Experimental and Computational Micro-Characterization Techniques in Wood Mechanics

WG2: (Hygro)-mechanical behaviour of wood

This Working Group is dedicated to further developing structure-function relationships in relation to the hygromechanical behaviour of wood and to determining hygro-mechanical properties of the cell wall material and its constituents. Thereby, the focus is put on in-situ measurements to obtain information about the behaviour within the natural molecular environment and spatial arrangement. This will avoid the uncertainties inherent to extracted material. Current in-situ testing possibilities will have to be improved or adapted to wood for this purpose. Considerable progress is expected from the application of emerging micro-characterisation techniques in this respect. Especially for new testing methods, the influence of the sample preparation on the test results will be studied.

Investigations will span the elastic, viscoelastic, viscoplastic, and mechano-sorptive behaviour as well as moisture transport and sorption at different temperatures. Particular emphasis will be placed on elucidating the mutual interference of the mechanical state and the moisture state of a sample. For example, the effect of a stress gradient on the water transport behaviour will be analysed as well as the influence of moisture changes on micromechanical properties. Mechanical properties will be determined by micro-tensile testing, nanoindentation, Scanning Acoustic Microscopy, and Atomic Force Microscopy, the latter in combination with Ultrasonic Force Modulation Spectroscopy. The softening of hemicelluloses and lignin at elevated temperatures will also be investigated. For the determination of moisture transport and sorption properties, conventional methods such as the cup test will be applied as well as more advanced methods such as Dynamic Vapour Sorption.

Another promising approach is to combine micromechanical in-situ tests with ultrastructural investigations (e.g. spectroscopic and X-ray methods). Such tests, which have already been mentioned in the description of Working Group 1, will contribute to an improved understanding of the molecular deformation mechanisms and allow for the identification of the load bearing networks. Also the combination of existing characterisation techniques with advanced environmental control systems is expected to deliver further insight in the behaviour of the cell wall material and its constituents. Thus, the performance of spectroscopic or microscopic investigations of wood during moisture uptake, transport or drying will help to elucidate the interaction of moisture with the cell wall polymers and the location of accessible regions in the cell wall. These results will be of great value for the work performed within the scope of Working Group 1. In return, models for the microstructure and anatomical data, which are required for the design and evaluation of the (hygro)-mechanical tests, will be received from Working Group 1.

Again modified samples will be used. Micromechanical tests on modified cell wall or fibre samples will enable to study the contribution of the individual cell wall components to the overall response of the sample. Reverse identification methods to be developed within Working Group 3 will be employed to estimate the properties of the components from the overall response of the sample.

Furthermore, combining Nuclear Magnetic Resonance or Near Infrared Spectroscopy with mechanical pre-treatment of the samples is expected to indicate how the mechanical loading alters the bonds between water molecules and the wood polymers.