Biomedical Engineering Reference
In-Depth Information
combination with carbon fibers. There is also an
inherently strong interface between carbon fiber and
matrix that occurs when they are blended and heated
together such that the polymer melts and coats the
fibers, then cools, crystallizes, and solidifies. As has
been described in the general case earlier, the
strength of the interface between PEEK and
the reinforcing fibers is important in determining the
overall strength of the material, especially in the case
of short fiber reinforced materials because it
is
through the interface that load transfer occurs.
3.2.3 Role of the Fiber
The role of the fiber is to provide reinforcement.
To achieve a material with enhanced strength and
stiffness, the additive needs to be capable of carrying
load. It must, therefore, be stronger and stiffer than
the host polymer matrix and be of suitable geometry
to enable efficient load transfer.
Fibers fulfill these criteria extremely well and are
remarkable in that their mechanical properties can
be significantly better than the same generic mate-
rial in bulk form. For example, bulk glass has
a tensile strength of between 27 and 62 MPa,
whereas freshly drawn glass fibers can have
strengths in excess of 3500 MPa. This is as a result
of there being far fewer flaws in glass fibers than
otherwise are present in the bulk material. Under
tensile stress, especially in brittle materials such as
glass, flaws of sufficient size (which can be micro-
scopically small) can quickly develop into cracks of
fatal dimensions that can quickly propagate and
result in brittle fracture. The strength of the material
can be significantly increased by avoiding such
flaws. A full account of the science behind the
manufacture and mechanics of fibers is beyond the
scope of this chapter and, for the interested reader,
much more information on this subject can be found
in published literature
[12]
.
Needless to say, there has been a lot of effort
expended over the last 50
Figure 3.4
Process, form, and function.
compared with the reinforcing component. For
example, some carbon fibers have a tensile strength
of 5000 MPa and modulus of elasticity of around
270 GPa compared with
<
150 MPa and
<
5 GPa for
the matrix, respectively. As in the string example
described earlier, it is the fibers that take up the
applied load and provide strength and stiffness,
whereas the matrix holds the fibers in position rela-
tive to each other.
The polymer matrix binds with the fibers to create
a unified material, such that fibers and matrix func-
tion cooperatively to carry the applied load. The
matrix may also serve to encapsulate the fibers to
provide a barrier to chemical attack, protect the
fibers from mechanical damage, or provide a mech-
anism for toughening the material system. This is
achieved by providing a means of blunting any
developing cracks, or deflecting cracks, or redirect-
ing them along the fiber/matrix interface to absorb
increasing amounts of energy, thereby increasing the
work of fracture. The role of the matrix and the
importance of the interface between the matrix and
reinforcing fiber in fracture have been the subject of
analysis over decades. A useful early account is
provided by Kelly
[11]
.
years to develop engi-
neered fibers and improve their mechanical strength
and stiffness. This has resulted in a broad range of
products that are now commercially available,
including materials such as glass, carbon, boron, and
a variety of organic synthetic fibers.
More recent research has introduced nanofiber
materials
[13]
more especially based on carbon for
commercial application in engineered products,
although such materials, however remarkable, are not
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3.2.2 PEEK Polymer Matrix
PEEK polymer has been described in Chapter 2 of
this handbook and needs no further introduction as
a thermoplastic polymer; however, special attention
is given here to the material as a constituent ingre-
dient of compounds and composites.
The strength, stiffness, toughness, and biocom-
patibility of PEEK make it an ideal polymer matrix in
