Optical fibre sensors for monitoring of welding residual stresses

Transient and residual strains in the heat affected zone (HAZ) of a TIG arc welded aluminium–magnesium alloy test plate have been measured on line using fibre Bragg grating sensors. The optical fibre sensor operates as a reflection filter: when a light beam travelling along the fibre optic reaches the Bragg grating, only a certain wavelength will be reflected. The reflected wavelength will change if any strain is applied to the fibre sensor. An optical fibre sensor was bonded on the bottom surface of the welding plate; during the weld thermal cycle, the strains experienced by the sensor were correlated to those by the plate. Thermal strains were measured on line during the welding procedure. Subtracting from the total strain the contribution of the fibre thermal expansion, it was possible to determine the pattern and rate of both dynamic and residual strains developed in the HAZ of the welded specimens. The final goal is to act actively during welding in order to minimise the residual stresses and distortions.