Swelling interactions of earlywood and latewood across a growth ring : global and local deformations

The sorption and swelling behavior of heterogeneous spruce wood samples is documented with high-resolution phase-contrast X-ray tomography combined with a dynamic vapor sorption analysis. Three samples extracted at the growth ring boundary contain almost equivalent volumes of earlywood and latewood tissues. The anisotropic swelling behavior of cellular wood material with different geometries and porosities is reported. In particular, the swelling anisotropy ratio in heterogeneous tissues is lower than the one seen in homogeneous wood specimens. This means that, due to the restraining effect of latewood on earlywood and vice versa, swelling of combined wood reduces and becomes less anisotropic. Hysteresis is no longer present when the affine swelling/shrinkage strain is considered as a function of moisture content, indicating that the same amount of moisture entering or exiting the cell wall material leads to the same swelling/shrinkage deformation of the cell material. Nevertheless, looking at non-affine deformations, using a modified B-spline algorithm, a local bending with respect to the latewood-earlywood interface is documented. In addition, the role of rays in the cellular structure in restraining the tangential swelling of thin-cell-walled earlywood is revealed. Finally, the swelling behavior of a greenwood sample is studied at cellular scale. The first desorption curve shows shrinkage strains that are not recovered under hygroscopic loading, and a similar, but of lesser magnitude, restraining effect is seen