Nondestructive testing of the decay condition of ancient building roof boarding based on infrared thermal imaging

  Objective  The roof boarding is an important part of the roof of ancient wooden structure building. Due to rain leakage, water seepage and other reasons, the hidden area where the roof boarding contacts the back of the tarpaulin is prone to decay and damage. Based on the principle of infrared thermal imaging, this paper carries out theoretical and experimental research on decayed roof boarding detection, which lays a preliminary foundation for the formation of nondestructive detection and evaluation technology for decayed ancient building roof boarding based on infrared thermal imaging.

  Method  Firstly, the heat transfer process of ancient building roof boarding was analyzed, and the calculation formula of average temperature difference between the internal surfaces of roof boarding in decayed area and non-decayed area was deduced; then, four kinds of decay grade of roof boarding specimens were selected, experimental devices were made, the reference method was used to measure the emissivity of specimens with infrared thermal image, and the detection experiment of the temperature change of internal surface of four decay grade roof boarding specimens was conducted.

  Result  The theoretical research results showed that the best detection time of infrared detection of roof boarding decay condition could be calculated based on the knowledge of wood density and specific heat capacity. The experimental results showed that at the same time, the higher the decay level of the roof boarding was, the higher the temperature of its internal surface was and the darker the color of the infrared image was; the internal surface temperature difference between the decayed roof boarding and the normal roof boarding will gradually increase with the start of heating. But it will decrease with the further increase of heating time after reaching a certain extent. The best infrared detection time of measured decay roof boarding in the experiment was in good agreement with the best detection time calculated theoretically. The best detection time for the decay roof boarding was not affected by the temperature difference between the two sides of roof boarding and the ambient temperature, which was generally 20−30 min. The greater the temperature difference between the two sides of the roof boarding was, the greater the temperature difference between the internal surfaces of the decayed roof boarding and the normal roof boarding was, and the better the infrared detection effect was.

  Conclusion  The theoretical and experimental results show that it is feasible to detect the decay condition of ancient building roof boarding based on infrared thermal imaging.