Dimensional stability of Douglas fir and mixed beech–poplar plywood: experimental measurements and simulations

Abstract

A finite element method (FEM) numerical model predicting the warping of plywood panels from a description of the intraveneer properties in response to moisture content changes is presented and compared with experimental measurements. Thirteen three-ply plywood panels of 40×40 cm² (1.9-mm thickness per ply) were manufactured from Douglas fir or from mixed beech and poplar. Maps of the longitudinal and tangential swelling as well as the wood density were established on the veneers before the glueing process. The shape was measured on the panels conditioned first at 17% moisture content (MC), then at the fiber saturation point, in order to calculate the maximum displacements along six profiles (three longitudinal and three tangential) on each panel. These experimental data were compared with the results of simulations based on two models. First, they were compared to a "heterogeneous" model in which all measured veneer properties were used as inputs. In this case the predicted displacements showed a close correlation (R ²=0.74) to the measured ones, but were remarkably underestimated. Second, they were compared to a "homogeneous" model in which the three layers of each panel were only described by their mean density and average swelling coefficients. For this comparison the prediction of the displacements was much less accurate in the longitudinal direction but was still satisfactory in the tangential direction.