Development of an imaging material testing system based on the photothermal measurement method

Description.: An excitation source (1) emits optical radiation which irradiates the test specimen consisting of a coating (5a), substrate (5b) and defect (5c) over a large area and heats a large surface area (3). The triggered thermal waves (4a) are reflected (4b) at the interface to the substrate and/or at the defect. The thermal radiation (6) emitted by the surface is picked up by e.g. an infrared camera (7). The evaluation of the infrared radiation detected by the camera and the control of the excitation source is performed on a computer (8).

Original German title: „Entwicklung eines bildgebenden Materialprüfsystems auf Basis des photothermischen Messverfahrens“
 

Photothermal measurement is an established method for non-contact measurement of the coating thickness of paints or powder coatings on metal or plastic substrates [1,2]. It is based on the fact that thermal waves are excited by an exciting light source inside a test specimen. On contact with interfaces, these show the same behavior as, for example, acoustic or optical waves, in particular they are reflected [3,4]. The phase angle difference between the excitation signal and the reflected signal can be used to determine the position of the interface. Such an interface does not necessarily have to be between the coating and the substrate, it can also be between two coating layers in a multi-layered coating system or a material and a defect, for example an air inclusion [2,5].

First of all, it is necessary to derive a mathematical model that serves for simulation and includes the physical effects of thermal wave interference. The measurement task can then be formulated as an inverse problem. Subsequently, a stable solution algorithm has to be developed and implemented for this inverse problem. After numerical tests, the validated method will lead to a software that will be integrated into the measurement system. A sketch of the measuring system for a coated substrate with a defect is given by the figure on the right.

 

References

[1] D. P. Almond and P. M. Patel, Photothermal Science and Techniques, 1st ed. (Chapman & Hall, London, 1996).

[2] P. M. Patel, D. P. Almond, and H. Reiter, Thermal-Wave Detection and Characterisation of Sub-Surface Defects, Appl. Phys. B 43, 9 (1987).

[3] W. Macke, Statistik Und Thermodynamik, Vol. 6 (Akad. Verlagsgs. Geest & Portig, Leipzig, 1962).

[4] W. Greiner, Klassische Elektrodynamik, 6th ed. (Verlag Harri Deutsch, Frankfurt a.M., 2002).

[5] F. Lepoutre, D. Fournier, and A. C. Boccara, Nondestructive Control of Weldings Using the Mirage Detection, J. Appl. Phys. 57, 1009 (1985).



Acknowledgements

This research is funded by the European Fund for Regional Development from the Operational Program EFRE Saarland 2014-2020 with the objective "Investments in Growth and Employment" (“Investitionen in Wachstum und Beschäftigung”).