Analysis of the hygroexpansion of a lignocellulosic fibrous material by digital correlation of images obtained by X-ray synchrotron microtomography: application to a folding box board.
Résumé
This study provides original experimental data on the microstructural mechanisms of the hygroexpansion of a material made up of lignocellulosic fibres. A paperboard made up of several layers was chosen and subjected to relative humidity variations during X-ray microtomography scanning. The 3D images of the evolving media were analysed using a digital image correlation technique to measure the displacement field within the studied material. This technique allowed the hygroexpansion of the studied material and of each layer of this latter to be analysed in the in-plane and out-of-plane directions. Results show that the hygroexpansion is highly anisotropic. The microstructural hygroexpansive mechanisms for the pore and fibre phases could also be revealed. They have been shown to depend strongly on the fibre content of the fibrous layers. This analysis provides also useful information concerning the size of the Representative Elementary Volume (REV) for the hygroexpansion phenomenon of dense lignocellulosic fibrous networks. In view of the obtained results, the relevancy of common theoretical models used to predict the hygroexpansion of materials such as papers and boards is also discussed.