Experimental and numerical determination of the hygroscopic warping of cross-laminated solid wood panels

Abstract

The moisture-induced warping of three-layered crosslaminated solid wood panels made of Norway spruce was studied. The panels were exposed to different climate conditions at 65% and 100% relative humidity at the two panel faces. The results showed increasing cup deformation with an increasing relative thickness of the outer layers. The annual growth ring orientation was found to have a significant influence on the magnitude of the cup deformation. Measurements and numerical simulations of the moisture distribution within the panel were made in order to provide data for numerical simulations of the warping. A distinctive moisture profile with a conspicuous influence of the adhesive bond lines was found. The coefficient of diffusion of the adhesive bond lines was determined from the measurements and simulations. The mechanical material model used for the warping simulations takes into account elastic strain, moisture-induced swelling, and mechano-sorptive strain. The simulations showed good agreement with the warping test results. The most important material parameters for the cup deformation, which were identified in a parametric study of a panel with vertically oriented annual rings, are the moduli of elasticity and the swelling coefficients in the longitudinal and radial direction. Furthermore, the mechano-sorptive coefficient in radial direction was found to have a significant influence on warp.

Moisture-induced stresses and distortions in spruce cross-laminates and composite laminates

“…The shape stability (warping) was determined by inducing a moisture gradient within the panels. The test method is shown in greater detail in Gereke et al (2009a). Two cup deformations were calculated according to Gereke et al (2009a): cup xz and cup yz .…”

“…The test method is shown in greater detail in Gereke et al (2009a). Two cup deformations were calculated according to Gereke et al (2009a): cup xz and cup yz . These terms refer to cupping along the x-axis (parallel to the fiber direction of the outer layers, Figure 2) and the y-axis (perpendicular to the fiber direction of the outer layers).…”

“…A one-dimensional material model based on Fick's law of diffusion -as established in Gereke et al (2009a) -was used to simulate the moisture distribution. The relationship between moisture content v w-x and the diffusion coefficient of the glue lines, D adh wm 2 s -1…”

“…In the present study, these deformations and stresses were investigated by means of experimental tests and numerical simulations of spruce cross-laminates and composite laminates. Unlike previous studies (Gereke et al 2009a), the spruce middle layer was substituted by wood composites. This weakens the middle layer compared to spruce in the longitudinal direction of the solid wood variant.…”

“…This is advantageous for practical applications. Gereke et al (2009a) evaluated the moisture distribution within three-layered cross-laminates made of spruce wood and the hygroscopic warping induced by a moisture gradient was studied. The authors also examined the diffusion coefficient of the polyurethane glue lines and validated a threedimensional mechanical material model against moistureinduced deformations of cross-laminated spruce panels.…”