In this paper, fatigue tests were carried out under the repeated two-step and random loading condition using the spot welded joints which had the base metal categorized into 590MPa class in order to consider the effect of their loading condition on fatigue characteristics of the joints. As the results of fatigue tests under the repeated two-step loading condition, it was found that the fatigue limit of the spot welded joints was vanished. Therefore, we found that it was necessary to consider

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... effect of the damage caused by the loading below the fatigue limit on the fatigue damage of the spot welded joints. When the random loading fatigue tests were carried out, it was confirmed that evaluation of the cumulative damage value considered with the mean loading effect had validity though the result of them considered without it indicated the un-conservative estimation. Thus, it was necessary to estimate the mean loading effect on the cumulative damage value of the spot welded joints under the random loading condition. Moreover, for considering the local mean stress involved with the deformation of the spot welded joints, it will be achieved to make the precise cumulative damage value evaluation.