Defect Detection and Residual Stress Measurement in Friction Stir Welds using Laser Ultrasonics

The laser-ultrasonic technique is proposed for defect detection and residual stress measurement in friction stir welds (FSW). When combined with the synthetic aperture focusing technique (F-SAFT), very good performances are achieved for detecting lack of penetration in butt joints, the detection limit coinciding with the conditions of reduced mechanical properties. Also, the detection of kissing bonds seems to be possible in lap joints with frequencies up to 200 MHz. Another application is the measurement of residual stresses induced by the FSW process. The method is based on monitoring the velocity change of the laser generated surface skimming longitudinal wave, propagating just below the surface and being found much more sensitive to stress. The residual stress profile measured across the weld line is in good agreement with results from a finite element model and from strain gauge measurements.