The welding defects in friction stir welds (FSW) such as kissing bond and wormhole were inspected by non-destructive techniques based on X-ray inspection and ultrasonic testing with phased-array probe. Although the X-ray inspection could detect wormhole defects, kissing bonds were recognized only by the ultrasonic testing. The sizes of kissing bonds were estimated based on the maximum echo height of the ultrasonic testing, while the estimated sizes were smaller than the actual ones observed on the fatigue fracture surfaces. Fatigue tests were performed using welds with defects, revealing that the fatigue strengths were significantly reduced due to the early fatigue crack initiation from defects. The prediction of fatigue life of weld was conducted based on the Paris law of fatigue crack propagation rate. The predicted lives of the welds with defects correlated with the actual ones. The ultrasonic testing led to the linear relationship between the estimated and actual sizes of kissing bond, thus it would be possible to predict the fatigue life of the weld with defects by non-destructive inspection.
