Non-Destructive methodology for the evaluation of residual stresses by using Active Infrared Thermography measurements

In this work, a Non-Destructive approach based on an Active Infrared Thermography model is proposed for the evaluation of residual stresses produced by surface treatments. In particular, the thermal diffusivity was chosen as a Non-Destructive thermal index of the investigated surface treatments, and it was carefully evaluated by using Lock-In Thermography measurements. The Active Infrared Thermography model was developed using a similarity to coatings concept that exploits the idea of equivalent thermal diffusivity. In addition, the most relevant factors involved during the Non-Destructive analyses were taken into consideration by means of an Analysis of Variance. The experimental activities were conducted following a complete Design Of Experiment approach composed of both Taguchi Design and Response Surface Methodologies, allowing for the laser parameters calibration with the aim of identifying the interaction between microstructures, residual stresses and thermal diffusivity. This approach resulted in a novel Non-Destructive methodology for the evaluation of residual stresses based on Lock-In Thermography and hardness measurement. The obtained results were compared with the measurements provided by using the X-Ray diffraction meter, allowing errors on the residual stresses evaluation less than 5 %. The validity of the proposed Non-Destructive approach was presented for cylindrical samples and gears with different geometries. Moreover, different surface treatments were also investigated in this work, the induction hardening and the carburizing surface treatments respectively.