Fatigue Behaviour of Aluminum Friction Stir Welds Under Highway Bridge Conditions

Abstract

Friction stir welding (FSW) is a solid state joining process performed by rotating a cylindrical tool with a short protrusion between the two metal pieces to be joined. The combination of frictional and deformation heating leads to the consolidation of the joint. This welding method is rapidly growing in popularity in many applications, particularly in aluminum alloys for transportation vehicle (rail cars, ships) and bridge applications. Across North America, over 150,000 bridges have been identified as “structurally deficient” or “functionally obsolete”. Since FSW has the potential to have a positive influence on their durability and economics, the Aluminum Association of Canada (AAC) has identified the possibility of replacing promoting aluminum bridge decks as a means of replacing existing deficient concrete decks. However, currently available codes and guidelines for aluminum welded joints only address structures made with conventional welding methods. Therefore, bridge designers are lacking the necessary knowledge to use FSW joints in their designs. The main objective of this thesis is to present a fatigue testing study to support the development of improved “performance-based” code provisions for the quality control and fatigue design of FSW joints by examining the durability of FSW joints with prescribed flaws. In order to obtain the experimental results, various intentionally flawed aluminum FSW samples were fabricated for fatigue testing under constant amplitude (CA) and simulated in-service variable amplitude (VA) loading conditions. A statistical analysis of the results has been performed to assess the influence of the various defect types. It has also been shown how finite element (FE) analysis using the software ABAQUS can be used to assess the influence of the defects on the local stresses within the welded joints. Lastly, it is shown how the fatigue performance of the welds can be predicted using linear elastic fracture mechanics (LEFM). The results of this research will contribute to an improved understanding of the behaviour of imperfect FSW joints under fatigue loading conditions, which simulate in-service vehicular bridge VA loading. The main conclusions of this research include the following: 1) The worst fatigue lives were observed in the specimens with “kissing bond” defects at the weld root (on the order of approximately 1 mm in depth), 2) toe flash, undercut, and worm hole defects, as well as surface improvement by polishing were seen to have a much lower influence on fatigue performance, 3) a novel “lap joint” specimen simulating an extruded bridge deck joint was also observed to fail at the root at a nominal stress level lower than that of a properly-welded butt joint.