Performance of high-frequency mechanical impact treatment for bridge application

Licentiate thesis, 2017

This thesis investigates the performance of high-frequency mechanical impact (HFMI) treatment for implementation on new bridge designs. Fatigue strength improvement with HFMI can enable lightweight design of bridges and allow the utilisation of the benefits of high-strength steels. Experimental work on HFMI-treated joints with thick main plates relevant for bridges are, however, scarce and comprehensive studies on the thickness effect few. Studies of various bridge types were performed in this thesis showing that 20% material saving is possible in the load-carrying members through post-weld treatment and the use of high-strength steel (fy > 355 MPa) where necessary. Limitations of the application of HFMI treatment on bridges were also identified regarding the degree of improvement and choice of steel grades. The thickness effect was studied on the basis of an established database of 582 fatigue test results of different types of HFMI-treated joints from 28 studies. It was shown that the thickness effect becomes weaker than what is recommended for as-welded joints. Fatigue experiments were conducted on a typical fatigue-prone detail in bridges with load-carrying plates of 40 and 60 mm which showed a significant fatigue strength improvement as a result of HFMI treatment, exceeding recommended fatigue strengths given by the International Institute of Welding. Based on the fatigue experiments, a weak thickness effect was derived for non-load-carrying transverse attachment joints where the attachment and weld sizes are kept constant.