Ultrasonic study of the influence of moisture content on the viscoelastic behavior of tropical wood

Wood is a viscoelastic material whose mechanical behavior depends on its moisture content. Several experimental studies using ultrasonic tests have reported an increase in the real part of Young’s modulus beyond the fiber saturation point (FSP). This finding contradicts results from other studies conducted at lower frequencies where Young’s modulus remains constant beyond the FSP. This apparent contradiction motivated the present work, which investigates the relationship between wood viscoelasticity and moisture content. First, an enriched experimental protocol was developed to measure the velocity and damping rate of ultrasonic waves in four tropical wood species, allowing to calculate the evolution of Young’s modulus (the real part of the complex viscoelastic modulus) as a function of moisture content. The results of the experimental measurements confirmed trends and values previously reported in the literature: wave velocity and apparent modulus increase above the FSP, highlighting the influence of water content on the dynamic mechanical response of wood. In a second part, a viscoelastic model was proposed to integrate both the evolution of density as a function of moisture content and the dependence of ultrasonic velocity on internal friction. The model reproduced the experimental increase observed in the real part of Young’s modulus above the FSP. This results highlight the importance of accounting for viscoelastic phenomena in wood beyond the FSP.