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

Abstract

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 ΔT 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 ΔT 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 ΔT and Ve for various hole defect sizes. Then, using finite element modeling, the prediction models of ΔT 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 ΔT (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 ΔT 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 ΔT was similar to the experimental ΔT 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.