Page 243 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

222 Chapter 9 Composite Materials: Structure, General Properties, and Applications

Other Fibers. Other fibers used in composites are silicon carbide, silicon nitride, alu-
minum oxide, sapphire, steel, tungsten, molybdenum, boron carbide, boron nitride, and
tantalum carbide. Whiskers also are used as reinforcing fibers (see also Section 22.10).
Whiskers are tiny needle-like single crystals that grow to 1 to 10 /.cm in diameter, with
high aspect ratios (the ratio of fiber length to its diameter) ranging from 100 to 15,000.
Because of their small size, either whiskers are free of imperfections, or the imperfections
they contain do not significantly affect their strength, which approaches the theoretical
strength of the material (size effect). The elastic moduli of whiskers range between 400
and 700 GPa, and their tensile strength is on the order of 15 to 20 GPa, although this
varies with the material.

9.2.2 Fiber Size and Length

Fibers are very strong and stiff in tension. The reason is that the molecules in the
fibers are oriented in the longitudinal direction, and their cross sections are so
small-usually less than 0.01 mm in diameter-that the probability is low for any
significant defects to exist in the fiber. Glass fibers, for example, can have tensile
strengths as high as 4600 MPa, whereas the strength of glass in bulk form (Section
8.4) is much lower.
Fibers generally are classified as short (discontinuous) or long (continuous).
The designations “short” and “long” fiber are, in general, based on the following
distinction: In a given type of fiber, if the mechanical properties improve as a result
of increasing the average fiber length, then it is called a short fiber. If no such im-
provement in properties occurs, it is called a long Hber. Short fibers typically have
aspect ratios between 20 and 60, long fibers between 200 and 500.
Reinforcing elements also may be in the form of chopped fibers, particles, or
Hakes, or in the form of continuous rot/ing (slightly twisted strands) fibers, woven
fabric (similar to cloth), yarn (twisted strands), and mats of various combinations.
Various hybrid yarns also are available.

9.2.3 Matrix Materials

The matrix in reinforced plastics has three principal functions:
I. Support the fibers in place and transfer the stresses to them so that the fibers
can carry most of the load (see Example 9.1).
2. Protect the fibers against physical damage and the environment.
3. Reduce the propagation of cracks in the composite by virtue of the greater
ductility and toughness of the plastic matrix.
Matrix materials are usually t/oerrnoplastics or t/vermosets and commonly con-
sist of epoxy, polyester, phenolic, fluorocarbon, polyethersulfone, or silicon. The most
commonly used are the epoxies (80% of all reinforced plastics) and the polyesters
(less expensive than the epoxies). Polyimides, which resist exposure to temperatures
in excess of 300°C, continue to be developed for use as a matrix with carbon fibers.
Some thermoplastics, such as polyetheretherketone (PEEK), are also used as matrix
materials; they generally have higher toughness than thermosets, but their resistance
to temperature is lower, being limited to 100° to 200°C.

9.3 Properties of Reinforced Plastics

The mechanical and physical properties of reinforced plastics depend on the type,
shape, and orientation of the reinforcing material, the length of the fibers, and
the volume fraction (percentage) of the reinforcing material. Short fibers are less