Moisture content, temperature, and grain orientation in timber and their effects on the viscoelastic properties and the characteristics of ultrasonic-guided waves

Timber is an essential natural material used in engineering applications, and its performance should be enhanced to ensure safe and long-term usage. This work studies the effect of moisture content (MC), temperature, and grain orientation on timber’s viscoelastic properties. Accordingly, a non-destructive testing method is developed using a non-contact laser Doppler vibrometer (LDV) on timber structures. A dynamic mechanical analyzer (DMA) is employed to measure timber’s viscoelastic properties with 0% to ~ 22% MCs and over − 20°C to 100°C for longitudinal, tangential, and radial specimens. Additionally, the dominant ultrasonic-guided wave (GW) is explored while studying the propagating modes in 10% to 27% moist timbers. The phase velocities are computed and compared to the analytical curves generated from the mechanical properties extracted from the DMA data. The results showed that the storage modulus, loss modulus, and tan δ decrease with increasing temperature and MC. Longitudinally, the storage modulus reduced by 27.5% at 0% MC and by 57.7% at 18% MC within the studied temperature range. The reduction is larger transversely than longitudinally, and the lowest storage modulus is observed radially. The relaxation peaks shifted to lower temperatures as MC increased. LDV results proved A₀ mode is dominant, and its phase velocities matched analytical results. The change in phase velocity is proportional to MC and temperature, due to the MC effect on timber’s mechanical properties. The presented results showed high promise in terms of characterizing the viscoelastic behavior and the high sensitivity of A₀ mode along various directions of grain growth and MC.