Undercuts are typical weld defects in steel welded structures, and they are considered to decrease fatigue strength. In this study, shape and size of undercuts including in cruciform welded joints are measured with the laser displacement sensor, and an influence of the undercut on fatigue strength is investigated through fatigue tests on cruciform welded joints with various shape and size of undercuts and without undercuts. The influence of undercut shape and size on stress concentration is also discussed through three dimensional finite element stress analyses on joint models with actual shape and size of undercuts.
