Development of microscopic thermography for small defects

研制了一种适用于复合材料和层压材料中细小缺陷和损伤检测的热成像无损检测系统。对试样加热和冷却后的表面红外热像进行了分析。采用传统的扫描式热像仪，可选配包括显微镜透镜在内的放大透镜，获得热像。采用人工分层试样，对缺陷尺寸的分辨率进行了实验和数值分析。研究发现，样品的热导率对分辨率有重要影响。高效冷却的使用是提高分辨率的关键。在空气冷却条件下，GFRP和CFRP试样亚表面的最小分层缺陷尺寸在直径4mm左右。对于厚度特别薄（厚度为60微米）的铜-聚酰亚胺层压柔性薄膜，能够检测到直径为1毫米的较小缺陷。提出了一种新的更有效的冷却技术，即用红外透明固体板与加热后的试样接触。该方法在透明板与样品接触时，通过透明板获得样品表面的热像。在GFRP和CFRP样品中都证明了缺陷检测的成功。

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.