Biomedical Engineering Reference
In-Depth Information
Time t
12.6
Creep data is typically recorded in terms of the strain in a
sample as a function of time. Some instantaneous elastic strain is
followed by a gradual accumulation of plastic strain, until failure
occurs.
as time goes on, but in most prostheses subsidence is a bad thing and, in fact,
the rate of early subsidence (measured radiographically) has been used as
a predictor of eventual failure (Perillo-Marcone
et al
., 2004). Human body
tissues (bone and, to a much greater extent, soft tissues) are subject to creep
effects, which must be taken into account when conducting stress analysis
of implants (see below).
12.2.4 Stress-corrosion cracking
Stress-corrosion cracking (SCC) has some similarities with fatigue, in
that it involves the creation and growth of cracks. The difference is that,
whilst in fatigue the essential ingredient is a cyclic stress, SCC can occur
under a constant stress, because of interactions between the stress and the
chemical environment. Fortunately, SCC only happens under some very
specific combinations of material and environment. However, several of
those combinations are commonly present in medical devices. For example,
stainless steels are susceptible to SCC in saline solutions and some polymers
display the phenomenon in the presence of iodine. Ceramic materials,
although characterized by chemical inertness, nevertheless can experience
SCC. Ceramicists refer to this phenomenon by the rather confusing name
of static fatigue.
In principle one can measure SCC in simple tests similar to those used for
measuring fatigue: a sample of material can be exposed to a constant stress
along with a chemical environment, the time to failure can be recorded, as can
the growth rate of cracks. However, it is very difficult to predict SCC in real
applications. This is because it proceeds by a variety of different mechanisms
and involves a very intimate synergy between the mechanical and chemical
