Mechanical performance of densified delignified wood under tension: the influence of structural characteristics

Abstract

Densified delignified wood (DW) is a novel engineering material with ideal specific strength based on the delignification and densification of natural wood (NW). However, the dependence of mechanical performance on structural characteristics (e.g. density, ρ) of DW has not been systematically elucidated, which limits the practical application of DW in engineering fields. To bridge this gap, this work investigates the strength, ductility and fracture toughness of DW with various ρ under tension parallel and perpendicular to the grain. The elastic modulus (E) and tensile strength (S) increased as ρ increased in both longitudinal (L) and tangential (T) directions. Fracture strain (ε_f) increased as density increased in the T direction, while a first decreasing and then increasing trend occurred in the L direction. This trend is the opposite in the work of fracture (W). A first decreasing and then increasing trend is observed of W in the T direction due to the coupling effect of delignification and densification. The mechanical performance of DW is found to be governed by the two-stage densification process during the preparation of DW. A transition density (ρ_n) is proposed herein to distinguish the two densification stages. When ρ is less than ρ_n, void collapse restricts gains in E, S and W while reducing ε_f due to loss of pore cushioning. When ρ is larger than ρ_n, cell-wall densification enhances fiber packing and hydrogen bonding, simultaneously improving all four mechanical properties. This work is expected to develop a deeper understanding of the multiscale mechanical design and mechanical behavior of cellulosic and wooden materials.