Page 377 - Forensic Structural Engineering Handbook

11.8 MATERIAL-SPECIFIC FORENSIC ANALYSES

FIGURE 11.1 Ductile deformation and fracture of a bridge wind chord. Note
the spalled paint in the vicinity of the deformation.

precludes the intervention of mitigating measures. Consequently, many brittle fractures
result in significant structural damage, catastrophic failure, and/or serious injury.
As shown in Fig. 11.3, brittle fracture is typically characterized by flat fracture surfaces
with little or no associated inelastic deformation. Additionally, brittle fracture surfaces
exhibit features known as chevron marks, which by virtue of their orientation, can be
used to identify the direction of fracture propagation and, more importantly, the site of
fracture initiation. On a microscopic level, brittle fractures exhibit cleavage facets,
shown in Fig. 11.4, a fracture mechanism that requires little energy to propagate cracks
through the steel microstructure.
Typically, brittle fractures initiate from stress concentrations such as weld flaws in
steels exhibiting low fracture toughness. It is important to note, however, that when local
constraint is severe, brittle-like fractures can occur in steel structures that possess good duc-
tility and toughness. For example, brittle fractures have occurred in ship decks, heavy plate