Wood and natural fibers exhibit an advantageous combination of good mechanics at comparably low density. Nevertheless, comparing absolute strength and stiffness, wood is clearly inferior to materials such as metals and engineered composites. Since there is a strong correlation between wood density and wood mechanical performance, densification by transversal compression suggests itself as a route towards improved mechanics. Partially delignified densified spruce veneers with excellent tensile properties were produced by means of an alkaline (AL) and an organosolv (OS) approach. Plywood specimens were manufactured using treated veneers glued with a phenol-resorcinol-formaldehyde adhesive and were compared with plywood samples made of native spruce veneers (Ref) and spruce veneer densified after plasticization by water impregnation (H₂O). Roughly, the bending strength and the modulus of elasticity of plywood from partially delignified densified wood were improved by a factor of 2.4 and 3.5, respectively. Interlaminar shear strength did not match this improvement after partial delignification. Together with excessive thickness swelling, this might be a drawback of partially delignified densified wood in need for further research.

Regarding mechanics, wood is one of the most efficient materials available. Due to its unique combination of composite and cellular microstructure, it exhibits superb specific mechanical properties which exceed many man-made materials. However, concerning absolute mechanical properties, wood is often inferior to metals and novel engineered composites. Since many wood properties are strongly correlated with its density, densification is a promising pathway towards improved absolute mechanics. Spruce, beech and poplar wood were densified in a two-step process. First, amorphous wood polymers were partially extracted according to an alkaline (AL) and an organosolv (OS) protocol. Subsequently, partially delignified veneers were densified by hot pressing in tangential direction. After densification, average densities increased to 1.00–1.20 g cm⁻³. FTIR analysis confirmed chemical changes, mostly in the bands attributed to hemicelluloses and lignin, of chemically treated and hot-pressed veneers. To evaluate the modification process regarding mechanics, tensile and bending properties were characterized and revealed promising results. Compared to untreated control specimens, stiffness and strength tested in tension and bending improved, regardless of wood species and pretreatment. Regarding average tensile properties, the OS treatment improved stiffness, up to 40 GPa, whereas the AL treatment improved strength, up to 300 MPa. Set-recovery tests showed, that chemically treated and densified samples exhibited a better dimensional stability compared to H₂O-soaked and compressed specimens. However, 24 h water soaking resulted in excessive thickness swelling. This might be a major drawback of partially delignified and densified wood and should be considered in a material selection process.

Partial delignification and densification provide a pathway to significant improvement in the mechanical performance of wood. In order to elucidate potential effects of this treatment on the mechanical anisotropy of wood, partially delignified and densified spruce wood veneers were characterized at varying degrees of off-axis alignment. While the tensile strength and the modulus of elasticity (MOE) were clearly improved in parallel to the axis of wood fibers, this improvement quickly leveled off at misalignment angles ≥30°. For transverse tensile strength, the performance of alkaline-treated and densified wood was even inferior to that of untreated wood. Microscopic examination revealed the presence of microscopic cracks in treated wood, which are assumed to be responsible for this observation. It is concluded that impaired transverse tensile properties are a weakness of partially delignified and densified wood and should be considered when a potential usage in load-bearing applications is intended.

The plastic deformation of wood perpendicular to the grain is gaining increasing importance due to advancements in forming technologies and the densification of wood. This study investigates how two hardwood species, i.e., beech (Fagus sylvatica) and birch (Betula pendula), respond to compression in the radial direction and examines the structural changes they undergo during both elastic and plastic deformation. Stress–strain curves at different moisture contents (dry to wet) and temperature conditions (20 to 140 °C) were recorded. In-situ observations at high moisture content and temperatures by means of different microscopic techniques are practically unfeasible. Therefore, the specimens were analysed ex-situ microscopically after the test. In addition to the compression of transversely oriented fibres and vessels, special attention was paid to the deformation behaviour of the wood rays. The results suggest that the wood ray cells carry a relatively higher proportion of the load in the radial loading direction than the surrounding vessels and fibres. This observation is supported by the higher percentage of deformed vessels, seen in the microscopy, in areas where the rays developed kinks, usually in the early wood of beech and anywhere in the cross-section of birch. The weaving of rays around big vessels introduced shear strains under compressive stresses at the kinked rays’ area. Thus, shear deformation is more evident in early wood than in late wood regions of wood. However, when the wood was tested at elevated moistures and temperatures, the material demonstrated a ductile response, namely the absence of localised shear deformations. Notably, wet beech and birch specimens heated to 100 °C and above exhibited pronounced thickness recovery and there was slightly irreversible buckling of rays and vessel deformations. Therefore, under such conditions, wood behaves like a “sponge” and is expected to be successfully processed without introducing clear damage to the material. This characteristic holds promise for replication in the development of bio-based energy-absorbing materials.

Due to very significant improvements in the mechanical performance of solid wood, densification after partial or full delignification has received renewed attention. While studies predominantly focus on isolated cases, e.g., one wood species densified in one anatomical direction, systematic investigations of the effect of wood structure on the densification process are seldom performed. The present study compares the deformation patterns of one representative softwood (spruce) and hardwood (poplar), respectively, in transverse compression. In terms of variables, the native and partially delignified state, wet and dry conditions, and three different orientations of wood grain are considered. It shows that the structural inhomogeneity of spruce wood, with low-density earlywood and high-density latewood, governs its densification pattern and hinders defect-free densification. Contrarily, diffuse-porous poplar is structurally more homogeneous, which together with more efficient softening by delignification leads to favorable densification behavior. Overall, delignification led to a significant softening of both wood species in transverse compression, which greatly enhanced the softening effect of high wood moisture content.