Modeling of the hygromechanical warping of medium density fiberboard

The general objective of this study was to develop and validate a finite element model for the simulation of the hygromechanical warping of MDF panels.; The finite element model was based on an unsteady-state moisture transfer equation, a mechanical equilibrium equation, and an elastic constitutive law. The study was organized into two steps. The first step was to determine experimentally the parameters required for utilization of the finite element model: sorption isotherms, diffusion coefficients, moduli of elasticity, shear moduli, Poisson's ratios, linear expansion coefficients and thickness swelling coefficients. The relation of all parameters with density was determined from MDF panels without density profile with average density levels of 540 kg/m3, 650 kg/m3 and 800 kg/m3. The sorption isotherms were determined from specimens conditioned to relative humidities of 50%, 65% and 80%. Model validation was done during the second step. To validate the model, the dynamics of warping was measured for 560 mm x 560 mm x 12 mm MDF panels with various density profiles, in the case of moisture adsorption from one face of the panels. The experimental warping results were compared to simulation results obtained by the finite element method.; The results from the study showed, that equilibrium moisture content was related to density and presented a maximum hysteresis at a moisture content of about 2%. When specimen density increased the effective water conductivity decreased. The effective water conductivity decreased with an increase in moisture content in adsorption, while it increased in desorption. Coefficients of diffusion showed similar trends. Panel moduli of elasticity and shear moduli increased with panel density and decreased when panel moisture content increased. There was a correlation between density and linear expansion, linear expansion coefficient, thickness shrinkage coefficient, linear contraction and linear contraction coefficient. The linear contraction coefficient was higher than linear expansion coefficient.; The maximum level of warp was higher for panels with higher average density and/or sharper density profile. When exposed to fluctuations in the ambient relative humidity, panel curvature changed from convex to concave due to the hysteresis in the expansion coefficient in adsorption and desorption. Similarly, a permanent concave warp developed when the panel vertical density profile was skewed towards the sealed surface.; The agreement between the experimental results and the finite element results confirmed the validity of the proposed model in the conditions and for the MDF properties considered in this work.