Experimental evidence for a mechanical function of the cellulose microfibril angle in wood cell walls

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Abstract
Wood is a natural fibre composite with a hierarchical cellular structure of a specific strength and a specific modulus of elasticity that can be compared with those of other common construction materials. Each wood cell is typically built of cellulose fibrils spiralling around the macroscopic fibre direction. While it is natural to assume a relation between the microfibril angle (MFA) and the mechanical properties, a good correlation has up to now only been established for single fibres, where a larger extensibility was found for fibres with larger MFA. In the present paper, we show for the first time that this relation even exists for thin (200 μm) sections of wood, which provides strong evidence for the fact that the MFA optimizes the extensibility of wood. In a combination of tensile tests with structural investigations by small angle X-ray scattering on the same sample of Picea abies, we found a remarkable increase in maximum strain with increasing MFA, and also a change in the elastic moduli.