Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee October 11-14, 2010, Geneva, Switzerland Paper NT-2 1 of 9 Full Set of Elastic Constants of Spruce Wood Cell Walls Determined by Nanoindentation Johannes Konnerth 1,* , Christoph Buksnowitz 1 , Wolfgang Gindl 1 , Karin Hofstetter 2 , and Andreas Jäger 2 1 Institute of Wood Science and Technology, Department of Material Science and Process Engineering, BOKU - University of Natural Resources and Life Sciences, Vienna 2 Institute for Mechanics of Materials and Structures, Faculty of Civil Engineering, Vienna University of Technology, Vienna, Austria In the last years Nanoindentation (NI) has become a frequently used tool in material science. Since Wimmer et al. (1997) introduced NI in wood science for the characterisation of micro- mechanical properties, the validity of the obtained data and their significance is a matter of debate. As NI theory was initially developed exclusively for homogenous and isotropic materials, the interpretation of results from heavily anisotropic wood cell walls is not straightforward. Therefore the indentation modulus M (also called reduced modulus) typically determined by NI- tests is not comparable to the longitudinal elastic modulus obtained with other experimental techniques. As a consequence, NI on wooden cell walls has been mainly used for comparative purposes. In order to overcome this limitation, we present a new approach capable of identifying the stiffness tensor components of the secondary cell wall S2 for quantitative purposes. The indentation modulus obtained by NI is a product of the stiffness tensor components. Under the assumption of transverse isotropy for the wood cell wall, it depends on five elastic parameters and also on the angle between the direction of indentation and the longitudinal cellulose microfibril axis. A model built by Jäger et al. (2010) was applied in order to draw a link between the indentation modulus M and the stiffness tensor. In this manner, the unknown elastic parameters can be back-calculated via minimization of the error between the modulus predicted by the model and the modulus measured in NI experiments. In order to find an explicit solution, NI tests have to be performed in at least five different angles between load direction and micro fibril orientation. The best solution of the algorithm delivered appropriate values for all elastic constants of the wooden cell wall. Keywords: Nanoindentation, mechanical properties, wood, cell wall, analytical modelling, stiffness tensor *Corresponding author johannes.konnerth@boku.ac.at Peter Jordan Strasse 82 1190 Vienna Tel.: +43 1 47654 4250 Fax: +43 1 47654 4295