Hysteretic swelling of wood at cellular scale probed by phase-contrast X-ray tomography.

We investigated the three-dimensional, microscopic, dimensional changes of Picea abies (L. Karst) wood samples due to controlled steps of the ambient relative humidity. The study was performed at the wood cellular scale by high-resolution synchroton radiation phase-contrast X-ray tomographic microscopy (srPCXTM). Tomographic images were taken after the samples achieved moisture equilibrium at five adsorption and four desorption steps. For spruce latewood, swelling and shrinkage are found to be larger, more hysteretic and more homomorphic than for earlywood. Furthermore, while latewood undergoes similar strains in the transverse directions, earlywood radial strains are less than a third of the tangential strains. The less homomorphic and smaller swelling/shrinkage of earlywood in radial direction is found to be caused by the presence of rays.