Page 17 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch01.qxd 9/29/05 4:59 PM Page 1.15

PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

STRUCTURAL STEELS, STEELMAKING, AND FABRICATION 1.15

approximate conversions (see ASTM A370). Because of its simplicity, the hardness test is widely used
in manufacturing operations to estimate tensile strength and to check the uniformity of tensile strength
in various products.

1.9 EFFECT OF COLD WORK ON TENSILE PROPERTIES

In the fabrication of structures, steel plates and shapes are often formed at room temperatures into
desired shapes. These cold-forming operations cause inelastic deformation, since the steel retains its
formed shape. To illustrate the general effects of such deformation on strength and ductility, the ele-
mental behavior of a carbon-steel tension specimen subjected to plastic deformation and subsequent
tensile reloadings will be discussed. However, the behavior of actual cold-formed structural mem-
bers is more complex.
As illustrated in Fig. 1.5, if a steel specimen is unloaded after being stressed into either the plas-
tic or strain-hardening range, the unloading curve follows a path parallel to the elastic portion of the
stress-strain curve. Thus a residual strain, or permanent set, remains after the load is removed. If
the specimen is promptly reloaded, it will follow the unloading curve to the stress-strain curve of the
virgin (unstrained) material.
If the amount of plastic deformation is less than that required for the onset of strain hardening,
the yield stress of the plastically deformed steel is about the same as that of the virgin material.
However, if the amount of plastic deformation is sufficient to cause strain hardening, the yield stress
of the steel is larger. In either instance, the tensile strength remains the same, but the ductility, mea-
sured from the point of reloading, is less. As indicated in Fig. 1.5, the decrease in ductility is nearly
equal to the amount of inelastic prestrain.
A steel specimen that has been strained into the strain-hardening range, unloaded, and allowed to age
for several days at room temperature (or for a much shorter time at a moderately elevated temperature)

FIGURE 1.5 Stress-strain diagram (not to scale) illustrating the
effects of strain-hardening steel. (From R. L. Brockenbrough and
B. G. Johnston, USS Steel Design Manual, R. L. Brockenbrough &
Associates, Inc., Pittsburgh, Pa., with permission.)

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