Visible, Infrared and Incoherent Light Transport Optical Techniques for in Situ Material Testing
Three experimental non intrusive techniques are used simultaneously for the metrological monitoring of the deformation of a material in a tensile load machine. The first technique is a video-extensometer (Videotraction®) based on ink spots tracing. The image treatment is fast enough to allow an active control of the machine. It furnishes the true stress-true strain curves for a given specimen according to a desired loading path. An infrared camera is placed on the opposite side of the sample and records some picture of the temperature field evolution during the whole experiment. Thanks to a numerical technique developed for solving the inverse problem of heat source reconstruction, it is possible to get the dissipated power versus strain curve. This brings a new information, of thermodynamic nature, reflecting the thermal activity of the internal mechanisms of deformation. Finally, on the same side of the sample, a third optical technique is used. This technique applies to turbid samples (neither absorbent nor completely transparent to visible light). A laser impact the sample on a very small spot (50 μm). An incoherent steady-light transport (ISLT) occurs within the turbid sample. Thanks to a high resolution camera, the backward scattering image (10 mm) is analysed through a theoretical modelling of incoherent light transport in disordered materials. It allows to follow the concentration of the scattering objects as well as their anisotropy during the experiment thus yielding additional information about what happens in the microstructure of the sample during the test.