Biomedical Engineering Reference
In-Depth Information
fracture toughness parameter is a function of the appar-
ent crack propagation stress and the crack depth and
shape. It is called the critical stress intensity factor (K
lc
)
and has units of Pa
A
m
3/2
(
Meyers and Chawla,
1984
). For materials that exhibit extensive plastic de-
formation at the crack tip, an energy-based parameter,
the J integral, can be used. The energy absorbed in
impact fracture is also a measure of toughness, but at
higher loading rates (
Newey and Weaver, 1990
).
morN
$
O
Time
B
Ultimate tensile
strength range
Effect of fabrication on strength
A general concept to keep in mind when considering the
strength of materials is that the process by which a ma-
terial is produced has a major effect on its structure and
hence its properties (
Newey and Weaver, 1990
). For
example, plastic deformation of most metals at room
temperature flattens the grains and produces strength-
ening while reducing ductility. Subsequent high-
temperature treatment (annealing) can reverse this
effect. Polymers drawn into fibers are much stronger in
the drawing direction than are undrawn samples of the
same material.
Because strength properties depend on fabrication
history, it is important to realize that there is no unique
set of strength properties of each generic material (e.g.,
316L stainless steel, polyethylene, aluminum oxide).
Rather, there is a range of properties that depends on the
fabrication history and the microstructures produced.
Endurance limit
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
Cycles to failure
Fig. 3.1.2-11 (A) Stress versus time in a fatigue test. (B) Fatigue
curve: fatigue stress versus cycles to failure.
Conclusion
Careful attention to these details is required if laboratory
fatigue results are to be successfully transferred to bio-
medical applications.
The determination of mechanical properties is not only an
exercise in basic materials science but is indispensable to
the practical design and understanding of load-bearing
structures. Designers must determine the service stresses
in all structural members and be sure that at every point
these stresses are safely below the yield strength of the
material. If cyclic loads are involved (e.g., lower-limb
prostheses, teeth, heart valves), the service stresses must
be kept below the fatigue strength.
Subsequently where the properties and behavior of
materials are discussed in detail, it is well to keep in mind
that this information is indispensable to understanding
the mechanical performance (i.e., function) of both bi-
ological and manmade structures.
Toughness
The ability of a material to plastically deform under the
influence of the complex stress field that exists at the tip
of a crack is a measure of its toughness. If plastic de-
formation does occur, it serves to blunt the crack and
lower the locally enhanced stresses, thus hindering crack
propagation. To design ''failsafe'' structures with brittle
materials, it has become necessary to develop an entirely
new system for evaluating service worthiness. This
system is fracture toughness testing and requires the
testing of specimens with sharp notches. The resulting
Bibliography
Billmeyer, F.W. (1984).
Textbook of
Polymer Science
. John Wiley and Sons
Inc., New York.
Hummel, R.E. (1997).
Understanding
Materials Science.
Springer-Verlag,
New York.
Kingery, W.D. (1976).
Introduction to
Ceramics
. John Wiley and Sons Inc.,
New York.
