Simultaneous Temperature and Deformations Measurements Using Long-Wave Infrared Speckle Interferometry: A Novel Hybrid Technique for Industrial Nondestructive Testing

The use of long-wave infrared (LWIR) CO2 lasers and microbolometer arrays in digital holographic and speckle interferometry shows interesting features for full-field displacement/deformation metrology and nondestructive testing. The first one is that the long wavelength yields the measurement of larger displacements together with decreasing the sensitivity to environmental perturbations. Therefore these interferometric methods can be applied more easily in industrial field applications. Another advantage was put forward in the European project FANTOM which is presented in this paper. In addition to specklegrams, the thermal background is captured simultaneously. Therefore in displacement metrology one can correlate uniquely the temperature and displacement information at the same time and in each pixel. We describe the successful achievements of the project. Thorough understanding of optics and speckle interferometry crossed with state-of-art in thermam imaging systems allowed to study this new hybrid technique in deep and tackle some specific constraints for reaching the best performances.
As a result, a mobile instrument has been developed and validated on various industrial structural testing facilities and inspection areas. It has been used in thermomechanical measurements as well as in detection of defects in aeronautical composites. In particular we show that the two parts of the signal (the thermal part on one hand and the interferograms showing the deformation on the other hand) yield complementary information. A very interesting result is that it allows increasing the probability of detecting defects, for some of them are better seen in the thermal signal, whereas others appear only through local surface deformation.