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Wood shrinkage response to tensile stresses in convective drying Lazarescu, Ciprian


This study intends to investigate the effect of tensile stresses, moisture content and air engineering parameters on shrinkage behavior of short thin and thick western hemlock specimens. The main goal is to increase the quality of dried wood products. The experimental design was structured on two levels: study the effect of tensile stresses on artificially restrained small wood strips and correlate these experiments with drying tests made on short pieces of lumber. Four matched wood strips specimens were subjected to different restraints during a drying process: zero restraint (free shrinkage), two static restrains of 3daN and 6daN, respectively and a dynamic restraint controlled by the drying process. The resulting shrinkage was dynamically measured by pairs of resistive transducers located on the middle part of each specimen. The same type of transducers was positioned around short pieces of lumber dried to similar drying conditions. The results allowed correlating the amount and rate of moisture loss depending on air parameters, determining the fiber saturation point, and studying the elastic and visco-elastic components of the restrained shrinkage process. Interconnected variables like temperature and moisture content were also shown to significantly impact the desorption process. The derived mechano-sorption analytical fit yielded high coefficients of determination (R²=0.83-0.85, p<0.05) for both structural directions tangential and radial, respectively. The results demonstrated that a temperature as high as 80ºC combined with a low humidity could reduce the tensile stresses generated in the early stages of the drying process. Overall the tangential stresses in quarter-sawn specimens are reduced to half compared with flat-sawn specimens.

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