Laser ultrasonics for defect detection and residual stress measurement of friction stir welds

Abstract

The laser-ultrasonic technique is investigated for defect detection and sizing as well as for residual stress measurement in welds obtained by friction stir welding (FSW). When combined with the Fourier domain synthetic aperture focusing technique, 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 when probing with ultrasonic frequencies up to 200 MHz. Residual stresses induced by the FSW process can also be probed by laser ultrasonics. 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.

Keywords:

• friction stir welding
• laser ultrasonics
• synthetic aperture focusing technique
• kissing bond
• residual stress
• surface skimming longitudinal wave

Acknowledgements

The authors would like to thank Martin Lord for their assistance in instrumentation for all aspects of this project, and Pascal Doran for his help in developing the FEM of the FSW process.

Notes

1. Now at Institut Maupertuis, Contour Antoine de St-Exupéry, Campus de Ker Lann, 35170 Bruz, France.

Additional information

Notes on contributors

Laurent Dubourg

1