Page 380 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.11

Irrespective the type of structure, buckling occurs almost exclusively in long slender
members, particularly long columns and plates with large width-to-thickness ratios.
Buckling can occur globally over an entire member such as a column, or locally in a stiffener
or web of a plate girder. In all instances, buckling is generally characterized by a rapid change
in geometry and excessive distortion. Inelastic deformation usually follows the initial elastic
instability.
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It has been shown that the dominant variable affecting the buckling strength of a col-
umn, other than its slenderness and eccentricity/distortion, is the magnitude and distribution
of fabrication-induced residual stresses. The net effect of residual stresses is that they create
a stress distribution, which can be asymmetric, thereby creating eccentric loading in the col-
umn and eventual instability under external compressive loading. Consequently, when eval-
uating buckling type failures, the presence of residual stresses must be considered.

Fatigue

The aforementioned failure modes usually occur under static loading conditions or during
a single load event wherein the applied load increases until it exceeds a critical load. In con-
trast, most structures are subjected to repeating loads of varying magnitude, which are most
often below yield strength and design-stress levels. Such repeated or cyclic loading is
referred to as fatigue and occurs in bridges, buildings, nuclear power plants, aircraft, ships,
railcars, trucks, and medical devices, to name a few.
Fatigue failure is characterized by the initiation and growth of one or more cracks. Cyclic
loading generates microscopic inelastic damage at regions of local stress concentration (i.e.,
fillet weld toe, lack-of-penetration flaw, flame-cut penetration, bolt holes, etc.). If sufficient
inelastic damage accumulates then a small crack develops, which then propagates through the
structure in a direction that is usually perpendicular to the local principal stress. Frequently,
fatigue crack growth will continue until the crack attains a critical size wherein failure of the
member occurs. Fatigue crack initiation and subsequent growth is a function of the applied
cyclic stress range and number of loading cycles. Fortunately, fatigue is a progressive dam-
age mechanism and is often identified before significant structural damage arises.
As shown in Fig. 11.5, fatigue failures are typically characterized by flat fracture sur-
faces, little or no associated macroscopic inelastic deformation, and crack growth bands
(beach marks) on the fracture surface. In addition, fatigue fractures frequently exhibit
markings, referred to as ratchet marks, which can identify the site of crack initiation.

FIGURE 11.5 Fatigue fracture of a bolt (top) and railroad axle (bottom). Both fractures are characterized by
a smooth dull appearance, beach marks (crack arrest lines); and ratchet marks indicating the fracture origins.