Page 401 - Forensic Structural Engineering Handbook

11.32 MATERIAL-SPECIFIC FORENSIC ANALYSES

The most common type of cracking experienced in structural weldments is hydrogen-
induced underbead or cold cracks. These cracks are a result of residual stresses, the pres-
ence of hydrogen, and a base metal composition that results in a hard heat-affected zone
microstructure. These cracks often form hours, possibly days, after welding and are located
in the base plate heat-affected zone (under the weld bead) such that they cannot be easily
seen. The most commonly employed preventive measure to avoid these cracks is to make
sure the joint is free of moisture and grease and to use adequate preheat prior to welding.
The International Institute of Welding (IIW) 45 has developed a predictive equation that
indicates the susceptibility of a given steel to hydrogen-induced cracking.
Mn Cr+Mo+V Ni+Cu
C =+ + + (11.16)
C
eq
6 5 15
where C is the carbon equivalent and C, Mn, Cr, etc. are the weight percentages of the
eq
alloy elements in the steel.
A steel with a carbon equivalent of 0.45 percent or more is typically susceptible to
hydrogen-induced cracking, and some level of preheat prior to welding will be required.
This typically starts at 150°F (65°C) but may exceed 300°F (150°C) for very thick mem-
bers and steels with carbon equivalents of 0.70 percent or more. The AWS D1.1 code lists
recommended minimum preheats for structural steels. Typically, many fabricators apply
some preheat to all joints to ensure that the joints are at least dry prior to welding. Another
important factor in preventing hydrogen-induced cracking is proper care of welding elec-
trodes and other consumables, that is, using low-hydrogen electrodes and fluxes and mak-
ing sure the manufacturers’ recommendations about keeping them moisture-free to ensure
their low-hydrogen characteristics are followed. Another possible cause of fatigue and brit-
tle fracture in weldments is lack of fusion in the joint. This usually results from either poor
welding procedures or poor execution, and typically it leaves a crack-like unfused plane in the
joint. Poor weld joint design can also lead to a crack-like defect. Experience indicates that one
of the common causes of lack of fusion (and other defects that can lead to fractures) is the use
of backing bars to form the root of the weld bead in joints welded from one side, especially if
they are not subsequently removed and the root repaired. Backing bars left in place are a com-
mon initiator of both fatigue and brittle fracture because they contain both lack-of-fusion dis-
continuities and cracks. Weld designs using backup bars are permitted in many welding codes
but are much harder to execute successfully than most weld designers realize.
Welded design can create potential causes of fatigue and fracture even if the joint is
“defect-free” because, unless ground smooth, they are a geometric discontinuity that pro-
mote the growth of fatigue cracks and sometimes subsequent brittle fracture. Design codes,
especially the AASHTO Bridge Design Code and BS7608, include provisions for assign-
ing specific fatigue lives for various types of welded details. These life expectancy estima-
tions include the effects of both the gross detail geometry and the effect of the unavoidable
residual-stresses and microscopic weld toe discontinuities that are usually present. It is sur-
prising that there are large segments of the structural community who design and operate
live loaded structures, but are unaware of the provisions, and more important, the engi-
neering data that form the basis for these codes. Experience indicates that one of the more
common causes of the fatigue failure of bridges, transportation vehicles, power transmis-
sion equipment, and many other structures subjected to cyclical loading is inadequate
design of welded details, not inadequate welding. Mechanical connections may be a cause
of structural failure, but brittle fractures from this source are rare. Bolted or riveted joint
failures are usually associated with inadequate design, but bolt failures due to fatigue crack-
ing at thread roots or bolt holes does occur in long-life service. Unless the materials joined
are relatively low in toughness, or no inspection is made of the structure in service, these
cracks can usually be identified before significant fractures occur. High-strength bolts,