Abstract:Thermography was performed on stainless steel 316L and aluminium alloy 7010 samples as they were submitted to a sinusoïdal mechanical stress (traction/compression). For each stress amplitude value the temperature data were recorded in 5 s. A specific signal demodulation procedure was used to extract the first two Fourier components and the mean temperature rise. From their particular dependence on the stress amplitude, characteristic stress values could be derived. In some instances, these values are very close to the fatigue limit of the considered material.

A technique for lock-in thermography that we recently developed was applied for the measurement of the in-plane principal diffusivities of orthotropic materials. The surface is point heated with a modulated laser beam and a 2-D synchronous detection is performed of the elliptical thermal field which develops around it. Diffusivities are inferred from the slope of the phase lag profiles. The approach was first validated with a duralumin (isotropic) sample. It was then applied on a unidirectional C/epoxy sample.The aspect ratio of the surface isotherms is imposed by the anisotropy of the considered material. Therefore, possible in-depth variations of the anisotropy unavoidably modify the aspect ratio distribution at the surface. Based on this observation, an inversion method was recently proposed to monitor the internal fibre orientation changes in composites. Results obtained with a continuous fibres C/epoxy sample and with an injection moulded short fibres sample are reported.

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The amplitude and phase space distributions of EM (electromagnetic) fields (X-and Ku bands) are imaged and measured using microwaves interferometry revealed by photothermal films and lock-in infrared thermography. Such EM fields imaging is a powerful tool for NDE (non Destructive Evaluation) of dielectric and radar absorbing materials. The absorption of an electromagnetic (EM) field E, H, by materials of complex permittivity, E = E" -iE", complex permeability, 11 = 11' -ill", and real part of the conductivity, d, produces heat with a dissipated volume power given by the well known formula: Nomenclature(1) where E and H are the modulus of the electric and magnetic fields. The resulting heating can be detected by IR thermography (technique called EMIR, for ElectroMagnetic InfraRed). It is controlled by the materials thermal properties and by heat transfer to the surroundings. The analysis of the space distribution and time evolution of the temperature increase allows to identify the heat sources, Pabs(x,y,z), which are controlled by the strength, space distribution and orientation of the EM field, and by the EM properties of the material, which acts as an array of photothermal sensors. If these properties of the material are known, identification of these distributed heat sources can finally lead to the knowledge of the intensity distribution of the incident EM field. The identification is optimized when the photothermal transducer is a thin film.Such a technique is a close mixing of electromagnetism and thermics. The use of both thin resistive photothermal films and lock-in IR thermography allows to image, in a quantitative way, the intensity of EM fields, with a dynamic range sufficiently extended for EM purposes. Reference [1] describes the optimisation of these films and the performances of the technique: space and time modulation transfer functions and sensitivity to the electric field. Need of a phase measurementThe main limitation of the technique was essentially due to the fact that the photon-heat conversion only allows to measure field intensity (square of the amplitude). Such a measurement impedes to reconstruct the full field, since the phase remains unknown. It is obvious that an EMIR method allowing phase measurements would be a real progress, especially for nearfield characterization of antennas. It is the reason why phase measurement was needed. Guided by optical techniques which very often use interferometric arrangements, the idea was to produce microwaves interferences for several phase values and to reveal them by photothermal films. By combining several intensity measurements it is possible to deduce both amplitude and relative phase distributions. Here the method will be very shortly presented, since we have recently presented it, with more details, in ref. [2,3].

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