Table 6-2.--Approximate Physical Property Changes of Carbon Steel as the Carbon Content Changes
Carbon Content in
Tensile Strength
Yield Point
Elongation in
SAE
AlSl No.
Percentages
Lbs/sq. in
Lbs/sq. in.
Percentages
24,000
30
.06
1006
43,000
-10
26,000
28
1010
47,000
30,000
25
1020
.20
55,000
.30
37,500
20
1030
68,000
.40
42,000
18
1040
76,000
.50
15
49,500
1050
90,000
1060
.60
98,000
54,000
12
1070
.70
56,000
12
102,000
1080
.80
112,000
56,000
12
.90
67,000
1090
122,000
10
1095
.95
120,000
66,000
10
crack that grows with continued loadings until
that can absorb a lot of energy before breaking.
Toughness does not exist in metals that do not have
high tensile strength; however, metals that are both
Since the small cracks may not be noticed, the
strong and hard tend to have less toughness than
metal may fracture with a suddenness that can be
metals that are softer and have less tensile strength.
dangerous, as in fast-moving vehicles or high-speed
machinery. Special inspection techniques have been
Toughness is definitely related to the property of
developed to spot small cracks before the material
plasticity; materials must be plastic in order to be
tough.
fails. Fatigue failures are due to the repeated
application of tensile stresses or shear stresses, which
tend to pull the material apart. However, a cycle that
PLASTICITY
consists of alternating equal stresses in tension and
compression, called a fully reversed cycle, is usually
Materials that can withstand extensive permanent
used to obtain the endurance limits of a particular
deformation without breaking or rupturing are said to
material.
be highly plastic. Note the use of the word permanent
in this statement; the term plastic deformation is used
HARDNESS
to indicate a PERMANENT change of shape.
Modeling clay is an example of a highly plastic
The property of hardness has been defined as the
material since it can be deformed extensively and
ability of a material to resist penetration. Because
permanently without rupturing. Clay could scarcely
there are several methods of measuring hardness, the
be called tough, however, even though it is highly
hardness of a material is always specified in terms of
plastic.
the particular test that has been used to measure this
Plasticity is in some ways the opposite of
property.
To get a simple idea of the property of hardness,
A material that is brittle will break without showing
consider lead and steel. You can scratch lead with a
deformation. Such a material is not very plastic. A
pointed wooden stick, but you cannot scratch steel
material that is highly elastic will return to its original
with such a stick. Steel is harder than lead.
shape after strain; consequently, such a material does
not show a high degree of plasticity (below the elastic
TOUGHNESS
limit for the substance). Most metals are elastic, rather
than plastic, up to the elastic limit; above the elastic
Toughness is the property that enables a material
limit, they tend to have the property of plasticity.
to withstand shock, to endure tensile stresses, and to
be deformed without breaking. Another way of
Plasticity, like many other properties, is relative.
expressing this is to say that a tough material is one
To some degree, all substances are plastic. Even glass,
6-9