Fatigue Enhancement of Undersized, Drilled Crack-Arrest Holes

Abstract

Fatigue cracks occur in steel bridges from repeated loads. If allowed to continue to grow, eventually the fatigue cracks will require either expensive repairs or reduction of traffic loads on the bridge, or they may lead to the failure of the bridge. The objective of this research was to develop a new, cost-effective technique that can be used to arrest the growth of fatigue cracks before they develop to an extent that more expensive repairs are required. It is well known that drilling a hole (crack-stop hole) at each end of a fatigue crack can arrest the growth of the fatigue crack. This new technique consisted of cold-expanding a crack-arrest hole thereby cold-working the material around the hole and subjecting the cold-worked material to ultrasonic vibration. It was hypothesized that this process would increase fatigue crack initiation life three ways. First, the compressive force used to cold-expand the hole would result in residual compressive stress fields around the hole when the radial force was removed. Second, the cold-expansion would cause strain-hardening and cold-working with a concomitant increase in the yield and ultimate strengths of the steel. Third, the ultrasonic vibration from the PICK treatment would further increase the resistance to fatigue propagation by increasing the yield and ultimate strengths and increasing the radial extent of the residual compressive stress field. These expected results and their effects on fatigue crack initiation were investigated through a proof-of-concept testing program using reduced-scale, laboratory models and by mathematical modeling.