The reduction and the scattering of fatigue strength of the butt welded joint with defects consisting mostly of blow holes were clarified through fatigue tests on 60 specimens for each of the three kinds of steels: JIS SM50Y, SM58Q, and H. T. 80. The specimens were prepared by machining from a full size butt welded plate so as to include weld defects intentionally in the groove.
The quality of butt weld, or the defect severity, was evaluated on the basis of the ratio of the total area of weld defects on the fractured surface to the cross sectional area.
The results showed that the fatigue strength decreased remarkably when the defects were included and the severity was small, and the rate of decrease was accelerated for the joints of higher strength steel. The fatigue strength of butt joints containing surface weld defects was 20% lower than that of the joints with inner defects. The coefficient of variation of fatigue strength varied from 10 to 13% among three kinds of butt welded joints with the defect severity of 1% or less.