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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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