Emil Engelund Thybring

Linking observed tensile creep and recovery in

Norway spruce with molecular

level mechanisms of deformation

Motivation

Loading a piece of wood

Mechanical properties at the nano-scale Elastic properties of amorphous wood polymers depend on hydrogen bonding Nissan’s theory for hydrogen bonded solid materials

Mechanical properties at the cell wall level Elastic properties based on lamellar composite theory Shearing deformation based on Eyring’s deformation kinetic theory

Experiment

Constant tensile load (creep) for 90 min followed by stress-free relaxation (recovery) for 90 min

Experiment Creep and recovery strain normalized with elastic strain Moisture has an effect on the response No effect of temperature

Creep Recovery

Recovery

Creep

Linearly increasing difference between creep and recovery curves

Computer modelling Full recovery predicted by computer model

Computer model

Physical experiment

Removing linear creep of middle lamella, cell creep is fully reversible as predicted

Effect of water is attempted incorporated by decreasing elastic properties and increased shear rate

Removing linear creep of middle lamella, cell creep is fully reversible as predicted

Effect of water is attempted incorporated by decreasing elastic properties and increased shear rate Not enough to explain effect of water!

Computer modelling

Experiments hint at both reversible (cell) and irreversible (middle lamella) time-dependent deformations The effect of water can be explained by changing mechanical properties alongside a change in microfibril orientation

In conclusion...

ET Engelund, L Salmén (2012) Holzforschung, DOI 10.1515/hf-2011-0172

ET Engelund (2011) Wood-water interactions, PhD thesis, Technical University of Denmark