Application of high-frequency mechanical impact treatment for fatigue strength improvement of new and existing bridges

Doktorsavhandling, 2023

Fatigue is known to be a progressive localized permanent damage which occurs in metallic structures that are subjected to repetitive loading. This type of damage occurs even when the components are subjected to stresses well below those causing failure in a single load application. Therefore, the fatigue process ultimately causes cracking which leads to structural failure. This makes the phenomenon of fatigue one of the major threats to the durability and safety of metallic structures. In steel bridges, weldments are essential for joining structural members, but they are usually fatigue-prone links. For that reason, providing a solution to this problem would extend the life expectancy of existing bridges, and make the design of new bridges more efficient.

High-Frequency Mechanical Impact (HFMI) is a relatively new post-weld treatment method that aims to increase weldments' fatigue strength by inducing beneficial compressive residual stress at the weld toe. However, this method has not yet been implemented in the bridge industry on a large scale. This is attributed to several reasons. First, the existing bridges may have accumulated certain damage which makes the efficiency of the treatment in fatigue life extension very ambiguous. Besides, corrosion may also pose a threat to the fatigue strength of HFMI-treated weldments. In other words, the effect of existing damage and corrosion on the fatigue life should be investigated. Moreover, the stability of the beneficial compressive residual stresses in treated weldments needs to be ensured throughout the design life of the bridge. This includes loading conditions that cause relaxation of these stresses such as loading cycles with high-stress ratios or overloads. These need to be incorporated into the design process in a simplified way.

This work focuses on providing rules, recommendations and guidelines that would enable a safe and efficient application of HFMI treatment on existing and new bridges. To be more specific, this thesis is divided into two parts. The first focuses on HFMI treatment in existing structures, which is studied experimentally and numerically. It is found that the fatigue design life assigned for HFMI treatment can be claimed regardless of the number of applied loading cycles before the application of the treatment. However, the weldments should be free of cracks deeper than 2.25 mm, or preferably crack-free. If a crack exists at the surface, HFMI treatment should be preceded by TIG remelting to remove the whole crack. Besides, the effect of corrosion on the fatigue performance of HFMI-treated weldments is proven. Nonetheless, the proposed design curves for HFMI-treated details can be used even if the bridge exists in a corrosive environment. To reduce the corrosion effect, paint should be applied, and sharp groove edges shall be removed by light grinding. 

The second part of the thesis focuses on the design rules of new HFMI-treated bridges. This includes the consideration of the mean stress and overload effects. The research is performed through analyses of measured truck/train loads. A design framework is proposed to include the mean stress effect via one variable called λHFMI. The method is characterized by its simplicity which makes the easy-to-use by bridge designers. Besides, statistical evaluations have shown that the characteristic combination of load actions associated with the serviceability limit state is the best-studied option to incorporate the overload effect in road bridges.