Fatigue test on Large-scale Welded Girder Specimens with Hot Dip Galvanizing

—Fatigue Strength of Welded Joints by Hot Dip Galvanizing (Report 2)—

Abstract

Galvanizing is one of the effective corrosions protection methods, and many galvanizing steel bridges have been constructed until now. It is also considered that residual stress of welded portion is reduced by the annealing effect due to immersing welded member into hot dip galvanizing tank of about 450°C. The stress concentration at the weld toe as an origin of fatigue crack is reduced if a part of stress is transmitted in the galvanizing layer. Authors already confined facts that tensile residual stress of the welded portion was significantly relieved by immersing in galvanizing tank. It was also recognized by finite element analysis that stress concentration factor at steel was reduced by galvanizing layer. However, the fatigue strength of the galvanized joints became low compared with the as-welded joints. As the reason why the fatigue strength lowers, it was considered that the fatigue crack initiates in an early stage from the discontinuity at the alloy layer or rough boundary of galvanizing layer and steel. This study aims at clarifying fatigue strength of welded joints by galvanizing at Report 1. For this purpose, web-gusset welded joints and scallop details that have especially low fatigue strength in the joint used for steel bridges and are worried about occurrence of fatigue cracks are taken up, and the large-scale girder specimens were made of SM490YB and were galvanized under standard condition of hot dip galvanizing, then fatigue tests were carried out on these specimens. Furthermore, microstructures on the alloy layer between zinc and steel or welds have been observed and fatigue cracks origin has been closely examined.