Biomedical Engineering Reference
In-Depth Information
support, and locomotion without their reinforcing
collagen fibers. Likewise, fibers alone would be of
little benefit without their matrix, which binds the
fibers together, keeping them in place and providing
a mechanism for load transfer to them from the
outside physical environment.
The principles of fiber reinforcement are described
in textbooks on composite materials [8,9] but can be
exemplified by considering a collection of parallel
strings, held between the hands as a loose rope. In
tension, the fibers making up the strings are capable
of carrying load and it may take considerable force to
break them. However, in compression, with no
“matrix” to bind the fibers together, the separate
strings easily buckle with almost negligible load.
Take the same set of strings and now impregnate and
bond them together with a polymer adhesive resin
and it becomes a different situation. The cured
adhesive matrix fixes the strings together and
provides a means of transferring load, so the “rope” is
rigid in both tension and compression, although as
one would expect, when placed under sufficient
compressive load the rope will eventually fail by
buckling as the strings separate and in all probability
this will be at significantly lower load than when
failure occurs under tension.
There is a strong and direct correlation between
the physical properties of the reinforcing fibers
(making up the string in this example), the amount
(volume fraction) of fibers, the length and orientation
of the fibers, the nature of the polymeric matrix
material, and the interface between matrix and fiber.
As a demonstration of the latter point, imagine taking
the same strings mentioned earlier, but this time coat
the strings in low-molecular-weight wax before
applying the adhesive resin, such that the wax
prevents the polymer matrix from penetrating the
string and from effectively bonding the strings
together as a unit. Now push along the axis of the
rope and there will be a significant reduction in
compressive strength compared with the previous
case. This is as a result of the now very weak inter-
facial bond between the fibers and matrix. As will be
discussed later in this chapter, it is important, there-
fore, to have a strong and effective interfacial bond
between fiber and matrix at the microscopic level to
realize the full reinforcing potential of the fibers.
In an engineering context, with the correct
combination of fibers, matrix, and fiber orientation
relative to the direction of applied load, the
mechanical properties of the fabricated artifact can
be significantly enhanced as the material of
construction may be specifically tailored to meet the
needs of the engineered design. These are the prin-
ciples of fiber-reinforced composite materials.
The same principles apply to compounds except
that here the additive is either in the form of dispersed
particles or short (mm and sub-mm length) fibers that
in molded parts can be almost randomly oriented.
The greater degree of randomness in the orientation
of short fibers and the fact that the fibers are short in
comparison with their diameter of a few microns
limit the amount of property enhancement achieved
compared with their continuous fiber counterparts.
However, despite their limitations, short fiber rein-
forced polymers constitute an important class of
materials, partly because of their relative ease of
processing and partly because they present usefully
enhanced mechanical or physical properties compared
with unfilled materials; as such they have been
the subject of significant research and application.
Helpful background information on the technology of
short fiber reinforced materials can be found in the
literature [10] .
In designing reinforced polymers, the objective is
to attain desirable macroscopic material properties
by combining materials at a microscopic level in the
appropriate form, in optimized proportions, such that
they operate in synergy whereby the resulting mate-
rial can be processed by a convenient route to make
artifacts that function according to the design
requirements. This interrelationship is illustrated in
Fig. 3.4 .
The successful adoption of any material in any
sphere of application depends on how it functions
mechanically in comparison with other available
materials and whether the material brings added
benefits in terms of processing, design freedom, or
specific functionality. For implantable materials,
such functionality may include modulus matching to
bone, or the ability to provide enhanced radiographic
imaging.
A composite material (and compound), then,
comprises a polymer matrix, additive in the form of
a powder, flakes, or fibers, and an interface between
them.
3.2.1 Role of the Matrix
In polymer composite systems, the matrix is
typically the component with the lowest
tensile
strength and stiffness
(modulus of
elasticity)
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