Experimental micromechanical characterisation of wood cell walls

International audienceThe properties of wood and wood based materials are strongly dependent on the properties of its fibres; i.e. the cell wall properties. The ability to characterize these in order to increase our understanding of structure-property relationships is thus highly important. This article gives a brief overview of the state of the art in experimental techniques to characterize the mechanical properties of wood at both the level of the single cell and that of the cell-wall. Challenges, opportunities, drawbacks and limitations of single fibre tensile tests and nanoindentation are discussed with respect to the wood material properties

Density profiles of novel kraft pulp and TMP based foam formed thermal insulation materials observed by X-ray tomography and densitometry

X-ray tomography and densitometry (XRT and XRD) were applied to characterise wood fibre based insulation materials, which were produced by the foam forming technology. XRT is a high resolution approach with long measurement times of around 29 h, while XRD measurement needs only a few minutes. The determination of density distribution of boards in the thickness direction was the focus of this study. Both approaches visualised well the impact of raw materials and manufacturing processes on the structure of the panels. The density profiles were dependent on the pulp applied for panel production, and the processing conditions were also influential. Air flow resistance correlated with the maximum density measured inside the board. Both XRT and XRD revealed similar trends, which are useful for the characterisation of insulation materials.</p

Experimental micromechanical characterization of wood cell walls

Understanding the mechanical response of wood to external loads is essential to understand (and predict) the stability of living trees or equally important for the behaviour of wood when it is used for construction purposes. In general, wood is a hierarchically structured material: a sophisticatedarchitecture at several length scales, from the mm to the nm level, allows ingenious control of various functions. This highlights the importance of studying wood at several length scales. Here we will focus on wood micromechanical data which can serve as - besides a deeper understanding of the material properties in general - important input parameters in applications such as manufacture of fibre-based products like cardboard and paper which helps this resource to be used in targeted, efficient, economic and sustainable ways.In the presentation, a brief overview of the state of the art of two frequently used methods that allow experimental characterization of mechanical properties at the cell and cell wall level, namely single fibre microtensile tests and nanoindentation, will be given.Since 1959, the year when Jayne published his paper “Mechanical properties of wood fibres” [1], the first experimental procedure describing single wood fibre testing, many different procedures have been developed. Key developments will be summarized.Nanoindentation, in comparison with tensile tests, is a rather young experimental technique that was applied on wood in 1997 by Wimmer et al. [2] for the first time. Again, key developments since then will be summarized.For both techniques sample preparation is not straightforward. Whereas single fibre tests demand the isolation of tracheids, fibre tracheids or libriform fibres, nanoindentation requires flat surfaces and an adapted indentation size compared to the cell wall thickness (i.e., correct choice of the loadingunloading conditions and measurements correction if necessary). Challenges, limitations and drawbacks will be discussed, also with respect to controlling relative humidities.Finally, examples for micromechanical properties will be given and a summarizing collection of data from literature, both for microtensile tests and nanoindentation, will be presented