Biomedical Engineering Reference
In-Depth Information
The fatigue process can be much accelerated if the material already
contains cracks or other defects introduced during manufacturing, because
the time taken for cracks to initiate is effectively bypassed. Design features
which concentrate stress, such as notches and sharp corners, have a similar
effect. So it is also useful to carry out tests on specimens which are pre-
cracked, measuring the rate of crack growth, defined as the increase in crack
length
a
per cycle, usually written d
a
/
dN
. as might be expected, d
a
/d
N
is a
function of the applied stress range, but it is also a function of the length of
the crack. These two parameters are combined to form the stress intensity
range, D
K
; the theoretical derivation of this parameter is beyond the scope
of this chapter. The interested reader is referred to textbooks on engineering
materials (Ashby and Jones, 1980; Hertzberg, 1989) or fracture mechanics
(Janssen
et al
., 2002). As Fig. 12.2(b) shows, D
K
correlates to the crack
growth rate (d
a
/d
N
is also a function of
R
although this is not shown here)
so that, knowing the applied stress and crack length, one can estimate how
fast the crack will grow at any given point in time and thus, by integration,
predict the total life of a pre-cracked component. This approach is commonly
used in those structures which can make use of crack-growth monitoring
through regular inspection, such as aircraft. Currently this is not possible for
orthopaedic devices but the approach can still be a useful way to assess the
effect of manufacturing defects and design features. For a more thorough
treatment of the effect of stress concentrations in engineering components,
the reader is referred to another recent publication (Taylor, 2007).
12.2.2 Wear
Wear, defined as the removal of material caused by contact and relative
motion between two surfaces, is a major problem for joint implants such
as the hip and knee. The physics of the problem is more complex than for
fatigue because there are many different mechanisms of wear. More details
can be found in textbooks devoted to the subject of tribology; a recent article
by Jin
et al.
gives a very useful introduction to the tribology of implants
(Jin
et al
., 2006). The underlying causes, however, are always the same: if
two components are placed in contact, with a relatively low nominal stress
between them (defined as the compressive load divided by the contact area),
then high local stresses exist owing to the fact that no surface is perfectly flat.
At high magnification (Fig. 12.3) the surfaces will be seen to make contact
only at their highest points, local stresses can exceed the stress needed to
cause yielding or cracking in the material. If relative motion now occurs
between the surfaces, shearing forces will cause small particles to break free
from one or both surfaces.
If one material is much softer than the other, for example the metal/polymer
and ceramic/polymer combinations that are common in joint implants, then
