Page 625 - Carrahers_Polymer_Chemistry,_Eighth_Edition

588 Carraher’s Polymer Chemistry

HRSI
AFRSI
LRSI
FRSI
AFRSI
HRSI

RCC HRSI

FRSI RCC
HRSI and LRSI
LRSI

FIGURE 18.7 Location of various advanced materials, including composites, employed for heat protection
in the Space Shuttle Orbiter. The descriptions of the materials are given in Table 18.7.

dense packing of the aggregates and good interfacial contact, both achieved by having a mixture of
aggregate sizes—thus the use of large gravel and small sand. The sand helps fill the voids between

the various larger gravel particles. Mixing and contact is achieved with the correct amount of water.
Enough water must be present to allow a wetting of the surfaces to occur along with providing some
of the reactants for the setting up of the cement. Too much water creates large voids and weakens
the concrete.

18.8.1.2 Fiber-Reinforced Composites

Mathematically, the critical fiber length necessary for effective strengthening and stiffening can be
described as follows:

Critical fiber length = (Ultimate or tensile strength times fiber diameter/2) times the fi ber-

matrix bond strength OR the shear yield strength of the matrix—which ever is smaller.
Fibers where the fiber length is greater than this critical fiber length are called continuous fi bers

while those that are less than this critical length are called discontinuous or short fi bers. Little trans-
ference of stress and thus little reinforcement is achieved for short fibers. Thus, fibers whose lengths

exceed the critical fiber length are used.

Fibers can be divided according to their diameters. Whiskers are very thin single crystals that
have large length to diameter ratios. They have a high degree of crystalline perfection and are essen-
tially flaw free. They are some of the strongest materials know. Whisker materials include graphite,

silicon carbide, aluminum oxide, and silicon nitride. Fine wires of tungsten, steel, and molybdenum
are also used but here, even though they are fine relatively to other metal wires, they have large

diameters. The most used fibers are “organic fibers,” which are either crystalline or amorphous or

semicrystalline with small diameters.
18.8.1.3 Processing of Fiber-Reinforced Composites
These exists a wide variety of particular operations but briefly they can be described in terms of fi la-

ment winding, preimpregnation of the fiber with the partially cured resin, and pultrusion. Pultrusion

is used to produce rods, tubes, beams, and so on with continuous fi bers that have a constant cross-
sectional shape. The fiber (as a continuous fiber bundle, weave or tow) is impregnated with a ther-

mosetting resin and pulled through a die that shapes and establishes the fiber to resin ratio. This
stock is then pulled though a curing die that can machine or cut producing the fi nal shape such as

filled and hollow tubes and sheets.

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