Fatigue of Aluminum Welds in Canadian Highway Bridges
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Aluminum is the most common metal in the world and its high strength to weight ratio, along with excellent corrosion resistance, can provide efficient solutions for the design and rehabilitation of highway bridge structures. A reduction in a structure’s self-weight, when using aluminum, is advantageous for the rehabilitation of existing structures requiring an increased live load capacity and for rapid bridge replacements whereby larger, lightweight components can be installed with limited disruption to traffic. Aluminum structures and components offer the potential for lower life-cycle costs due to the favourable corrosion resistance, allowing for less maintenance over the life of the structure. One significant disadvantage of aluminum is that it is more susceptible to fatigue damage in relation to steel. Being a newer design material for bridge structures, compared to steel, and due to its limited use in the past, limited fatigue testing has been conducted to date. Bridge design codes and specifications employ different approaches for establishing fatigue design (S-N) curves for aluminum structures. The British and European design standards use a two-slope design curve, with a shallower slope in the high cycle range, implying that fatigue damage accumulates at a different rate at lower stress ranges. The Aluminum Association in the United States uses a more conservative approach, assuming a single-slope design S-N curve, by simply extending the curve past the constant amplitude fatigue limit at the initial slope. Limited testing under variable amplitude loading in the high cycle range has been completed to date, where a second slope could be warranted. A new chapter of the Canadian Highway Bridge Design Code (CSA-S6) on aluminum structures is currently under development. The research presented herein provides recommendations regarding the correction factors required for fatigue design of aluminum. In addition, fatigue testing and fracture mechanics analysis studies are performed to further investigate the use of a two-slope S-N curve for the fatigue design of aluminum highway structures.