Page 39 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 1.5 Selection of Materials I7

chair, (c) aluminum vs. copper wire, (d) plastic vs. steel car bumper, (e) plastic vs.
metal toy, and (f) alloy steel vs. titanium submarine hull.
The two examples that follow give typical details of the major factors involved
in material substitution in common products.

EXAMPLE l.2 Baseball Bats

Baseball bats for the major and minor leagues are
generally made of wood from the northern white ash
tree, a wood that has high dimensional stability, a
high elastic modulus and strength-to-weight ratio,
and high shock resistance. Wooden bats can, how-
ever, break during their use and may cause serious
injury. The wooden bats are made on semiautomatic
lathes and then subjected to finishing operations for fm
appearance and labeling. The straight uniform grain
required for such bats has become increasingly diffi-
cult to find, particularly when the best wood comes
from ash trees that are at least 45 years old.
For the amateur market and for high school and
college players, aluminum bats (top portion of Fig. 1.5)
have been made since the 1970s as a cost-saving
alternative to wood. The bats are made by various
metalworking operations, described throughout
Part III. Although, at first, their performance was
not as good as that of wooden bats, the technology
has advanced to a great extent. Metal bats are now
made mostly from high-strength aluminum tubing, as
well as other metal alloys. The bats are designed to FIGURE L5 Cross sections of baseball bats made of
have the same center of percussion (known as the aluminum (top two) and composite material (bottom
sweet spot, as in tennis racquets) as wooden bats, and two).
are usually filled with polyurethane or cork for
Developments in bat materials now include
improved sound damping and for controlling the
composite materials (Chapter 9) consisting of high-
balance of the bat.
strength graphite and glass fibers embedded in an epoxy
Metal bats possess such desirable performance
characteristics as lower weight than wooden bats, resin matrix. The inner woven sleeve (lower portion of
Fig. 1.5) is made of Kevlar fibers (an aramid), which add
optimum weight distribution along the bat’s length,
strength to the bat and dampen its vibrations. These bats
and superior impact dynamics. Also, as documented
perform and sound much like wooden bats.
by scientific studies, there is a general consensus that
metal bats outperform wooden bats. Source: Mizuno Sports, Inc.

EXAMPLE L3 U.S. Pennies

Billions of pennies are produced and put into circula- 1793-1837 100% copper
tion each year by the U.S. Mint. The ,materials used 1837~1857 95% copper, 5% tin and zinc
have undergone significant changes throughout history, 1857-1863 88% copper, 12% nickel
largely because of periodic material shortages and the 1864-1962 95% copper, 5% tin and zinc
resulting fluctuating cost of appropriate raw materials. 1943 (WW II years) Steel, plated with zinc
The following table shows the chronological develop- 1962-1982 95% copper, 5% zinc
ment of material substitutions in pennies: 1982-present 97.5% zinc, plated with copper