Timothy A. Philpot - Mechanics of materials _ an integrated learning system-John Wiley (2017)

52
MECHANICAL PROPERTIES
OF MATERIALS
80
70
Stress (ksi)
60
50
40
30
20
10
58 ksi
Yield strength
Proportional limit
0.2% Offset
Necking
of tension
specimen
0
0.0020 0.0060 0.0100
0.0000 0.0040 0.0080 0.0120
Strain (in./in.)
FIGURE 3.9 Yield strength determined by offset method.
FIGURE 3.10 Necking in a
tension specimen.
region of the specimen, forming a contraction, or “neck.” This behavior is referred to
as necking (Figure 3.10 and Figure 3.11). Necking occurs in ductile materials, but not
in brittle materials. (See discussion of ductility, to follow.)
Fracture
Many ductile materials break in what is termed a cup-and-cone fracture (Figure 3.12). In
the region of maximum necking, a circular fracture surface forms at an angle of roughly
45° with respect to the tensile axis. This failure surface appears as a cup on one portion of
the broken specimen and as a cone on the other portion. In contrast, brittle materials often
fracture on a flat surface that is oriented perpendicular to the tensile axis. The stress at
which the specimen breaks into two pieces is called the fracture stress. Examine the relationship
between the ultimate strength and the fracture stress in Figure 3.4. Does it
seem odd that the fracture stress is less than the ultimate strength? If the specimen did not
Jeffery S. Thomas
FIGURE 3.11 Necking in a
ductile metal specimen.
FIGURE 3.12 Cup-and-cone failure surfaces.
Jeffery S. Thomas