Civil Engineering Reference
In-Depth Information
Fracture
energy
Brittle or
cleavage
fracture
Ductile or
shear fracture
Temperature
Strain rate
Triaxial stress
FIGURE 2.2
Fracture toughness transition behavior of typical bridge structural steel.
energy is released (Fisher, 1984; Barsom and Rolfe, 1987). In steel railway bridges,
this fracture can be initiated below the yield stress.
Design- and fabrication-induced cracks, notches, discontinuities, or defects can
create stress concentrations that may initiate brittle fracture in components in ten-
sion.Welding can also create hardened heat-affected zones (HAZ), hydrogen-induced
embrittlement, and high residual tensile stresses near welds. All of these may be of
concern with respect to brittle fracture. Rolled sections might contain rolling inclu-
sions and defects that may also initiate brittle fracture. Other factors that affect brittle
fracture resistance are galvanizing (hot-dip), poor heat treatments, and the presence
of nonmetallic alloy elements. Brittle fracture most often occurs from material effects
in cold service temperatures, high load rates, and/or triaxial stress states (Figure 2.2).
Normal railway bridge strain rate application is relatively slow (in comparison
to, e.g., machinery components or testing machines). Brittle fracture can, however,
be caused by high strain rates associated with large impact forces from live loads.
∗
Triaxial stress distributions and high stress concentrations can be avoided by good
welding and detailing practice. Thick elements are often more susceptible to brittle
fracture due to the triaxial stress state. Normalizing, a supplemental heat treatment,
can be beneficial in improving material toughness through grain size reduction in
thick elements (Brockenbrough, 2006). Adequate material toughness for the coldest
service temperature likely to be experienced by the bridge (generally a few degrees
cooler than the coldest ambient temperature) is critically important.
Temperature changes the ductile to brittle behavior of steel.A notch ductility mea-
sure, the Charpy V-Notch (CVN) test, is used to ensure adequate material toughness
against brittle fracture at intended service temperatures.A fracture control plan (FCP)
should ensure that weld metals have at least the same notch ductility as the specified
base metal and some specifications indicate even greater notch toughness require-
ments for welds in fracture critical members (FCM). CVN testing is performed to
∗
Caused by poor wheel and/or rail conditions or collision.
