In-situ synchrotron studies of texture in wood during mechanical tests.

J. Keckes¹, I. Burgert ², M. Müller ³, K. Kölln ³, S. V. Roth ⁴, S. E. Stanzl-Tschegg ⁵, P. Fratzl²

¹Erich Schmid Institute for Materials Science, Austrian Academy of Sciences and Institute of Metal Physics, University of Leoben, Austria

²Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany

³Institute for Experimental and Applied Physics, University Kiel, Germany

⁴European Synchrotron Radiation Facility, Grenoble, France

⁵Institute of Meteorology and Physics, University of Agricultural Sciences, Vienna, Austria

The exceptional mechanical properties of biological materials reside in their complex architecture at all hierarchical levels supported by specific molecular mechanistic phenomena. In order to understand the structure-property relationship in wood, in-situ synchrotron diffraction studies on individual wood cells and on wood foils combined with tensile tests were performed at ID1 and ID13 beam-lines of ESRF, respectively.

The wood foils of the dimensions 5 x 50 x 0.2 mm and the individual wood cells of about 20 µm in diameter and 500 µm in length were strained in tensile stages under various strain rates monitoring strain-stress development and collecting x-ray diffraction patterns using a 2D CCD detectors. For the experiment on individual cells, a very precise piezoelectric tensile stage with simultaneous stress/strain control was used while the samples were examined with beam of 2 mm [1].

The diffraction data obtained from both wood foils as well as from wood cells demonstrated significant changes in texture due to straining. By relating the mechanical data with the texture information from cells and from foils, it was possible to separate deformation mechanisms inside the cell‑wall from those mediated by cell‑cell interactions. The data indicate the presence of a dominant recovery mechanism in the cell-wall which reforms the amorphous matrix between helical cellulose microfibrils, recovering its mechanical function. This mechanism dominates to the tensile behaviour of different wood tissues. Moreover, the comparison of the mechanical and the microstructural results from various wood types allowed to draw relevant general conclusions regarding the role of different microstructural features of wood on its mechanical behaviour as well as to deduce wood architecture units progressively optimised during evolution.

[1] J. Keckes, I. Burgert, K. Frühmann, M. Müller, K. Kölln, M. Hamilton, M. Burghammer, S.V. Roth, S. Stanzl-Tschegg & P. Fratzl (2003), Cell-wall recovery after irreversible deformation of wood, Nature Materials 2, 811-814.