Ceramics and composites - Orthopaedic Biomaterials in Research and Practice - page 211

Continuous
fiber

Pre-peg
fabrication

Assembly Machining

Component

Resin matrix

Compounding Molding Machining

Particulate
filler

FIGUre 8.1 Processing of composites.

2. The filler must have a density near enough to that of the matrix
that “settling” of the filler during processing does not occur. There
is no firm rule for this; matrices with high viscosities and good
wetting abilities may be successfully used in the face of large den-
sity differences.

3. The matrix-filler composition must be able to be processed.
Addition of the filler to the matrix rapidly increases the viscosity
of the mixture. Thus, some processes, such as injection molding,
may become very difficult or impossible when large amounts of
filler are used.

Continuous fiber fillers are used primarily with thermoset matrices,
such as epoxies. It is the usual practice to form sheets with fibers in a
single orientation, called pre-pregs , and then to combine the sheets as
is called for in the design. The final body then may be machined, as in
the production of screw holes in a fracture plate (Figure 8.2). Care must
be taken with such steps, as they break fibers and may produce areas of
relative weakness in the finished devices. One mode of failure of such
devices is separation along pre-preg interfaces, called delamination ,
resulting from inadequate cohesion of the overall part.

Mechanical
properties

There are many possible structures that a composite material may take,
depending on the shape of the reinforcing phase (particles, fibers, etc.)
and its spatial arrangement within the matrix. The simplest of these is a
random arrangement of short fibers in a uniform matrix. The properties
of such PMCs are difficult to predict exactly, but some indication may
be obtained from examination of matrices with 10-30 volume percent
(v/o) chopped S-glass filler. Table 8.3 provides a sampling of such data.
Although composite materials and structures must be tested for accu-
rate determination of mechanical properties, mathematical analyses
have been performed on a variety of defined structures with extremely
complex and relatively accurate solutions derived for elastic moduli and
failure strengths. The simplest solution, but one that illustrates most of