Fatigue and Fracture Research Group
 

Fatigue Enhancement of Undersized, Drilled Crack-Stop Holes


Funded by the Kansas Department of Transportation (KDOT) and the KU Transportation Institute

Preventing and repairing fatigue cracking in steel bridges is a major problem facing Bridge Engineers at the Kansas Department of Transportation (KDOT).  Repeated loadings due to vehicles driving across bridges can cause fatigue cracks to form in susceptible regions of steel bridge girders.  Because of the potential for damage, KDOT regularly inspects its bridges for cracking.  If a crack is found, a common repair method is to drill a round hole at the end(s) of the crack.  This type of hole is termed a crack-stop hole, and if drilled to a large enough diameter, they are capable of halting growth of the crack.  This repair method is effective, is very inexpensive to implement, and causes little to no interruption to the traveling public. 

 

Cut-away of a bridge girder showing a crack-stop hole in a geometrically constrained region.

The proper size of the crack-stop hole diameter can be easily calculated.  However, elements on bridge girders that are often the most prone to fatigue cracking are often in "congested" areas on the bridge girder.  It can be impossible to drill a crack-stop hole to the proper diameter in these regions on the bridge girder, resulting in undersized crack stop holes.  The repercussion to this shortcoming is that the undersized crack-stop hole will not be effective in halting crack growth; the crack is likely to re-initiate after additional load cycles. 

This study is aimed at developing a technique to improve the performance of undersized crack-stop holes.  A tool is being developed that will repeatedly load and unload a plug that fits tightly inside the undersized crack-stop hole.  The plug will force compressive stresses into the steel bridge girder around the crack-stop hole's circumference.  Since fatigue cracks tend to grow only under tension forces, any locked-in compressive stresses are beneficial to reducing the net tension force experienced by the crack, resulting in longer fatigue life of the repair.

The bench tool being developed is being tested in the laboratory by treating and testing holes in steel specimens.  Results from treated undersized crack stop holes are being compared to results from untreated, undersized crack-stop holes.