Moisture-induced stresses in cross-laminated wood panels

When cross-laminated wood panels are exposed to moisture variations, the crosswise bonding of the layers may cause problems. Shape distortions may reduce the serviceability. Drying stresses cause cracks on the surface, which act as a potential target for water accumulation and disrupt the natural wood surface. Moisture-induced deformations may lead to problems in areas where two panels are connected, such as gap opening or failure due to compression. It was the objective of the present study to determine and to describe moisture-induced stresses and deformations and to find possibilities to reduce them. The moisture field throughout laminated wood panels, which is substantially characterised by the glue line diffusivity, is an essential basis for the investigation of panel response on moisture impact. Thus, the moisture behaviour of laminated spruce panels was investigated by means of sorption and cup measurements and finite element simulations of the moisture transport. The diffusion coefficient of the glue line was evaluated from the measurements. A high resistance to moisture diffusion of the glue lines was detected. From the results it was concluded that the governing process of moisture transport through wood adhesive joints is water-vapour diffusion. In laboratory tests, the shape stability of cross-laminated spruce panels was investigated by applying a moisture difference on both large panel faces. The internal stress state was studied in experiments where panels were exposed to a single moistening step. The stresses were determined by release of strain and dynamic determination of the modulus of elasticity. Numerical simulations completed these tests. A three-dimensional material model considering elastic deformation, moisture-induced swelling and mechano-sorptive deformation was applied. The results show significant influences of annual ring angle, layer ratio, middle layer material and pre-stresses on both hygroscopic warping and internal stresses. In parameter studies, the influences of input parameters on the results of numerical simulations were studied. The mechano-sorptive effect was found to have a strong influence on the results. Furthermore, stiffness and coefficients of hygroexpansion of the wood and the diffusivity of the glue lines affect the results of the simulations. From the findings of this thesis it was concluded to apply vertical annual rings and to avoid horizontal annual rings in the preparation of lamellas. Pre-stresses in the outer layers may be a valid method to reduce the consequences of moisture variations but production costs and relaxation of the pre-stresses are critical factors. When applying wood composites in the middle layer, then oriented strand board is most preferable. A large moisture diffusivity of the connection system is capable to reduce warping.

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