Fatigue enhancement of welded coverplates using carbon-fiber composites

Low strength-to-weight ratios and crack-growth-inhibiting natures of fiber reinforced polymers (FRPs) make them ideal materials to repair and strengthen fatigue-critical details in steel bridges. FRP materials can have distinct strength advantages over steel when loaded in their optimal orientation, and fiber composite materials such as graphite (carbon)-epoxy and Kevlar-epoxy can outperform steel when subjected to uniform tension. External layers of composite material can be attached to a fatigue-critical detail with the goal of providing an alternate load path, and consequently reducing the stress demand. If the reduction in the stress range is significant, it can lead to a substantial increase in the fatigue life of fatigue-prone details in steel bridge members. This technique has been studied in riveted joints with limited success in recent years. However, use of CFRP materials as a fatigue enhancement technique has proven challenging in past investigations due to localized delamination failures experienced at the bond between the steel and composite materials. Recent research at the University of Kansas has investigated the use of CFRP materials to stiffen fatigue details in steel bridge girders. In the study external layers of CFRP material were bonded to a fatigue-critical detail to provide an alternate load path and reduce the stress demand. To prove the viability of this concept as a repair and strengthening technique, a composite overlay, or doubler element, was bonded over a fatigue-critical weld (AASHTO category E') in a steel test specimen with the goal of reducing the peak stress at the weld. The test specimen was subjected to fatigue cycles in bending until the composite delaminated from the steel and/or the steel substrate experienced a fully propagated fatigue crack. This paper describes the development of this composite doubler element.