Effect of Thermal Modification on the Correlation Between Ultrasonic Wave Propagation Velocity and Physico-Mechanical Properties of Wood

Thermal modification alters the chemical composition and cell wall structure of wood, particularly through hemicellulose degradation, which may affect physical–mechanical properties and the applicability of nondestructive evaluation (NDE). This study investigated the relationship between ultrasonic wave propagation velocity and the physical–mechanical properties of Eucalyptus sp., Pinus sp., and Cedrela sp. subjected to thermal modification at 140, 180, 220, and 240 °C, in addition to the control treatment. Ultrasonic velocity was measured at 500 and 90 kHz according to ASTM E494-20, while strength and stiffness properties were determined by compression parallel to the grain, tension parallel to the grain, static bending, and Janka hardness tests according to NBR 7190-3. Associations were evaluated using Pearson correlation and Fisher’s test, and Random Forest models assessed predictor importance. Ultrasonic velocity was positively correlated with all properties, with higher correlations for modulus of elasticity (r = 0.92) and apparent density (r = 0.82–0.85) and lower correlations for tensile strength (r = 0.60–0.66). From the control to 240 °C, the correlations decreased most for compressive strength (25%–28%), Janka hardness (22%) and MOR (20%), and least for MOE (6%–7%). Density and genus were the most important predictors. Overall, thermal modification reduced the strength of the correlations, and tensile strength was the property least well predicted (R2 = 0.45). These findings highlight the need to consider treatment severity, genus, density, and frequency when interpreting ultrasonic measurements.