CEMP-E
TI 809-26
1 March 2000
affected base metal. Reinforcement, if appropriate for the application, should be considered in lieu of
repair or replacement.
m. Brittle Fracture. Brittle fracture is a failure that occurs in the steel or weld without appreciable
deformation or energy absorption. Not all fractures are brittle, as the material may have undergone
considerable straining and deformation prior to fracture. Sufficient ductility should be provided in joint
design and detailing, and toughness in materials selection, so that brittle fracture will not occur. Many
joint designs assume the ability to deform and redistribute stress throughout the connection. Standard
design and detailing practices are typically adequate for building structures. Extreme loading conditions,
cold temperature environments, high seismic risk, unusual materials, and fatigue applications may
require more care in the selection and construction of connections and their details. Notches, whether
inadvertent or inherent in the design, greatly increase the risk of brittle fracture. Care should be taken to
avoid transversely loaded sharp notches and joint transitions, particularly in areas such as weld toes.
Backing bars should be removed in some applications because the notch inherent at the root pass
between backing bar and steel may initiate a crack in the weld, HAZ or base metal. Where it is assumed
that plastic behavior will be required to provide ductility and energy absorption, such as seismically-
loaded structures, sufficient length of base material should be provided in the assumed area of plastic
yielding to allow this to occur, and notches that would serve as crack initiators should be avoided in this
area. Notch-tough materials reduce the risk of brittle fracture.
4. DESIGN FOR CYCLICALLY LOADED STRUCTURES (FATIGUE).
a. General. The fatigue strength of a welded component is a combination of a stress range and a
number of cycles (N) that causes failure of the component. The stress range is the total range between
the maximum and minimum applied stresses. Stress range does not require stress reversal, only a
variation in stress. The fatigue life of a component, also called the endurance limit, is the number of
cycles to failure. The fatigue life of a welded joint is affected by the stress range at the location of crack
initiation, and the fatigue strength of the detail, primarily a function of its geometry. In welded joints,
fatigue life is generally not affected by applied stress level or the strength of the material.
(1) Traditional fatigue design is based upon high-cycle fatigue, generally in the range of 20,000
cycles to 100,000 cycles and up. However, low-cycle fatigue may also occur in cases of extreme stress
and strain, such as seismic events or unanticipated out-of-plane bending from applied stresses or
distortion. Applications that may experience low-cycle fatigue require design and detailing specific to the
application that exceed the general fatigue design provisions of the codes.
(2) The S-N curves used for fatigue design provides an assumed relationship between fatigue life
and stress range, and are commonly plotted on a logarithmic scale as a straight line. At the upper left
end of the straight line, at the low endurance limit, the ultimate material strength is exceeded and failure
occurs from static stress. At the lower right end of the curve, the high-endurance range, the stress ranges
are generally too low to initiate crack propagation. The design S-N curves used to design structural
members have been established approximately 25% below the mean failure values. Several design
codes are now replacing the design S-N curves with the equations used to generate the plotted curves.
(3) The fatigue strength of different welded details varies according to the severity of the stress
concentration effect. Those with similar fatigue life characteristics are grouped together into a Stress
Category, identified as Classes A through F, with subcategories for special cases. There are several
details that fall within each class. Each detail has a specific description that defines the geometry. The
details and stress categories are classified by:
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