Fatigue failure is the culmination of a progressive damage process which occurs in metallic structures or components that are subjected to cyclic loading. The most common example of fatigue failure in everyday life is when a metallic paper clip breaks into two pieces after repeated bending. In this case, the applied stresses exceed the elastic limit of the material, causing significant deformations in the paper clip and resulting in fatigue failure after just a few numbers of load cycles.
In bridges, cyclic loading occurs every time a vehicle passes the bridge. The generated stresses are usually far below the elastic limit of the material, so the deformations are invisible to the naked eye. Still, the fatigue process goes on and can ultimately - after millions of load cycles that occur in the course of many years - result in cracks which cause structural failure. In steel bridges, weldments are essential for the manufacturing and joining of the structural members and components, but welds possess unfavourable properties which makes them the weakest links in the structure in terms of fatigue. Therefore, fatigue of welded joints is a fundamental part in the design of steel bridges and decisive for the material consumption and the choice of steel grades.
High-frequency mechanical impact (HFMI) treatment is a fatigue enhancement method which improves the fatigue properties of welds. The improvement comes mainly from the introduction of compressive residual stresses which enhance the resistance against fatigue cracking and dramatically extends the fatigue life (the endurable number of load cycles). In addition, fatigue improvement with HFMI is steel grade dependent, meaning that the fatigue strength increases with increased steel grade. For steel bridges, this entails a great potential for material saving as the required amount of steel is closely related to the fatigue performance of the weldments. In this thesis, it was shown calculation wise that 17-23 % steel could be saved based on three case-study bridges. Hence, HFMI treatment can allow for lighter steel bridges with substantial cost savings and environmental benefits as a result.