Biomedical Engineering Reference
In-Depth Information
350
Lot 1
300
250
200
Lot 2
150
0
2
4
6
Time (weeks)
8
10
12
FIGURE 2.16
Flexural strength of dense alumina rods after aging under stress in Ringer's solution. Lots 1 and 2
are from different batches of production. (From Krainess F.E. and Knapp W.J. 1978.
J. Biomed. Mater. Res.
12:245.
With permission.)
Studies of the fatigue behavior of vapor-deposited pyrolitic carbon fibers (with a thickness
of 4000−5000 Å) onto a stainless-steel substrate showed that the film does not break unless the substrate
undergoes plastic deformation at 1.3 × 10
−2
strain and up to 1 million cycles of loading. Therefore, the
fatigue is closely related to the substrate, as shown in Figure 2.17. Similar substrate-carbon adherence is
the basis for the pyrolitic carbon-deposited polymer arterial grafts (Park and Lakes, 1992).
The fatigue life of ceramics can be predicted by assuming that the fatigue fracture is due to the slow
growth of preexisting flaws. Generally, the strength distribution, σ
i
, of ceramics in an inert environment
can be correlated with the probability of failure
F
by the following equation:
1
1 −
=
s
s
i
o
Ln Ln
m
Ln
(2 .1)
F
4
3
2
1
0
0
1
2
3
4
5
6
7
log
10
N
FIGURE 2.17
Strain versus number of cycles to failure (
○
= absence of fatigue cracks in carbon film;
●
= fracture
of carbon film due to fatigue failure of substrates;
□
= data from substrate determined in a single-cycle tensile test).
(From Shimm H.S. and Haubold A.D. 1980.
Biomater. Med. Dev. Art. Org.
8:333-344. With permission.)
