Page 381 - Forensic Structural Engineering Handbook

11.12 MATERIAL-SPECIFIC FORENSIC ANALYSES

Further, fatigue damage accumulates at regions of locally high stress concentration, which
in steel structures is most often at welds (fillet weld toes, lack-of fusion flaws, etc.) and pen-
etrations such as bolt and rivet holes. In particular, recent reviews 11,12 revealed the vast
majority of fatigue failures to have occurred at welded connections. In fact, none of the
investigated fatigue failures occurred in base metal that did not contain a weld, weld repair,
or significant stress concentration associated with the fatigue crack initiation site.

Corrosion and Stress Corrosion Cracking
Although its significance is often overlooked, the most common failure mechanism in steel
structures is damage due to corrosion (oxidation). Since unprotected steel corrodes when
exposed to oxygen, particularly in moist environments, the opportunities for structural steel
oxidization are endless. General corrosion is characterized by the formation of an oxide
(e.g., rust) and broad thinning over a significant region of the structure in an aggressive
environment. The variation in metal loss, for general corrosion, does not usually differ by
more than a factor of 2 or 3 over the surface area that has sustained the corrosion damage.
Rusting due to weather (atmospheric) exposure is a prime example of general corrosion, as
shown in Fig. 11.6. Localized corrosion encompasses a wide range of attack mechanisms

FIGURE 11.6 Severe generalized corrosion wastage of a structural steel member web.

all of which are limited to relatively small confined sites. Examples of localized corrosion
are shown in Fig. 11.7 and include pitting, crevice corrosion, and stress corrosion cracking.
Generally, it is the localized attack that presents the greatest potential for damage, as well
as being the most difficult for detection/inspection.
Stress corrosion cracking (SCC) is a sub-critical crack growth mechanism (i.e., crack
growth at a crack length that is less than the crack length necessary for unstable fracture to
occur) that occurs in steels under static loading conditions in the presence of an aggressive
environment. Often, SCC occurs without any indication of its presence and it can result in
catastrophic fracture with little or no warning. SCC requires three components to occur: a
susceptible material, an aggressive environment, and tensile stresses. An example of
SCC is the development of cracking and the subsequent fracture of main cable wires in