Objective Fully-concealed wood column is one of the important load-bearing components of ancient wood buildings. Due to its humidity and lack of ventilation, it often has defects such as decay and material deficiency caused by serious decay, and it is easy to lose the original carrying capacity and stability. The structure of fully-concealed wood column in walls makes the existing detection methods limited and easy to cause irreversible damage. Therefore, based on the principle of infrared thermal imaging, this study carried out theoretical and experimental research on the detection of material deficiency of fully-concealed wood column in walls, in order to lay a part of the preliminary foundation for the final establishment of the non-destructive inspection method of fully-concealed wood column defects in walls based on infrared thermal imaging.

Method Firstly, the heat transfer process of fully-concealed wood column in walls of ancient buildings was discussed from the theoretical point of view, and the temperature formula of the wood column area and the pure wall area under steady state was deduced, and the factors affecting the temperature change in the material deficiency defect area of the wood column were analyzed. Then, the test model was established, and the infrared inspection test of fully-concealed wood column in walls was carried out to collect the infrared image. Finally, by analyzing the infrared heat map, we discussed the relationship between the material deficiency defect size of fully-concealed wood column in walls and the temperature distribution of the wall surface.

Result (1) The theoretical research showed that the transverse thermal conductivity of wood was much smaller than that of the wall, and the temperature corresponding to the outer surface of the wood column wall was lower given the same heating temperature on the surface. The detection effect of the fully-concealed wood columns in the detection wall was mainly affected by the defect size, i.e., the defect height and the defect depth. (2) The test results showed that under the condition of steady heat conduction, the external surface temperature of the wall at the wood column was lower than that of the pure wall, i.e, the color of the infrared image at the former was lighter. The surface temperature of the wall became higher and the color of the infrared image became darker. The higher the defect height was, the greater the range of high temperature caused by defects was. The higher the defect depth was, the higher the temperature in the defect area was. (3) In the case of unsteady heat conduction, the temperature difference in the middle line of the wood column showed a strong positive correlation with the height (R² ≥ 0.964) and depth (R² ≥ 0.951) of wood column defects. (4) In the case of small wood column defects, the surface temperature of the wall didn’t change significantly due to the existence of defects.

Conclusion The height and depth of defects of fully-concealed wood column are the main factors affecting the detection effect of defects by infrared thermal imaging. It is possible to use infrared thermal imaging method to screen and evaluate the material deficiency defect of fully-concealed wood column in walls of ancient buildings.