A unified rheological model for wood under hygro-mechanical coupling

Wood is a natural and renewable structural material; however, its hygroscopic nature adversely affects mechanical performance, leading to the degradation of stiffness and strength during service. To accurately predict the long-term behavior of timber structures, the coupled effects of moisture variation and mechanical loading must be taken into account. In the present work, a comprehensive three-dimensional constitutive model is developed to consistently represent the anisotropic elasto–plastic, viscoelastic, and mechano–sorptive behaviors of wood. The proposed model effectively addresses the limitations of conventional formulations by incorporating appropriately selected viscoelastic components, thereby overcoming the underestimation of creep development at high stress levels, and the failure to predict creep evolution under ultimate loading conditions. Moreover, the nonlinear characteristics of mechano–sorptive creep induced by cyclic moisture variations are accurately reproduced. The model is implemented within a finite element framework and subsequently validated against benchmark simulations involving coupled load–moisture effects in the radial, tangential, and longitudinal directions. It exhibits good agreement with experimental observations in predicting the creep and mechano–sorptive creep behaviors of wood under loading in different directions and varying moisture contents, as well as in estimating its failure strength.