Page 533 - Carrahers_Polymer_Chemistry,_Eighth_Edition

496 Carraher’s Polymer Chemistry

chains past one another, that is, creep or plastic strain. Many samples undergo both reversible and
irreversible strain.
Flexural strength, or cross-breaking strength, is a measure of the bending strength or stiffness
or a test bar specimen used as a simple beam in accordance with ASTM D-790. The specimen is
placed on supports (Figure 14.13). A load is applied to its center at a specified rate with the loading

at failure called the fl exural strength. However, because many materials do not break even after
being greatly deformed, by agreement, the modulus at 5% strain is used as the flexural strength for

these samples.

14.4.2 TENSILE STRENGTH OF INORGANIC AND METALLIC FIBERS AND WHISKERS
The tensile strength of materials is dependent on the treatment and form of the material. Thus, the
tensile strength of isotropic bulk nylon-66 is less than that of anisotropic oriented nylon-66 fi ber.

Inorganics and metals also form fibers and whiskers with varying tensile strengths (Table 14.1).
Fibers are generally less crystalline and larger than whiskers.
Many of these inorganic fibers and whiskers are polymeric, including many of the oxides (includ-

ing the so-called ceramic fibers), carbon and graphite materials, and silicon carbide. Carbon and

graphite materials are similar but differ in the starting materials and the percentage of carbon.

Carbon fibers derived from polyacrylonitrile are about 95% carbon, while graphite fibers are formed
at higher temperatures giving a material with 99% carbon.

These specialty fibers and whiskers exhibit some of the highest tensile strengths recorded
(Tables 14.1 and 14.2) and they are employed in applications where light weight and high strength are
Testing machine movable
head

Sample specimen

FIGURE 14.13 Typical flexural strength test.

TABLE 14.1
Tensile Strengths of Inorganic and Metallic
Materials as a Function of Form

Material Form Tensile Strength (MPa)
Graphite Bulk 1,000
Fiber 2,800
Whisker 15,000
Glass Bulk 1,000
Fiber 4,000
Steel Bulk 2,000
Fiber 400
Whisker 10,000

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