MATERIALS NDE BY NONLINEAR FILTERING APPLYING HEAT TRANSFER MODELS

In active thermal Non Destructive Evaluation, properties of interest are obtained analysing time evolution of an informative parameter (e.g. excess temperature, absolute or relative temperature contrast, etc.). The fundamental problem to get rid of different kinds of noise is not deeply studied up to now. Popular filtering techniques, both in space and time domain, are discussed. The analysis of the thermal signal through a suitable heat transfer model has been suggested by many authors. This technique has been applied to NDT of layered materials if a simple analytical expression describing the real problem is found. New more powerful digital tools allow to extend this approach to other applications. For instance the extraction of useful information from raw data, in case of non selective maximum of the contrast signal, is taken into account. In this paper the use of non linear filter in time-domain is described to improve the signal to noise ratio of thermal-infrared NDE. The filter is based on the heat transfer model of the test procedure, taking into account actual boundary conditions. The description of the heat transfer allows to separate the useful signal from noise. Therefore the analytical models of classic thermal infrared NDE are reported. The suggested method allows to filter data and, at the same time, estimate useful quantities. The basic procedure is an iterative non-linear best fit of experimental data. Solving the direct problem it is possible to simplify the testing procedure, tuning the filter to the actual experimental conditions. The NDE accuracy is increased taking into account heat losses. Finally it is possible to choose the parameters which have to be estimated. In such a way the same approach can be used for different tests as thickness or absorbed energy or surface heat exchange coefficients measuring. Thermal diffusivity measurements are carried out according with the flash method and processed by the suggested procedure. A filter suitable for thermal infrared NDE on homogeneous porous materials is designed and tested for the moisture mapping. Experimental results are reported and discussed.