Fatigue and Fracture Research Group
 

Fatigue Behavior of Welded Connections Enhanced with UIT and Bolting


Funded by the Kansas Department of Transportation (KDOT)
 

Steel bridges commonly contain structural elements that are prone to damage caused by repeated loading from vehicular traffic.  Fatigue damage due to these cyclic loads tends to accumulate over time, with the primary adverse consequence being the formation of cracks in the steel structure.  Cracking presents a real problem to Kansas State Bridge Engineers, as they are responsible for finding, diagnosing, and repairing fatigue cracks.  Because of the great effort and cost involved in repairing cracked structures, KDOT is interested in identifying and implementing retrofit methods aimed at preventing fatigue damage from ever occurring. 

Four of the steel specimens used to evaluate the effectiveness of UIT and bolting as fatigue treatments

One retrofit method that has shown great potential for preventing fatigue cracks from forming in welded connections on steel bridges is called Ultrasonic Impact Treatment (UIT).  UIT is performed with a hand-held device that has needle tips that extend and retract at very high frequencies.  When the tip of the tool is applied to a welded surface, it strikes the weld very rapidly, and smoothes the weld geometry while also creating an indented surface.  One effect of the weld being permanently deformed is the formation of locked-in beneficial compressive stresses in the fatigue-vulnerable weld.

This study examined the effectiveness of Ultrasonic Impact Treatment on steel specimens that consisted of a cover plate welded to a larger plate representative of a bridge girder's flange.  This type of connection detail is common in aging steel bridges, and is a significant source of fatigue cracking.  Specimens were cyclically tested in bending until cracking occurred or the test reached a predetermined number of cycles.  Results of the research project showed that UIT was extremely effective in reducing fatigue cracking under high cyclic stresses.

Use of structural bolts as a mechanism to apply large compressive forces in the region of the weld was also investigated.  Bolts were tightened to prescribed tension forces, with the intention that the corresponding compression in the plates being squeezed would result in greater fatigue life in the weld.  It was found that addition of the bolts had no positive effect on fatigue performance, and in some cases, actually had a detrimental effect.   Combining UIT and bolts saw similar results to the UIT-only case, or slightly worse fatigue performance in some cases.