Drying Related Strain Development in Restrained Wood

The objective of this study was to investigate elastic, viscoelastic, mechanosorptive, and plastic strains developed by the drying process in perpendicular-to-grain restrained western hemlock specimens. Drying tests were performed on small, clear wood specimens sawn in such a manner that either the tangential or the radial direction was parallel to their length, where stress driven deformation rates were determined using resistive transducers. Different mechanical restraint schemes were designed by using weights for partly and a load cell for totally restrained specimens. The difference from previous research done in this domain was the restraining technique, which allowed stress to be induced by the drying process, thus, wood was forced to lift the weight as it reached the fiber saturation point. Drying rates were determined gravimetrically on free specimens having similar wood structure. Experiments with free and restrained specimens were carried out simultaneously while drying at 40, 60, and 80°C, down to 17, 12, and 6% target moisture contents. Overall, the magnitude of restrained shrinkage was successfully quantified and separated in free, combined viscoelastic and mechanosorptive and plastic strains. In all these preliminary experiments the dimensional change started at high overall moisture contents, temperature and environmental conditions being key factors. The results can be used to assess the stress indirectly by studying the restrained strain together with moisture content value.

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