Moisture-induced stresses in spruce cross-laminates

Eigenstresses occur in cross-laminated solid wood panels exposed to changes of moisture and are caused by hygroscopicity, volume change and orthotropy of the wood. Consequently, the shape distortions that result may reduce the serviceability and drying stresses may also cause cracks on the surface. It is the objective of this present study to determine and to describe moisture-induced stresses and to determine possibilities to reduce them. In laboratory tests, the stress state was studied by exposing panels 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 that the annual ring angle and pre-stresses are significant influential factors. In parameter studies, the influence of the input parameters on the results of numerical simulations was studied. The mechano-sorptive effect was found to have a strong influence on the results. Furthermore, the stiffness and coefficients of hygroexpansion of the wood affect the results of the simulations. It was concluded from the findings of this study that vertical annual rings should be applied and that horizontal annual rings should be avoided in the preparation of lamellas. Compressive 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.