Page 244 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 9.3 Properties of Reinforced Plastics 22

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FIGURE 9.5 The effect of the type of fiber on various properties of fiber-reinforced nylon (6,6).
Source: Courtesy of NASA.

effective than long fibers (Fig. 9.5 ), and their properties are strongly influenced by
temperature and time under load. Long fibers transmit the load through the matrix
better; thus, they are commonly used in critical applications, particularly at elevated
temperatures. The physical properties of reinforced plastics and their resistance to
fatigue, creep, and wear depend greatly on the type and amount of reinforcement.
Composites can be tailored to impart specific properties (such as permeability and
dimensional stability), to make processing easier, and to reduce production costs.
A critical factor in reinforced plastics is the strength of the bond between the
fiber and the polymer matrix, because the load is transmitted through the fiber-
matrix interface. Weak interfacial bonding causes fiber pullout and delamination of
the structure, particularly under adverse environmental conditions. Adhesion at the
interface can be improved by special surface treatments, such as coatings and cou-
pling agents. Glass fibers, for example, are treated with silane (a silicon hydride) for
improved wetting and bonding between the fiber and the matrix. The importance of
proper bonding can be appreciated by inspecting the fracture surfaces of reinforced
plastics, shown in Figs. 9.6a and b. Note, for example, the separation between the
fibers and the matrix; obviously, better adhesion between them improves the overall
strength of the composite.
Generally, the highest stiffness and strength in reinforced plastics are obtained
when the fibers are aligned in the direction of the tension force. The composite is
then highly anisotropic (Fig. 9.7); that is, it has properties, such as strength and stiff-
ness, that depend on direction. As a result, other properties, such as stiffness, creep
resistance, thermal and electrical conductivity, and thermal expansion, also are
anisotropic. The transverse properties of such a unidirectionally reinforced structure
are much lower than the longitudinal properties. For example, note how strong