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Ductility is the percent elongation reported in a tensile
test is defined as the maximum elongation of the gage
length divided by the original gage length. The measurement
is determined as shown in Figure
6.
Reduction of area is the proportional
reduction of the cross-sectional area of a tensile test piece at the plane of fracture
measured after fracture.
The reduction of area is reported as
additional information (to the percent elongation) on the
deformational characteristics of
the material. The two are used as indicators of ductility,
the ability of a material
to be elongated in tension. Because the elongation is not
uniform over the entire
gage length and is greatest at the center of the neck, the
percent elongation is not an absolute
measure of ductility. (Because of this, the gage length must
always be stated when the percent
elongation is reported.) The reduction of area, being
measured at the minimum diameter of
the neck, is a better indicator of ductility.
Ductility is
more commonly defined as the ability of a material to deform
easily upon the application
of a tensile force, or as the ability of a material to
withstand plastic deformation without
rupture. Ductility may also be thought of in terms of
bendability and crushability. Ductile
materials show large deformation before fracture. The lack of
ductility is often termed brittleness.
Usually, if two materials have the same strength and
hardness, the one that has the higher
ductility is more desirable. The ductility of many metals can
change if conditions are altered.
An increase in temperature will increase ductility. A
decrease in temperature will cause a
decrease in ductility and a change from ductile to brittle
behavior.
Cold-working also tends to make metals less
ductile. Cold-working is performed in a temperature region
and over a time interval to obtain plastic deformation, but
not relieving the strain hardening.
Minor additions of impurities to metals, either deliberate or
unintentional, can have a
marked effect on the change from ductile to brittle behavior.
The heating of a cold-worked metal
to or above the temperature at which metal atoms return to
their equilibrium positions will increase
the ductility of that metal.
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