INFLUENCE OF COLD WORK ON MECHANICAL PROPERTIES OF STEEL 37
“Structural Welding Code—Sheet Steel” (ANSI/AWS
D1.3) provides welding processes for shielded metal arc
welding (SMAW), gas metal arc welding (GMAW), flux
cored arc welding (FCAW), and submerged arc welding
(SAW).
The design of welded connections is discussed in
Section 8.3.
2.6 FATIGUE STRENGTH AND TOUGHNESS
Fatigue strength is important for cold-formed steel struc-
tural members subjected to vibratory, cyclical, or repeated
loads. The basic fatigue property is the fatigue limit
obtained from the S–N diagram (S being the maximum
stress and N being the number of cycles to failure)
which is established by tests. In general, the fatigue–tensile
strength ratios for steels range from 0.35 to 0.60. This
is for plain specimens; the fatigue strength of actual
members is often governed by details or connections. For
cold-formed steel members, the influence of repeated and
cyclic loading on steel sections and connections has been
studied at the University of New Mexico, the United
States Steel Research Laboratory,
2.11–2.13
the University of
Manitoba,
2.62
and elsewhere.
2.87–2.91
In 2001, the AISI Committee on Specifications developed
the fatigue design provisions on the basis of Klippstein’s
research work (Refs. 2.11, 2.12, 2.83–2.85) as summarized
by LaBoube and Yu in Ref. 2.72 and the AISC Specifica-
tion. These design criteria for cold-formed steel members
and connections subjected to cyclic loading are included in
Chapter G of the North American Specification.
In general, the occurrence of full wind or earthquake
loads is too infrequent to warrant consideration in fatigue
design. Therefore, Section G1 of the North American Spec-
ification states that evaluation of fatigue resistance is not
required for wind load applications in buildings. In addi-
tion, evaluation of fatigue resistance is not required if the
number of cycles of applications of live load is less than
20,000.
When fatigue design is essential, cold-formed steel
members and connections should be checked in accordance
with Chapter G of the North American Specification with
due consideration given to (1) member of cycles of loading,
(2) type of member and connection detail, and (3) stress
range of the connection detail.
2.86
Toughness is the extent to which a steel absorbs energy
without fracture. It is usually expressed as energy absorbed
by a notched specimen in an impact test. Additionally, the
toughness of a smooth specimen under static loads can be
measured by the area under the stress–strain diagram. In
general, there is not a direct relation between the two types
of toughness.
2.7 I NFLUENCE OF COLD WORK ON
MECHANICAL PROPERTIES OF STEEL
The mechanical properties of cold-formed steel sections are
sometimes substantially different from those of the steel
sheet, strip, plate, or bar before forming. This is because
the cold-forming operation increases the yield stress and
tensile strength and at the same time decreases the ductility.
The percentage increase in tensile strength is much smaller
than the increase in yield stress, with a consequent marked
reduction in the spread between yield stress and tensile
strength. Since the material in the corners of a section
is cold worked to a considerably higher degree than the
material in the flat elements, the mechanical properties are
different in various parts of the cross section. Figure 2.3
illustrates the variations of mechanical properties from
those of the parent material at the specific locations in a
channel section and a joist chord after forming tested by
Karren and Winter.
2.14
For this reason, buckling or yielding
always begins in the flat portion due to the lower yield stress
of the material. Any additional load applied to the section
will spread to the corners.
Results of investigations conducted by Winter, Karren,
Chajes, Britvec, and Uribe
2.14–2.17
on the influence of cold
work indicate that the changes of mechanical properties due
to cold work are caused mainly by strain hardening and
strain aging, as illustrated in Fig. 2.4,
2.15
in which curve
A represents the stress–strain curve of the virgin material.
Curve B is due to unloading in the strain-hardening range,
curve C represents immediate reloading, and curve D is
the stress–strain curve of reloading after strain aging. It is
interesting to note that the yield stresses of both curves C
and D are higher than the yield stress of the virgin material
and that the ductilities decrease after strain hardening and
strain aging. In addition to strain hardening and strain aging,
the changes in mechanical properties produced by cold
work are also caused by the direct and inverse B auschinger
effect. The Bauschinger effect refers to the fact that the
longitudinal compression yield stress of the stretched steels
is smaller than the longitudinal tension yield stress, as
shown in Fig. 2.5a.
2.17
The inverse Bauschinger e ffect
produces the reverse situation in the transverse direction,
as shown in Fig. 2.5b.
2.17
The effects of cold work on the mechanical properties
of corners usually depend on (1) the type of steel, (2) the
type of stress (compression or tension), (3) the direction
of stress with respect to the direction of cold work (trans-
verse or longitudinal), (4) the F
u
/F
y
ratio, (5) the inside
radius–thickness ratio (R/t), and (6) the amount of cold
work. In general, the increase of the yield stress is more
pronounced for hot-rolled steel sheets than for cold-reduced
sheets.