Biomedical Engineering Reference
In-Depth Information
and sulfides and it is known that at the tip of a crack sulfur is often detected and
suspected to promote the progress of the crack.
We a r
: In an artificial hip joint, the head (in stainless steel in this case study but
can be either in other alloys or in ceramics; see following chapters) is often artic-
ulating in a plastic (polyethylene) socket replacing the acetabulum. Wear produces
wear debris: metal and polymer particles. Debris can contribute to wear, when it
remains trapped between the articulating surfaces, it is called
third body wear
,or
to immunologic response by the host body if diffusing into the surrounding tissue.
Wear rate is very variable from one case to another: in the present case, the surgeon
did not have to replace the cup after 27 years, which is quite exceptional! In other
cases, it can go wrong after a few months. It is evident that wear also promotes
corrosion.
Although the evidence is not conclusive, the failure is most probably explained
by a combination of fatigue and corrosion. All questions raised in the preceding
paragraphs will be discussed systematically in the following sections and chapters.
2.2
Strength and Response to Load
Resistance, which we experience when trying to lengthen a metal or polymer rod by
a tensile force or to deform it by torsion or bending, is the translation at the macro-
scopic length scale of interatomic and/or intermolecular forces and kind and degree
of ordering at this atomic length scale. However, speaking in terms of
materials'
properties
, interatomic forces are only experienced in a direct way when measuring
the tensile strength of a simple linear chain of atoms or, in the limit, of a perfect
crystal. In this very hypothetical case, it would be found that the maximum force
between atoms is reached at separation of their centers by 1:25r
0
, r
0
representing the
atomic radius. These forces are responsible for the elasticity modulus (Table
2.2
).
Bond rupture takes place for a tensile force exceeding the bond force. Knowing the
real interatomic potentials, the ideal(?) strength of a material should be predictable
from first principles. Unfortunately, real life is more complicated: the result would
be grossly overestimated. Therefore, let us examine real crystals. Crystalline mate-
rials are important because metals dealt with in this chapter are all polycrystalline.
4
Molecular materials will be dealt with when discussing polymers.
By definition a perfect crystal is one in which
the atoms are arranged in a
pattern that repeats itself periodically in a self-similar infinite way in three dimen-
sions.
In real life, crystals are never infinite and even those consisting of one atomic
species, deviate from perfect periodicity.
5
Imperfections in crystals can be classified
4
In a later chapter amorphous materials (glasses) will be dealt with. For the time being we are not
aware of any bioapplication of monocrystalline materials, except indirectly silicon as a substrate
for bioelectronic devices.
5
The term
perfect crystal
is to some extent a misnomer because it refers only to geometric per-
fection. In Chap. 1, the structure of trees is so well adapted to fulfill its function that it can be
