Development of microscopic thermography for small defects

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A thermographic non-destructive inspection system, which is applicable for the examination of the small defects and damage embedded in composite and laminated materials, has been developed. The infrared thermal image of the surface of test samples is taken and analysed, when the samples are heated and cooled. A conventional scanning-type thermal imager was used to obtain the thermal image with some optional magnification lens including microscope lens. The resolution in defect size was examined both experimentally and numerically by using artificial delaminated samples. It was found that the thermal conductivity of the sample plays an important role for the resolution. The use of an efficient cooling is a key for the improvement of the resolution. A minimum delaminated flaw size in the sub-surface of GFRP and CFRP samples was around 4mm in diameter under air cooling. A smaller flaw as large as 1mm in diameter was able to detect for a copper-polyimide laminated flexible film whose thickness is exceptionaly thin (60 microns in thickness). A new and more effective cooling technique, in which an infrared transparent solid plate is in contact on the heated test sample, has been developed. The thermal image of the sample surface is obtained through the transparent plate as soon as the plate is in contact on the sample in this method. The successful flaw inspection was demonstrated both in GFRP and CFRP samples.