From view point of obtaining statistical informations on the fatigue strength of welded joints, a series of fatigue tests were carried out under bending stress conditions on the polished smooth and notched base metal specimens and on the butt joint specimens prepared by arc welding with covered electrode. The results were represented as the P-S-N diagrams and discussed by analyzing the distribution of relative strength deviation values from the mean S-N curve of the population.
The distribution in fatigue strength of the smooth base metal specimens was similar to that in Vickers hardness and skewed from the normal distribution, whereas those of the butt welded specimens were almost normal. The variation coefficient in fatigue strength was evaluated under reversed bending the smooth and notched base metal specimens both as about 2.8%, and for the welded specimens 5.0% under reversed bending and 7.3% under repeated bending. The variation in fatigue strength of the welded specimens was attributed in half part to that in the stress concentration due to the reinforcement. The variation introduced during fatigue crack propagation was sufficiently small compared with that for crack initiation in the base metal specimens.
