Page 248 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 9.5 Metal-matrix Composites 227

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carrier is
Plastic film
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i;'_ 3; KeV|ar® ‘ l*1 1 ‘ “‘

FIGURE 9.9 Schematic illustration of body armor, showing the
layers of woven fibers.

flexible body armor was Kevlar 29 (an aramid), which oxide and silica), or polyethylene plates strategically
has been improved through a number of versions. located to prevent penetration of ballistic particles to
Qther forms include Kevlar 49, Kevlar 129, and Kevlar critical areas. Designs currently being evaluated utilize
Protera-a form whose tensile strength and energy- fluids with suspended nanoparticles of silica. At low
absorbing capabilities have been improved through the strain rates, these fluids are inviscid and flow readily.
development of advanced spinning processes to At high strain rates typical of ballistic particles, these
produce the fibers. Aramid fibers are used very fluids are very resistant to deformation and can
commonly in flexible body armor. Honeywell also provide additional protection. The fluid is contained
produces an aramid fiber-based body armor, but other by the woven fiber mesh (it acts like a sponge holding
designs, such as Akzo Noble’s TWARON aramid fiber; the fluid in place) and is contained by the outer fabric.
use over a thousand finely spun filaments that interact In addition, a composite military helmet has been
with each other to dissipate the impact energy. developed that, although weighing about the same as
Spectra fiber is used to make the Spectra Shield a conventional manganese-steel helmet, covers more
composite for use in body armor. A layer of Spectra of the head and offers twice the ballistic and
Shield composite consists of two unidirectional fragmentation protection. A composite helmet has a
layers of Spectra fiber arranged to cross each other nonwoven fiber construction made with Spectra fibers
at 0- and 90-degree angles and held in place by a in a thermosetting polymer matrix, which effectively
flexible resin. Both the fiber and resin layers are stops the bullet by flattening it as it strikes the first
sealed between two thin sheets of polyethylene film, layer of material.
which is similar in appearance to plastic food wrap.
Hard armor uses a number of designs, but Source: Courtesy of Pinnacle Armor, AlliedSignal Corp.,
typically consists of steel, ceramic (usually aluminum and CGS Gallet SA.

9.5 Metal-matrix Composites

The advantages of a metal matrix over a polymer matrix are higher elastic modulus,
toughness, ductility, and higher resistance to elevated temperatures. The limitations are
higher density and a greater difficulty in processing parts. Matrix materials in metal-
matrix composites (MMC) are usually aluminum, aluminum-lithium alloy (lighter
than aluminum), magnesium, copper, titanium, and superalloys (Fig. 9.10). Fiber ma-
terials are graphite, aluminum oxide, silicon carbide, boron, molybdenum, and tung-
sten. The elastic modulus of nonmetallic fibers ranges between 200 and 400 GPa, with
tensile strengths in the range from 2000 to 3000 MPa.
Typical compositions and applications for metal-matrix composites are given
in Table 9.3.