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Mechanism of ultra high cycle fatigue crack initiation in nugget zone of AA7075 friction stir welded joint

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Document pages: 10 pages

Abstract: The ultra-high cycle fatigue behavior of nugget zone of AA7075 friction stir welded joint was experimentally studied by ultrasonic fatigue test system (20   kHz) to clarify the crack initiation mechanism. It is found that without the traditional fatigue limit, the fatigue strength of nugget zone will continue to decline even if it exceeds 107 cycles. Fatigue cracks caused by welding defects at the bottom of friction stir welding seam. In addition, a special semicircular region can be formed around the crack initiation point, and its stress intensity factor is approximately equal to the threshold of fatigue crack growth rate. Finally, a simplified model is proposed to estimate the fatigue life by correlating the size of welding defects and applied stress. Prediction resultsts are in good agreement with the experimental results.

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