Fatigue crack detection by the use of ultrasonic echo height change with crack tip opening

There have been reports on fatigue cracking of steel bridges, especially old bridges such as constructed in the 1970s. At that time, less attention of weld quality was paid. Weld defects of the poor quality of welded joints can be crack initiation points. Judgment whether a fatigue cracking occurs or not from a weld defect is very important in terms of appropriate maintenance work under limited time and cost. This study aims at discriminating between fatigue crack and weld defect by ultrasonic nondestructive test. During fatigue crack growth, the opening phenomenon is known. Crack tip displacement increases according to the loading level. That can be observed by the echo height. Cruciform specimens of double bevel welded joints were prepared. Each of the specimens had an incomplete penetration by remaining root face. Tensile force was applied to the specimens. During loading, the ultrasonic instrumentation was set up and echo heights were measured. By the subtraction of the images between loading and non-loading conditions, the tip of a crack was localized successfully.