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 (Norwegian spruce), in particular the cellulose microfibril angle, has been shown to control not only the stiffness but also the extensibility of wood within a wide range. In order to further elucidate this effect, the deformation of wood under tensile load parallel to the longitudinal cell axis was studied 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 microfibril angle and deformation behaviour. The microfibril angle was shown to influence not only the extensibility in longitudinal direction but also the deformation perpendicular to the applied load. Moreover, the results showed that the energy absorption capacity is higher for specimens with larger microfibril angle. SEM pictures of the fractured samples indicated clearly the differences in the fracture process as the fracture zones of samples with low microfibril angle were smooth and the fracture zones of samples with high microfibril angle were heavily torn and deformed indicating a more ductile behaviour.