The effect of seismic loading on the fatigue strength of welded joints

Earthquakes sometimes damage steel structures. Structures which are not seriously damaged are still used after earthquakes but their fatigue strength may have been reduced by the large cyclic loadings imposed by the earthquakes. In order to clarify the effect of seismic loading on the fatigue strength of welded joints, high cycle fatigue and variable amplitude fatigue tests after a number of large initial strain cycles were performed. The large strain cycles formed a short crack at the toe of the weld in a low cycle fatigue that triggered a high cycle fatigue strength reduction. The high cycle fatigue limit of welded joints after initial strain cycles is governed by the threshold stress intensity factor of the short crack. The formation of short cracks also enhanced the damage accumulation for subsequent variable amplitude loading. It is important to keep all of the stress variations after earthquake below the fatigue limit of the cracked welded joints to avoid fatigue damage accumulation after an earthquake.