Page 22 - 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

1.20 CHAPTER ONE

1.12 FATIGUE

A structural member subjected to cyclic loadings may eventually fail through initiation and propa-
gation of cracks. This phenomenon is called fatigue and can occur at stress levels considerably
below the yield stress.
Extensive research programs conducted to determine the fatigue strength of structural members
and connections have provided information on the factors affecting this property. These programs
included studies of large-scale girder specimens with flange-to-web fillet welds, flange cover plates,
stiffeners, and other attachments. The studies showed that the stress range (algebraic difference
between maximum and minimum stress) and notch severity of details are the most important factors.
Yield point of the steel had little effect. The knowledge developed from these programs has been
incorporated into specifications of the American Institute of Steel Construction, American Association
of State Highway and Transportation Officials, and the American Railway Engineering and Maintenance-
of-Way Association, which offer detailed provisions for fatigue design.

1.13 BRITTLE FRACTURE

Under sufficiently adverse combinations of tensile stress, temperature, loading rate, geometric dis-
continuity (notch), and restraint, a steel member may experience a brittle fracture. All these factors
need not be present. In general, a brittle fracture is a failure that occurs by cleavage with little indi-
cation of plastic deformation. In contrast, a ductile fracture occurs mainly by shear, usually pre-
ceded by considerable plastic deformation.
Design against brittle fracture requires selection of the proper grade of steel for the application and
avoiding notchlike defects in both design and fabrication. An awareness of the phenomenon is important
so that steps can be taken to minimize the possibility of this undesirable, usually catastrophic, failure mode.
An empirical approach and an analytical approach directed toward selection and evaluation of
steels to resist brittle fracture are outlined below. These methods are actually complementary and are
frequently used together in evaluating material and fabrication requirements.
Charpy V-Notch Test. Many tests have been developed to rate steels on their relative resistance to
brittle fracture. The most commonly used is the Charpy V-notch test, which specifically evaluates
notch toughness, that is, the resistance to fracture in the presence of a notch. In this test, a small
square bar with a specified-size V-shaped notch at its mid-length (Type A impact-test specimen of
ASTM A370) is simply supported at its ends as a beam and fractured by a blow from a swinging
pendulum. The amount of energy required to fracture the specimen or the appearance of the fracture
surface is determined over a range of temperatures. The appearance of the fracture surface is usually
expressed as the percentage of the surface that appears to have fractured by shear.
A shear fracture is indicated by a dull or fibrous appearance. A shiny or crystalline appearance
is associated with a cleavage fracture.
The data obtained from a Charpy test are used to plot curves, such as those in Fig. 1.10, of energy
or percentage of shear fracture as a function of temperature. The temperature near the bottom of the
energy-temperature curve, at which a selected low value of energy is absorbed, often 15 ft⋅lb, is called
the ductility transition temperature or the 15-ft ⋅ lb transition temperature. The temperature at
which the percentage of shear fracture decreases to 50% is often called the fracture-appearance tran-
sition temperature. These transition temperatures serve as a rating of the resistance of different steels
to brittle fracture. The lower the transition temperature, the greater is the notch toughness.
Of the steels in Table 1.1, A36 steel generally has about the highest transition temperature. Since
this steel has an excellent service record in a variety of structural applications, it appears likely that
any of the structural steels, when designed and fabricated in an appropriate manner, could be used
for similar applications with little likelihood of brittle fracture. Nevertheless, it is important to avoid
unusual temperature, notch, and stress conditions to minimize susceptibility to brittle fracture.
In applications where notch toughness is considered important, the minimum Charpy V-notch value
and test temperature should be specified, because there may be considerable variation in toughness

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