Tag: compressive strength

In this study, the effects of a chemical-mechanical modification system on low-density poplar (Populus tremula L.) wood were investigated without external confinement. Wood samples were initially treated with an alkali-sulphite solution (2.5 M NaOH, 0.4 M Na₂SO₃ for 7 h) to induce partial delignification, after which they were densified at 100°C under 5 MPa pressure. Following this process, a significant increase in density to 1.22 g/cm³ (+172.6%) was recorded. Mechanical evaluations indicated substantial enhancements; specifically, Brinell hardness, bending strength (MOR), and modulus of elasticity (MOE) were improved by 346.3%, 67.4%, and 78.3%, respectively, confirming the successful consolidation of the cellulose network. However, a trade-off was revealed by microscopic analysis, and two distinct failure mechanisms were identified. First, large cracks (~150 µm) were formed in the core due to diffusion limitations that prevented full chemical penetration. Second, widespread micro-cracks (~4 µm) were triggered in the densified shell by the Poisson effect due to the lack of lateral confinement. These results suggest that while hardness is significantly improved by unconstrained densification, compressive performance is compromised by the resulting defects. While the modification was intentionally applied to 35 mm thick lumber to reveal diffusion limitations and Poisson-induced defects under unconstrained conditions, small, defect-free specimens were subsequently selected for mechanical testing to isolate and characterize the intrinsic properties of the successfully modified regions near the surface.