Influence of re-ultrasonic impact treatment on fatigue behaviors of S690QL welded joints

• Re-UIT increased the fatigue life if interval fatigue cycles were appropriate.
• The contribution of residual stress due to re-UIT to fatigue life was small.
• The folded defects due to UIT were found.
• Re-UIT effect relies on the competition between closing cracks and inducing damage.

The effects of re-ultrasonic impact treatment with different fatigue cycle intervals between two UITs on the fatigue life of S690QL cruciform welded joints were investigated. Fatigue life, residual stress, SEM micrographs of specimen cross profile, and fatigue fracture were determined. Fatigue determination results indicated that re-UIT increased the total fatigue life when the fatigue cycle interval between two UITs was appropriate. The contribution of residual stress produced by re-UIT to fatigue life was not significant at room temperature. The damages caused by UIT were confirmed with optical microscope and scanning electron microscope. Considering that UIT could close cracks, the effect of re-UIT on fatigue life strongly depended on the competition between the role of closing cracks and the formation process of new damage.

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