The significance of the elastic modulus of wood cell walls obtained from nanoindentation measurements

Nanoindentation is a powerful method for the study of hardness and elasticity at small scale. When applied to the measurement of the longitudinal elastic modulus of wood cell walls, where the longitudinal modulus is approximately 10 times the transverse modulus, nanoindentation yields values considerably below results obtained from single-fibre tensile tests and model calculations, respectively. In a nanoindentation experiment on a wood cell wall using a Berkovich-type indenter, the cell wall is loaded at an angle of approximately 25degrees due to the inclination of the faces of the indenter pyramid. In the zone deformed by the indenter, the resulting three-dimensional stress state is governed not only by the longitudinal (Young's) modulus, but also affected by the much lower transverse modulus, which leads to an underestimation of the longitudinal modulus. It is concluded that nanoindentation using current isotropic theory is therefore not suitable for the direct measurement of the longitudinal elastic modulus of wood cell walls. (C) 2004 Elsevier Ltd. All rights reserved.