Finite element simulation of the hygro-thermal response of wood during surface densification

The high temperature process of wood surface densification increases the wood density in the transverse direction under compression. This process could be of great interest for industry and represents the topic of recent research efforts addressed both to experimental studies and to investigate models and numerical methods of simulation. In this work a numerical analysis of the hygro-thermal behavior of wood under surface densification is proposed by using a three-dimensional model based on earlier literature approaches that consider the conservation equations for energy, air and water (bound water and vapor) mass. Phenomena of moisture transport include water vapor convection and diffusion and bound water diffusion in the solid wood, while free water is neglected for moisture contents below the fiber saturation point. Local thermodynamic equilibrium is assumed, with a sorption isotherm that couples the moisture contents in the solid and the vapor phase. To perform the simulations the model is implemented in the FEM based code Abaqus as user defined subroutine UEL.