J.O. Amando de Barros; J. Schwiedrzik; F.K. Wittel
Manufacturing 199 (2025) 109209
Wood’s increasing role as a structural resource in sustainable materials selection demands accurate character-
ization of its mechanical behavior. Its performance arises from a hierarchical structure, where the dominant
load-bearing component is the S2 layer of tracheid cell walls—a thick, fiber-reinforced composite of cellulose
microfibrils embedded in hemicelluloses and lignin. Due to the small dimensions and anisotropic nature of
the S2 layer, mechanical testing presents significant challenges, particularly in producing uniform stress and
strain fields. In this study, we apply micropillar compression (MPC) combined with digital image correlation
(DIC) to Norway spruce tracheids, enabling direct and model-free strain measurements at the cell wall scale.
Micropillars were oriented at different microfibril angles (MFAs), confirming the expected dependence of
stiffness and yield stress on ultrastructural alignment, with higher stiffness and yield stress at low MFAs.
For these under compression fibril-aligned kink bands occurred, while shear related failure was observed at
higher angles. A parameter study on the acceleration voltage of the Scanning Electron Microscope revealed
that electron beam exposure significantly degrades pillar integrity, which could explain data scatter and
mechanical underestimation in earlier MPC studies. By controlling imaging protocols and using DIC-based
strain measurements, we report the highest direct measurements of wood cell wall stiffness to date – up to
42GPa for MFA = 0◦ – closer matching micromechanical model predictions compared to previous results.
Findings are compared with Finite Element Method-based displacement corrections to establish a robust
protocol for probing soft, anisotropic biological composites’ mechanical behavior while clarifying longstanding
inconsistencies in reported results of wood MPC measurements.