A thermal imaging camera is used to obtain a brightness map of the thermal scene observed in the infrared spectral domain. The thermal imaging camera therefore gives a thermal image of a thermal scene.
The internal camera technology and built-it programmes can make this radiation visible through a reconstituted image. Transcribing the temperature will then produce a thermograph.
Such a test is non-destructive, quick, simple to use and has no contact with the target object (no disturbance).
Testing a part by measuring temperature enables surface thermal effects to be observed. This type of test is used traditionally to check electrical installations and the thermal insulation of buildings.
2. Active infrared thermography
Nevertheless, there is an alternative for NDT applications, where the component being tested does not usually generate heat spontaneously, whereby a well-controlled external thermal stimulation is applied to it to provoke the appearance of a gradient.
This method involves heating the surface of the material inspected in controlled fashion and measuring resulting changes in temperature using an infrared camera. This is called active infrared thermography.
In the case of defective parts, analysing the sequence of images can reveal discontinuities in the propagation of the heat. They can be caused, for example, by delamination, cracking or water infiltration.
A method such as this has the advantage of being non-destructive, contactless, quick, modular and easily customisable according to needs.
We are therefore looking to observe an alteration in the propagation of the heat flux in the part to be tested, which is conveyed by a contrast in the thermographs.
Based on the material to be analysed, its thickness and the nature and size of the defect sought, various excitation sources are used to heat the part to be tested.
Standards currently in force
NF A09-420 Non destructive testing - Infrared thermography - Characterisation of equipment
NF A09-421 Non destructive testing - Infrared thermography - Methods for characterisation of equipment
NF A09-400 Non destructive testing - Infrared thermography - Vocabulary concerning the designation of equipment
Text prepared by COFREND in conjunction with Patrick Bouteille and Bruno Bruez (CETIM).