Abstract
In this study, fatigue tests were carried out under variable stress amplitude in order to consider about effect of them on fatigue properties of the spot welded joints whose base metal was 590MPa class high strength steel. Under the repeated two-step loading conditions, it was clear that cyclic loading below the fatigue limit gave also fatigue damage to the spot welded joints since the fatigue limit of them was vanished. In addition, fatigue tests were carried out under the random loading condition. As the result of evaluation based on the modified Miner's rule, their cumulative fatigue damages had no accordance with the unity. In order to consider about this point, fatigue tests were carried out with the single overload. Then, it was found that absolute mean strain value increased involved with the single overload. This result suggests that residual plastic strain formed by the overload makes the mean stress increase. Therefore, it is anticipated to make a conservative estimation of fatigue life on the spot welded joints considering the effect of mean stress.
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