Marie Hartwig-Nair: Structure and hygroelastic properties of conifer branch wood: A multiscale approach

Abstract

Studies of  structure-property relationship of compression (CW) and opposite wood (OW) formed in conifer branches are rare, mostly due to their lack of application in construction. Instead most branch wood is today being used as fuel. However, utilising branches as material can contribute to a more efficient and sustainable use of forest biomass and reduce the demand of stem wood for engineered wood products. Furthermore, deeper insight in compression and opposite wood might inspire toward new engineering solutions by using principles prevalent in the tree branch.

This thesis investigates hygroelastic properties of compression and opposite wood in branches by modelling and experimental techniques at several hierarchical material levels.

First, mechanical optimisation of tree branches for bending by using compression and opposite wood in a beam model is analysed. One weakness of the analytical model is the lack of elastic properties of compression and opposite wood of branches. Hence, hygroelastic properties for these are determined by mechanical testing and micro-computed tomography.

Following that, swelling behaviour of CW and OW lignin is studied by Molecular Dynamics (MD) simulations and wide-angle X-ray scattering to understand the effect of their chemically distinct structure.

Lastly, a hierarchical multiscale model is established to study the effect of previously determined lignin swelling coefficient, as well as lignin content and microfibril angle on swelling properties of cell walls. Swelling coefficients and elastic properties obtained by MD simulations are used as an input for Finite Element modelling.

The branches composition of compression wood and a opposite wood indicates that it is optimised for bending resistance. The hygroelastic properties of the comprising tissues are obtained. The swelling of CW is much less anisotropic than CW. The structural differences in lignin of compression and opposite wood and their resulting different swelling coefficient do not lead to different swelling of the compression and opposite wood cell walls.

The experimental and modelling approaches in this thesis are not specific to branch wood and can be of interest in wood science in general to gain more insight into the effect of structural changes on moisture-wood interaction and hygroelastic properties.