Page 102 - Mechanical Behavior of Materials

Section 3.7 Composite Materials 103

Figure 3.25 Fracture surface showing broken fibers for a composite of Nicalon-type SiC fibers
in a CAS glass–ceramic matrix. (Photo by S. S. Lee; material manufactured by Corning.)

Figure 3.26 Laminated composites. Sheets having various fiber directions, as shown in (a),
may be bonded together. The ARALL laminate (b) is constructed with aluminum sheets
bonded to sheets of composite, with the latter being made of Kevlar fibers in an epoxy matrix.

cured sheets, which are called prepregs because they have been previously impregnated with the
epoxy resin. Appropriate heat and pressure are applied to complete the cross-linking reaction, while
at the same time bonding the layers into a solid laminate. Fibers commonly used in this manner with
an epoxy matrix include glass, graphite, boron, and the aramid polymer Kevlar. The microstructure
of a laminated composite can be seen in Fig. 3.27.
For polymer matrix fibrous composites, strengths comparable to those of structural metals are
obtained, as shown in Fig. 3.28(a). The values for ordinary polyester matrix fiberglass and the
lower strength structural metals are similar. But for epoxy reinforced with long fibers of S-glass or
graphite, the strength rivals that of the stronger steels. Values of stiffness (E) for high-performance
laminates are comparable to those for aluminum, but less than for steel. However, in considering
materials for weight-critical applications such as aircraft structure, it is more relevant to consider the