Abstract
"At the microstructural level, wood is an aggregation of cells: high aspect ratio ‘tubes’ around 2 mm in length and 100 µm in width. To produce medium density fibreboard (MDF) these cells are separated through thermo-mechanical means and are subsequently referred to as fibres. During the manufacture of MDF, adhesives are applied to these fibres, in a process called resination.
In the current work, such fibres have been mechanically characterised using a recently developed three-point bend method, which uses the arm of an atomic force microscope to both load the fibres and record the resulting displacements. Pine and beech fibres were characterised. The data was then evaluated using Weibull statistics, an analytical method which allows (a) the assignment of scale and variance factors and (b) can explore subsets present within a dataset (through signal decomposition), provided each subset is sufficiently numerous. Both the pine fibres (mean stiffness: 13.4 GPa, standard deviation: 10 GPa), and the beech fibres (mean stiffness: 4.8 GPa, standard deviation: 3.9 GPa), were found to be bimodal: they each contained two discreet subsets of fibres with scale factors of 5.3 and 24.3 GPa (pine) and 2.6 and 9.2 GPa (beech). It is suggested that this is a consequence of early and late growth fibres, which are structurally different.
Resination triggers a cascade of material properties changes to the fibres; resinated pine fibres (mean stiffness: 24.1 GPa, standard deviation: 15.1 GPa) were unimodal, with a single scale factor of 26.8 GPa. The process increased both the mean modulus and the variance. High and low stiffness outliers were present within the datasets. Based on several statistical inferences, it is proposed that the low modulus outliers were damaged (during production).
In summary, the work presents an interesting exploration of wood fibres through the lens of materials science, unique compared to the more usual biological context. The work contains several implications for the broader materials community and the hope of a timber driven future. "