Moisture-dependent dynamic elasticity of wood: A non-destructive evaluation under varying relative humidity conditions

This study investigates the influence of moisture content (MC), controlled via oven-drying and relative humidity (RH) at 45%, 65%, and 85%, on the dynamic elastic properties of poplar, fir, spruce, and pine wood. Ultrasonic non-destructive testing (NDT) was used to measure the sound velocity and calculate the dynamic modulus of elasticity (MOE) in the longitudinal (L), radial (R), and tangential (T) directions of the samples. The findings revealed that the highest sound velocity in the L direction was observed in fir (5018 m/s under dry conditions), followed by poplar (4935 m/s), spruce (4912 m/s), and pine (4637 m/s). Correspondingly, the dynamic MOE in the L direction reached up to 12412 MPa for spruce and 10751 MPa for pine. As RH increased, all species showed declines in sound velocity and elasticity, with the radial and tangential directions being more sensitive to moisture variation. Spruce demonstrated the highest reduction rate in elasticity with increasing RH, particularly in the radial and tangential directions, while fir showed relatively stable mechanical behavior. These results underscore the species-specific and anisotropic response of wood to moisture and confirm the effectiveness of ultrasonic NDT for characterizing hygro-mechanical performance. The outcomes contribute to enhanced material selection and their long-term durability assessments for wood applications exposed to variable environmental conditions.