F rancesca Curà

Politecnico di Torino, Torino, Italy



Active Thermography (AT), also called stimulated thermography, is a Non Destructive Technique (NDT) widely used to detect defects, concentrated and distributed, and to generally characterize damaging conditions in materials and components.

Unlike Passive Thermography, Active Thermography requires an external excitation source that may be optical (flash lamps, halogen lamps, laser) or inductive (electro-magnetic coils), as well as an infrared thermocamera as response sensor.

Object of this work is the definition of a method based on Active Thermography for detection and measurement of residual stresses on materials and components.

The problem to determine local residual stresses values is well known in industrial environment, where only destructive techniques are available to characterize residual stresses in components. The necessity to establish a nondestructive procedure is a primary issue.

The proposed method aims to relate thermal parameters (as, for example, the thermal diffusivity) of the system to surface residual stresses in the material and to provide a corresponding quantitative local characterisation.

To set up the method and to define the best procedure for measurements, an experimental activity was organized with a particular attention devoted to both specimens and excitation source choice.

Specimens were chosen without and with residual stresses, due to thermal and mechanical surface treatments. In particular, steels gears with and without thermal and mechanical treatment were analysed and stress entity characterised.

Different excitation sources were chosen, as laser source and lamps, and corresponding power and time period for excitation were tested.

The final aim of this activity was to make a cross check between experimental set up parameters and obtained results in terms of residual stresses distribution and entity for each case of study.



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