Theoretical and experimental investigation on hygroscopic warping of wood composite panels.

摘要

This thesis deals with the hygroscopic warping of laminated wood and wood composites. The theoretical analysis includes elastic, inelastic and visco-elastic approaches. Experiments were conducted on narrow cross laminated yellow-poplar beams and on two-ply beams constructed from laminas of medium-density-fiberboard and particleboard. The elastic strains and the swelling stresses that sustain warp under conditions of increasing moisture content were determined experimentally, using a specially designed restraining device.In the case of the yellow-poplar laminates it was found that in the transverse direction (tangential) only about 25 percent of the free hygroscopic expansion is transformed into elastic strain under conditions of complete restraint. In the longitudinal direction, yellow-poplar behaves elastically. These results were used to modelling the inputs into the elastic warping equation (inelastic approach) which greatly improved the accuracy of the theoretical warping predictions.In the case of laminate composites, the elastic equation without modification produced good agreements with measured warp.The visco-elastic approach, although were time consuming produced good agreement in both cases.A detailed discussion of the elastic warping equation for laminated beams is included. For a two-component multi-layer laminate warping is found to be a function of the ratio of the elastic moduli of the two materials, and to be proportional to the difference of the hygroscopic expansion of the two materials.This study will allow the formulation of relative simple guidelines for the mathematical modeling of hygroscopic warping of wood based laminates.