Biomedical Engineering Reference
In-Depth Information
wrong engineering as we would say now, causing metal fatigue were finally found
to be the origin of the catastrophic blow of the fuselage. Fatigue was certainly not
unknown as reason for failure, but the dramatic Comet crash (followed by a second
one three months later near Naples) made the engineering community extra alert to
the phenomenon. Here, only mechanical stresses were involved. The failure of the
hip prosthesis of the case study in Chap.
2
was caused by corrosion-assisted fatigue
crack growth. In fatigue cracks fresh unprotected metal is an easy prey to biofluids
and, as mentioned in the case of stainless steel, sulfur compounds are diffusing into
the cracks (and along the grain boundaries as well).
Stress corrosion cracking.
The sudden failure of normally ductile metals when
subjected to tensile stresses in a corrosive environment is called SCC. It is the result
of typically subcritical crack growth, hence under conditions where failure should
not occur. Although titanium alloys were recognized for their excellent corrosion
resistance, Ti-6Al-4V is also sensitive to this phenomenon. In 0.6 M KCl, the frac-
ture toughness
K
Ic
of Ti-6Al-4V is lowered from 60 to 20 MNm
3=2
in 0.6 M KCl
solution.
12
It is the
˛
-phase (h.c.p.), which is sensitive in this case [
105
].
3.3
Does It All Fit the Practice of Implants?
How does it look like in practice?
The chapter was introduced by case studies of retrieved modular hip prostheses.
Gilbert and colleagues reported about modular single-alloy prostheses, both parts
made of CoCrMo. In terms of simple corrosion potentials, no problem could be
expected at all but it was wrong. As shown on Fig.
3.1
b inside the taper and just
outside the head-neck junction, grain boundary attack happened. The phenomenon
is clearly illustrated in Fig.
3.10
: in (b) the intergranular paths are seen and in (d)
the fractured surface is partly covered with nonconducting debris, whitish in the
micrograph, which was identified as carbides deposited in the grain boundaries.
In (c), pits are visible on the free surface of the stem. These pits were formed by
egression of particles
out of the surface. The origin of the pits has a mixed (elec-
tro)chemical and mechanical origin. Carbide particles of 2-300
m were released
from the surface. They were nicely located in a grain boundary and the pits would
probably deepen in course of time. The egression might have been assisted by
corrosion underneath the particle.
The retrieved prostheses had all stems with a porous CoCr coating entailing a few
problems. On the electrochemical side, the application of a (porous) coating requires
heat treatment close to the melting temperature and provokes phase segregation;
moreover, the porosity increased the area of the cathode (the stem) with respect to
the area of the pits (anode) with subsequent increase of the anodic current density.
On the mechanical side, heat treatment may provoke voids at the grain boundaries.
12
For more about fracture toughness, see Chap. 9
.
