Page 15 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

STRUCTURAL STEELS, STEELMAKING, AND FABRICATION 1.13

The strain at which strain hardening begins ( st ) and the rate at which stress increases with strain
in the strain-hardening range (the strain-hardening modulus E st ) have been determined for carbon
and HSLA steels. The average value of E st is 600 ksi, and the length of the yield plateau is 5 to
15 times the yield strain. (T. V. Galambos, “Properties of Steel for Use in LRFD,” Journal of the
Structural Division, American Society of Civil Engineers, Vol. 104, No. ST9, 1978.)

Yield Point, Yield Strength, and Tensile Strength. As illustrated in Fig. 1.3, carbon and HSLA
steels usually show an upper and lower yield point. The upper yield point is the value usually recorded
in tension tests and thus is simply termed the yield point.
The heat-treated steels in Fig. 1.3, however, do not show a definite yield point in a tension test.
For these steels it is necessary to define a yield strength, the stress corresponding to a specified devi-
ation from perfectly elastic behavior. As illustrated in the figure, yield strength is usually specified
in either of two ways: For steels with a specified value not exceeding 80 ksi, yield strength is con-
sidered as the stress at which the test specimen reaches a 0.5% extension under load (0.5% EUL) and
may still be referred to as the yield point. For higher-strength steels, the yield strength is the stress
at which the specimen reaches a strain 0.2% greater than that for perfectly elastic behavior.
Since the amount of inelastic strain that occurs before the yield strength is reached is quite small,
yield strength has essentially the same significance in design as yield point. These two terms are
sometimes referred to collectively as yield stress.
The maximum stress reached in a tension test is the tensile strength of the steel. After this stress
is reached, increasing strains are accompanied by decreasing stresses. Fracture eventually occurs.

Proportional Limit. The proportional limit is the stress corresponding to the first visible departure
from linear-elastic behavior. This value is determined graphically from the stress-strain curve. Since
the departure from elastic action is gradual, the proportional limit depends greatly on individual
judgment and on the accuracy and sensitivity of the strain-measuring devices used. The proportional
limit has little practical significance and is not usually recorded in a tension test.
Ductility. Ductility is an important property of structural steels. It allows redistribution of stresses in
continuous members and at points of high local stresses, such as those at holes or other discontinuities.
In a tension test, ductility is measured by percent elongation over a given gage length or percent
reduction of cross-sectional area. The percent elongation is determined by fitting the specimen
together after fracture, noting the change in gage length and dividing the increase by the original
gage length. Similarly, the percent reduction of area is determined from cross-sectional measure-
ments made on the specimen before and after testing.
Both types of ductility measurements are an index of the ability of a material to deform in the
inelastic range. There is, however, no generally accepted criterion of minimum ductility for various
structures.
Poisson’s Ratio. The ratio of transverse to longitudinal strain under load is known as Poisson’s
ratio v. This ratio is about the same for all structural steels—0.30 in the elastic range and 0.50 in the
plastic range.

True-Stress–True-Strain Curves. In the stress-strain curves shown previously, stress values were
based on original cross-sectional area, and the strains were based on the original gage length. Such
curves are sometimes referred to as engineering-type stress-strain curves. However, since the orig-
inal dimensions change significantly after the initiation of yielding, curves based on instantaneous
values of area and gage length are often thought to be of more fundamental significance. Such curves
are known as true-stress–true-strain curves. A typical curve of this type is shown in Fig. 1.4.
The curve shows that when the decreased area is considered, the true stress actually increases
with increase in strain until fracture occurs instead of decreasing after the tensile strength is
reached, as in the engineering stress-strain curve. Also, the value of true strain at fracture is much
greater than the engineering strain at fracture (though until yielding begins, true strain is less than
engineering strain).

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