The objectives of this project were to develop and validate a finite element (FE) model of the hygroscopic warping of OSB panels and to suggest panel structures that ensure a higher level of stability during the storage, handling and use of panels.
A modification of the methodology developed at Université Laval for solid wood and already applied to MDF panels at Forintek Canada Corp. was used for the determination of diffusion coefficients in OSB. An existing finite element model developed jointly by Forintek Canada Corp. and Université Laval for the evaluation of MDF warping was adapted to the characteristic OSB structure. The finite element model was based on an unsteady-state moisture transfer equation, a mechanical equilibrium equation, and an elastic constitutive law. The experimental inputs were the mechanical properties E1, E2, E3, G12, G13 and G23 all as a function of moisture content, density and strand orientation; the expansion properties b1, b2 and b3 as a function of density and alignment; sorption isotherms and diffusion coefficient generated by producing a total of 50 laboratory OSB panels: 18 one-layer panels without density profile and strands oriented through the entire thickness (6 x 500 kg/m³, 6 x 625 kg/m³, 6 x 800 kg/m³), 24 three-layer panels with density profile (16 panels with density 625 kg/m³, aligned strands in surface, random strands in core, and 8 panels with density 625 kg/m³, different alignment in the two surface layers, random strands in core) and 8 panels with density 625 kg/m³, random strands. The panels had dimensions after trimming of 838 mm x 838 mm x 10.5 mm (33 in x 33 in x 7/16 in). To validate the model, warp was initiated and its dynamics was monitored by submitting 2 panels from each group to an 80% relative humidity.
The results showed that for all one-side sealed panels, the MC-increase in the zones close to the surface at the early exposure stages caused rapidly a convex deformation towards the exposed surface. When MC gradually homogenized across thickness, most of the panels returned close to their original flat shape. For panels with a flat density profile, the higher the average panel density, the higher the level of warp due to the effect of density on the expansion and swelling properties. Panels with oriented strands experienced higher strain differential and therefore developed stronger warp compared to panels with random strands. Panels with a one-layer structure experienced higher warp compared to panels with a three-layer structure. When the sealed surface layer was thicker than the exposed surface layer, or when the alignment in the sealed layer was higher than the alignment of the exposed layer, the panels continued to distort and their warp became negative, instead of stabilizing close to their original flat form.
The agreement between the experimental results and the finite element results confirmed the validity of the proposed model in the conditions and the OSB properties considered in this work. Simulations with the finite element method were performed corresponding to specific industrial applications and allowed the creation of a large database of results, which served for building the software package WarpExpert.