Fracture characteristics of poplar wood multiscale structures under longitudinal tensile loading

Objective Wood, as an engineering material with a natural multiscale structure, often fails under external loading. Although the multiscale structure plays a key role in wood failure, the failure features at different structural levels and their influence mechanisms are still not fully understood. This paper aims to clarify the multiscale failure characteristics of poplar wood under longitudinal tensile load, especially focusing on the influence of cell structure and arrangement direction. The goal is to provide a theoretical basis for evaluating the mechanical properties of poplar wood and improving its engineering application.

Method Poplar from Populus × canadensis plantation was selected as the research material. Tensile specimens were prepared from different regions within the growth ring. Structural features and mechanical differences at three scales: growth ring, cell, and cell wall were studied using SEM and AFM techniques. The fracture paths under longitudinal tension were analyzed to reveal the internal relationship between poplar wood structure and failure behavior.

Result (1) Growth ring scale: under longitudinal tension, shear stress occurred near the growth ring boundary due to large structural differences. This leads to fracture along the ring boundary or on the earlywood side near the boundary. (2) Cell scale: under axial loading, axial cells mainly showed transverse fracture perpendicular to the grain, forming flat or curved fracture surfaces. Separation occurred between ray cells arranged in the radial direction. After the crack spreading to the end of ray cells, it turned again along the axial direction, forming a step-like fracture surface. The differences in fracture types reflected the guiding effect of different cell arrangements on crack propagation. (3) Cell wall scale: regions with large variation in cell wall thickness were more likely to experience through-wall fracture, forming transverse cracks. When cracks spreading within the cell wall, the interfaces between S₁/S₂ or between S₂/S₃ layers were weak points due to differences in their microscopic structure. This showed the variation in bonding strength between layers and their response under stress. When cracks spreading along the grain between cells, delamination occurred at the interface between the compound middle lamella (CML)/S₁, showing that the CML has relatively weak bonding strength under longitudinal tensile loading.

Conclusion Under longitudinal tensile loading, the internal structure, cell morphology, and arrangement direction of poplar significantly affect the failure modes at different scales. Differences in structure and mechanical properties between similar units at the same scale are the main factors influencing internal stress distribution under external loads. In future studies, in-situ testing equipment can be used to monitor subtle changes in wood’s macromolecular structure under mechanical load in real time. This will help reveal the mechanical response mechanisms of wood at the molecular level.