The influence of microstructural characteristics and cell wall material properties on the mechanical behaviors of different tissues of sorghum stems

Sorghum stems comprise different tissue components, i.e., rind, pith, and vascular bundles in the rind and pith regions, of different cell morphologies and cell wall characteristics. The overall responses of stems to mechanical loadings depend on the responses of these tissues themselves. Investigating how each tissue deforms to various loading conditions will inform us of the failure mechanisms in sorghum stems when exposed to wind loadings, which can guide the development of lodging-resistant variants. To this end, numerical analyses were implemented to investigate the effects of cell morphologies and cell wall properties on the overall mechanical responses of the above four tissues under tension and compression. Microstructures of different tissues were constructed from microscopic images of the tissues using computer-aided design (CAD), which were then used for finite element (FE) analyses. Shell finite elements were used to model the cell walls, and the classical lamination model was used to determine the overall mechanical responses of cell walls having different fiber composite arrangements. The results from the numerical analyses helped explain how the loading (boundary) conditions, the cell microstructures, the mechanical properties of cell walls of different tissues, the cell wall thickness, the microfibril angle (MFA) of fiber composites of the cell walls, and the turgor pressure affected the overall mechanical responses of the tissues. Tissue stiffening or softening behaviors were attributed to different microstructural deformations, i.e., local or global buckling of cell walls, cell collapse, densifications of cells, or reorientation and rearrangement of cells. The mechanical properties and thickness of cell walls only affected the stiffness and load-bearing ability of the tissues. The turgor pressure affected the compressive responses but its effect on tensile responses was negligible. The MFA had a significant influence on the stiffness and load-bearing ability when the tissues were loaded along their longitudinal axis, but it had an insignificant effect on loading in the transverse direction. Tissues with smaller cell sizes and denser cells were stronger and stiffer than those with larger cell sizes. The numerical simulations also revealed that rind and rind vascular bundles were stiffer and had higher load-bearing ability than pith and pith vascular bundles.