Deformation and energy absorption of wood cell walls with different nanostructure under tensile loading

The nanostructure of the S2 cell wall layer in tracheids of Picea abies (No rwegian spruce), in particular the cellulose microfibril angle, has been sh own to control not only the stiffness but also the extensibility of wood wi thin a wide range. In order to further elucidate this effect, the deformati on of wood under tensile load parallel to the longitudinal cell axis was st udied in a contact-free way using a video extensometer. The combination of these measurements with small-angle X-ray scattering on the same microtome sections allowed us to establish a direct relationship between the microfib ril angle and deformation behaviour. The microfibril angle was shown to inf luence not only the extensibility in longitudinal direction but also the de formation perpendicular to the applied load. Moreover, the results showed t hat the energy absorption capacity is higher for specimens with larger micr ofibril angle. SEM pictures of the fractured samples indicated clearly the differences in the fracture process as the fracture zones of samples with l ow microfibril angle were smooth and the fracture zones of samples with hig h microfibril angle were heavily torn and deformed indicating a more ductil e behaviour. (C) 2001 Kluwer Academic Publishers.