Nonlinear Material Behavior of the Bridge Components - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 58

2.2.4 Fracture Toughness
Steel members used in bridges must have sufficient fracture toughness to
reduce the probability of brittle failure. The brittle failure may occur suddenly
under a load, which may be below the load level to cause yielding. It may be
initiated by the existence of a small crack or other forms of notch. Very high
concentration of stress occurs at the root of a natural crack. Any sudden change
that occurs in the cross section of a loaded member having a notch-like effect
can disturb the stress pattern and cause a local stress concentration. If the local
yielding at the tip of the crack or notch is insufficient to spread the load over a
large area, a brittle fracture may be initiated. Once initiated, the fracture prop-
agates at high speed driven by the release of the elastic strain energy in the struc-
ture. Linear-elastic fracture mechanics analysis is the basis for predicting brittle
fracture in structural steels. The stress intensity factor ( K I ) can characterize the
crack tip singularity. For a given plate geometry, the stress intensity present at a
crack tip is a function of the crack size and the applied stress. The material frac-
ture resistance is characterized by the critical stress intensity factor ( K Ic )thatcan
be sustained without fracture. When the applied stress intensity K I equals or
exceeds the material fracture resistance K Ic , fracture is predicted. The Charpy
V-notch (CVN) test can be used tomeasure the fracture toughness of structural
steel [ 2.15 ]. A small 10 10 mm bending specimen with a machined notch is
impacted by a hammer, and the energy required to initiate fracture is mea-
sured. CVN test data can be used to predict the K Ic fracture toughness. The
AASHTO [1.23, 1.24] specification classifies structural steel materials into
two categories, which are fracture-critical material and non-fracture-critical
material. Fracture-critical materials' fracture would cause collapse of the struc-
ture. The specification divides the United States into three temperature zones
for specifying fracture toughness of bridge steels. The zones are defined by the
lowest expected service temperature as shown in Table 2.7 . Itshouldbenoted
that the specified toughness requirements are higher with colder zones, thicker
components, higher grades of steel, and fracture-critical components.
In Europe, EC3 (BS EN 1993-2) [1.27] requires that structural steels
used for bridges should have the enough material toughness to prevent brit-
tle fracture within the intended design to prolong the working life of the

Table 2.7 AASHTO Temperature Zones for Specifying CVN Toughness
Lowest Anticipated
Service Temperature Temperature Zone

0 F and above 1

1to 30 F

2

31 to 60 F

3