Page 112 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 3.2 Density

KeV|ar49 ' High modulus _.
_
1 1
graphite
S-glass Boron
High-tensile High-tensile
graphite graphite
Boron Graphite/epoxy
High";;C:)Lm,:; Boron/epoxy
E-glass Kevlar 49

Graphite/epoxy E-glass
Boron/epoxy S-glass
Glass/epoxy Aluminum

Titanium Titanium
Steel Steel
Aluminum Glass/epoxy
\_ (_
0 5 10 15 20 25 0 5 10 15 20
Tensile strength/density (m >< 104) Elastic modulus/density (m >< 106)
(S) (b)

FIGURE 3.2 Specific strength (tensile strength/density) and specific stiffness (elastic modulus/
density) for various materials at room temperature. (See also Chapter 9.)

as described in Chapter 6, they are among the most commonly used metals for air-
craft and aerospace applications.
The range for specific tensile strength and specific stiffness at room tempera-
ture for a variety of metallic and nonmetallic materials is given in Fig. 3.2. Note the
positions of composite materials, as compared to those of metals, with respect to
these properties; these advantages have led composites to become among the most
important materials (see Chapter 9). At elevated temperatures, specific strength and
specific stiffness are likewise important considerations, especially for systems that
operate at these temperatures, such as automotive and jet engines and gas turbines.
Typical ranges for a variety of materials are given in Fig. 3.3.
Density is an important factor in the selection of materials for high-speed
equipment, such as magnesium in printing and textile machinery, many components
of which usually operate at very high speeds. To obtain exposure times of 1/4000 s
in cameras without sacrificing accuracy, the focal plane shutters of some high-
quality digital cameras are made of titanium. Aluminum is used with some cameras
for better performance in cold vveather. The lovv resulting mass of the components
in these high-speed operations reduces inertial forces that otherwise could lead to
vibrations, to inaccuracies, and even, over time, to part failure. Because of their lovv
density, ceramics (Chapter 8) are used for components in high-speed automated
machinery and in machine tools. On the other hand, there are applications where
weight is desirable; examples are counterweights for various mechanisms (using lead
or steel), flyvvheels, ballasts on yachts and aircraft, and Weights on golf clubs (using
high-density materials such as tungsten).