Temperature-humidity-time equivalence and relaxation in dynamic viscoelastic response of Chinese fir wood

Design, application and service life of modern engineered wood and its products are closely related to environmental temperature and relative humidity (RH). In this study, the frequency-dependent viscoelastic properties of Chinese fir wood were investigated under different hygrothermal conditions (temperature: 30-80 degrees C, RH: 0-85%), to verify the applicability of humidity-time and temperature-humidity-time equivalence principles to wood viscoelasticity (i.e., the equal effects of elevating temperature, increasing RH, or prolonging testing time on the changes of stiffness and damping). The relaxation of wood cell wall was compared under moisture equilibrium and non-equilibrium states. It was demonstrated that both the humidity-time equivalence and temperature-humidity-time equivalence principles were applicable for describing the evolution of wood stiffness. Master curves constructed by humidity-time equivalence and temperature-time equivalence principles were basically overlapped at a short-time region (frequency >0.01 Hz) when reference condition was 30 degrees C/0% RH. However, the humidity-time equivalence principle failed to predict wood damping properties, regardless of hygrothermal condition. The testing frequency (f(c), assigned to the transition of different relaxation processes) corresponded to the local minimum value of tan delta was ranged from 8 to 30 Hz and moved to the high frequency direction with increasing RH level. The f(c) value at moisture equilibrium state was lower than non-equilibrium state (no matter moisture adsorption or desorption). It was attributed the higher f(c) value at moisture non-equilibrium state to the unstable configuration structure of wood cell wall. These findings not only help understanding the wood-water relations, but also be relevant for the utilization and production processes of engineered wood and its products in the construction and building fields. (C) 2019 Elsevier Ltd. All rights reserved.