Detection of crack in painted flange gusset welded joint by ultrasonic test

According to the periodical inspection manual for civil infrastructures in Japan, the visual inspection plays a very important role to detect deterioration such as fatigue cracking. Since the steel bridges are covered with paint coating, a fatigue crack is often detected as a crack of the paint. Conventionally, when a paint crack is detected, the paint coating is removed and magnetic particle test is applied. However, it is not always true that there is a fatigue crack under the paint crack. This process is time-consuming, especially when no fatigue crack is detected. The objective of this study is to detect fatigue crack without removing the paint coating by nondestructive technique. Among nondestructive techniques, ultrasonic test was applied, because an internal crack can be detected. This study focused on flange gusset welded joint; lateral connection plates were welded to the base plate from both sides. After being welded, the specimens were painted by either fluorine resin paint or epoxy resin paint. Fluorine and epoxy paints are mainly applied for highway road and railway bridge, respectively. Fatigue tests were carried out for the specimens. During the fatigue test, when strain to monitor crack initiation was changed, ultrasonic test by creeping wave was conducted. The creeping wave is a longitudinal wave that travels very close to the test surface. A fatigue crack of 1.1 by 1.6 mm could be detected by the ultrasonic test.