Stress release during drying Evidence that release of internal stress contributes to drying strains of wood

Wood shrinks during drying with the departure of bond water. Along the fibre direction, the magnitude of this shrinkage is mainly governed by the orientation of cellulose microfibrils (MF) in the cell wall. However, tension wood has an unexpectedly high longitudinal shrinkage considering the fact that MFs are oriented nearly parallel to the cell direction. This effect is thought to be due to the gel collapse of G-layer, however, some species producing a tension wood without G-layer also exhibit a higher longitudinal shrinkage than normal wood. The aim of this study is to analyse the contribution of maturation stresses to drying shrinkage. Longitudinal and tangential drying shrinkage of tension wood and normal wood were measured on two sets of matched chestnut wood samples. The first set was directly ovendried, whereas on the second set a hygrothermal treatment released the maturation stress before oven-drying. The analysis of the strains during each step of the procedure reveals that part of the drying shrinkage is due to the release of internal stresses during the desorption process. Finally, a tentative schematic model is proposed taking into account the cumulative contributions to longitudinal drying shrinkage.