Lock-in thermography for residual stresses investigation in steel welded joints

A non-destructive method for the residual stresses evaluation in welded joints is here proposed. Conventional residual stress determination techniques are based on the destruction of the original material stress state that causes a relaxation, hence is not possible to use those methods on working components. The proposed method provides for an active lock-in infrared thermography analysis by means of a FLIR thermal camera and an exciting laser head for the evaluation of the thermal diffusivity. In general, it is possible to relate the thermal diffusivity to the diffusion length, which is the ability of a thermal wave with a known wavelength to penetrate inside the sample, using the phase contrast trend as a function of the distance from the laser spot. Through the variation in diffusivity is possible to investigate the level of anisotropy in the Heat Affected Zone (HAZ). On the basis of these considerations, tests were run by varying thermal source parameters, as laser power level and number of impulses in the time unit. The post-processing of the experimental data needs to face high noise to signal ratio and phase mapping extraction algorithm, therefore Single Value Decomposition (SVD)-denoising and lock-in amplifier techniques were implemented in a Matlab script. Thus, the diffusivity is expected to be dependent on the local characteristics in the joint and varying through the different directions Experimental results obtained on custom made welded joint samples are presented and discussed