Fatigue improvement of welded cover-plates using High Frequency Mechanical Impact Treatment

Many existing steel and composite bridges in Sweden need strengthening and upgrading in order to meet demands on higher axel loads. One method that is widely used to increase the bending resistance of beams is to add cover-plates. The solution adopted today is to bolt the plates to the existing structure to avoid welding and the accompanied reduction in fatigue strength brought by this detail which has very low fatigue strength. The use of bolted connections with many holes drilled on site and fitted bolts is very costly. Therefore, a research program has been initiated to study the possibility of using High-Frequency Mechanical Impact (HFMI) treatment to enhance the fatigue strength of the ends of welded cover-plates. This paper reports the results from fatigue tests done on beams with welded cover-plates, with and without HFMI-treatment. The beams and cover plates were made of S355 steel, and all tests were conducted with an R-ratio of 0.1. The results from preliminary numerical analysis to evaluate the risk for root cracking in these details are also reported. The results from testing show that HFMI-treatment can produce an improvement in fatigue strength of welded cover-plates with more than four detail categories. No root cracking was observed in any of the tested beams.