Study of the effect of hole defects on wood heat transfer based on infrared thermography

From a quantitative perspective using infrared thermography, this study examines the impact of various surface defect sizes on the heat transfer of wood under the condition of wood along-grain excitation. Firstly, the correlation between the defect size and the temperature change Delta T in the thermal steady state phase and the cooling coefficient Ve in the unsteady state phase were built by theory analyzed. And then, a theoretical analysis conducted under wood along-grain excitation conditions examines the theoretical relationship between the diameter D and depth H of the hole defect and the two thermodynamic properties Delta T and Ve. Second, using larch (Larix gmelinii) as the study's object, thermal excitation experiments and natural cooling experiments were conducted on wood specimens in the down-grain direction. The temperature field changes on the specimens' surfaces were recorded using a thermal imaging camera in order to obtain Delta T and Ve for various hole defect sizes. Then, using finite element modeling, the prediction models of Delta T and Ve are built, respectively, for the case of hole defects of various sizes. The model demonstrates that: A very strong negative correlation between the defect diameter D and the thermal steady-state temperature change Delta T (R2 > 0.946) and a very strong positive correlation with the cooling coefficient Ve (R2 > 0.985); The defect depth H has an extremely strong positive correlation with Ve (R2 > 0.946). In addition, as H increases Delta T increases first and then decreases, which is in line with the quadratic curve distribution (R2 > 0.991). Finally, the prediction model was compared with the experimental data for validation. The validation results revealed that the established model's Delta T was similar to the experimental Delta T variation pattern (R2 = 0.864) and that it was able to more accurately predict the pattern and value of Ve (R2 = 0.846, MAPE = 5.44%). The research results could lay some preliminary theoretical foundation for the formation of wood defect nondestructive testing technology based on infrared thermography.