Longitudinal eigenfrequency and dynamic modulus of elasticity (MOE) responses to thermal treatment intensity in beech and spruce

Thermal modification improves the dimensional stability and biological durability of wood, but it also alters mechanical properties depending on treatment intensity. This study investigated whether longitudinal eigenfrequency ( E f ) and dynamic modulus of elasticity (MOE dyn ) measurements can be used as a rapid, non-destructive method to assess treatment-induced stiffness changes of thermally modified wood by evaluating the relationship with the process-induced mass loss as an indicator of treatment intensity. While previous studies primarily focused on MOE dyn, this study additionally evaluated longitudinal eigenfrequency as a directly measured parameter that does not require density determination. For this purpose, Fagus sylvatica and Picea abies specimens were thermally modified at 160 °C, 175 °C, 190 °C, 205 °C, and 220 °C for 3 h under constant steam supply, with acoustic measurements performed before and after treatment. Longitudinal eigenfrequency was determined by impulse excitation and fast Fourier transform analysis, and MOE dyn was calculated from longitudinal eigenfrequency, specimen dimensions, and oven-dry density. Eigenfrequency showed a clear temperature-dependent response to thermal modification, with generally decreasing values at higher treatment intensities. The relationship between eigenfrequency change and mass loss displayed a stronger relationship in spruce ( R 2 = 0.76) than in beech ( R 2 = 0.36). Variability was greater at higher treatment temperatures, which may partly reflect the microscopic defects within the wood structure and their irregular distribution across specimens. MOE dyn showed a slight increase at mild treatment intensities but decreased progressively at higher temperatures. Under the investigated conditions, MOE dyn reflected treatment-induced stiffness changes more consistently than eigenfrequency alone. However, MOE dyn is calculated from density and is therefore not fully independent as a predictor of mass loss. Eigenfrequency alone offers a simpler parameter for rapid screening, and its relationship with mass loss was promising in spruce. In beech, however, this correlation was low, indicating species-specific variability and highlighting the need for further validation.