STEEL - Materials for Civil and Construction Engineers - page 113

of area. Typically, the test is performed at temperatures between 10°C and
35°C (50°F to 95°F).
The test specimen can be either full sized or machined into a shape, as
prescribed in the product specifications for the material being tested. It is
desirable to use a small cross-sectional area at the center portion of the
specimen to ensure fracture within the gauge length. Several cross-sec-
tional shapes are permitted, such as round and rectangular, as shown in
Figure 3.11. Plate, sheet, round rod, wire, and tube specimens may be
used. A 12.5 (1/2 in.) diameter round specimen is used in many cases. The
gauge length over which the elongation is measured typically is four times
the diameter for most round-rod specimens.
Various types of gripping devices may be used to hold the specimen, de-
pending on its shape. In all cases, the axis of the test specimen should be
placed at the center of the testing machine head to ensure axial tensile
stresses within the gauge length without bending. An extensometer with a
dial gauge (Figure 1.25) or an LVDT (Figure 1.29) is used to measure the de-
formation of the entire gauge length. The test is performed by applying an
axial load to the specimen at a specified rate. Figure 3.12 shows a tensile test
being performed on a round steel specimen using an LVDT extensometer to
measure the deformation.
As discussed in Chapter 1, mild steel has a unique stress-strain relation
(Figure 3.13). Here, a linear elastic response is displayed up to the propor-
tion limit. As the stress is increased beyond the proportion limit, the steel
will yield, at which time the strain will increase without an increase in stress
(actually the stress will slightly decrease). As tension increases past the
yield point, strain increases following a nonlinear relation up to the point of
failure.
Note that the decrease in stress after the peak does not mean a decrease
in strength. In fact, the actual stress continues to increase until failure. The
reason for the apparent decrease is that a neck is formed in the steel speci-
men, causing an appreciable decrease in the cross-sectional area. The tradi-
tional, or engineering, way of calculating the stress and strain uses the original
FIGURE 3.11 Tension test speci-
mens with round and rectangular
cross-sections.