Micromechanical Estimates for Elastic Limit States in Wood Materials, Revealing Nanostructural Failure Mechanisms

At the macroscale, wood materials show great variability and diversity. At the nanoscale, however, they exhibit common (universal) building blocks which build up universal organizational patterns over several length scales up to the macroscale. In the framework of continuum micromechanics, this building principle was recently expressed in quantitative terms, allowing for a prognosis of tissue-specific anisotropic elasticity properties of wood from tissue-specific chemical composition and porosity, and from universal elastic properties of the elementary constituents “amorphous cellulose,” “crystalline cellulose,” “hemicellulose,” “lignin,” and “water.” In this paper, we extend this investigation to tissue-specific macroscopic elastic limit states: We show that shear failure of the nanoscale building block “lignin,” which exhibits an isotropic, tissue-independent (“universal”) shear strength, is the dominant determinant of anisotropic macroscopic failure of wood under different loading conditions. In a continuum micromechanics setting, quadratic strain averages over material phases represent microstructural strain peaks, which are responsible for material phase failure. The good agreement of tissue-specific micromechanical predictions of macroscopic limit stresses with corresponding tissue-specific strength experiments underlines the role of lignin as the governing strength-determining component of wood.