Thermographic imaging is increasing being utilized for inspection and characterization of materials and structures. Thermal diffusivity imaging provides a means for quantiative characterization of a material which is independent of apparatus and measurement configuration. This enables more precise imaging of variations in the materials or structure needed to track changes resulting from fatigue or aging processes. To reduce the thermal images to a diffisivity image, a nonlinear least squares fitting routine is used to minimized the difference between measured time records to temperature and an analytical solution for the thermal response of the material to a thermal heat flux. This algorithm is applied point to time sequences of scanned IR imges. An advantage of this technique is that the entire time record is used, from the initial heating impulse until thermal equilibrium is reached, rather than just one or two points of the thermal response. To reduce the processing time the analytical solution is divided into two parts, one for the early time response and one for the late time response. Critical to accurate measurement of the thermal diffusivity, is the time intervals between images. A criteria is presented for establishing the optimum time interval, based on an initial estimate of thermal diffusivity and the thickness of the part. In addition, a relationship between signal to noise in the thermal images and variance in the diffusivity images is delineated. A method is presented for reducing thermographic data to diffusivity images using a nonlinear least squares routine. A primary focus of the method is an analytical representation of the thermal response of a single layer in a form so it is computationally efficient to calculate. Since this is a measurement of a physical parameter of the specimen, it is advantageous for measurements of changes in a specimen over long periods of time. An analytical expression for variance in the calculated diffusivity is given and shown to be in good agreement with computational experiments when the number of discretized points is greater than 8.


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    Title :

    Thermal diffusivity imaging of aerospace materials and structures


    Additional title:

    Abbildung der Temperaturleitfähigkeit von Luftfahrtwerkstoffen und -bauteilen


    Contributors:
    Winfree, W.P. (author) / Heath, D.M. (author)


    Publication date :

    1998


    Size :

    9 Seiten, 5 Bilder, 16 Quellen





    Type of media :

    Conference paper


    Type of material :

    Print


    Language :

    English




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