= 43.85,
m
= 13.21, and σ
o
= 55728 psi. (From
Ritter J.E. Jr. et al. 1979.
J. Biomed. Mater. Res.
13:261. With permission.)
