Page 379 - Forensic Structural Engineering Handbook

11.10 MATERIAL-SPECIFIC FORENSIC ANALYSES

FIGURE 11.4 Scanning electron micrograph showing cleavage on the surface of a brittle, low-
energy fracture.

fracture toughness at low temperatures and/or high loading rates is directly related to the
influence of these parameters on the yield strength. Low temperatures and high loading
rates tend to increase steel’s yield strength. Higher yield strength levels, in turn, generally
result in smaller inelastic damage zones at the notch tip and, therefore, apparent lower frac-
ture toughness and brittle behavior. Fortunately, many currently produced steels—by virtue
of improved manufacturing techniques (e.g., grain refinement)—do not exhibit such notch
sensitivity at low temperatures and/or high loading rates. Moreover, many brittle fractures
are influenced by the presence of weld related residual stresses. As such, a root cause
assessment of a brittle failure must account for the potential existence of residual stresses
in the crack driving force assessment.

Buckling/Instability
Buckling/instability of steel structures is the only failure mode that is dependent primarily
on the structure’s geometry and only secondarily on the properties of the steel. That is,
buckling is essentially an instability phenomenon rather than the actual separation or frac-
ture of material. This may be the result of inadequate bracing and lateral support, higher-
than-anticipated loading, or other factors that were not anticipated in the design. Moreover,
it is the only failure mode that occurs principally under local compressive loading, whereas
most other failure modes occur largely under tensile loading. In general, column buckling
9
behavior can be predicted by any number of Euler-like buckling relationships, such as the
following tangent modulus equation: 10
P cr = F = π 2 EI e (11.1)
A cr () 2
LA
where EI is the effective bending rigidity of the column.
e