The structure and mechanics of Moso bamboo material

Abstract

Although bamboo has been used structurally for millennia, there is currently increasing interest in the development of renewable and sustainable structural bamboo products (SBPs). These SBPs are analogous to wood products such as plywood, oriented strand board and glue-laminated wood. In this study, the properties of natural Moso bamboo (Phyllostachys pubescens) are investigated to further enable the processing and design of SBPs. The radial and longitudinal density gradients in bamboo give rise to variations in the mechanical properties. Here, we measure the flexural properties of Moso bamboo in the axial direction, along with the compressive strengths in the axial and transverse directions. Based on the microstructural variations (observed with scanning electron microscopy) and extrapolated solid cell wall properties of bamboo, we develop models, which describe the experimental results well. Compared to common North American construction woods loaded along the axial direction, Moso bamboo is approximately as stiff and substantially stronger, in both flexure and compression but denser. This work contributes to critical knowledge surrounding the microstructure and mechanical properties of bamboo, which are vital to the engineering and design of sustainable SBPs.

Keywords: bamboo; cellular solids; mechanical modelling; mechanical properties; microstructure.

Figure 1.

Density plotted against radial position, r, normalized by the culm thickness, a. (Online version in colour.)

Figure 2.

SEM micrograph of Moso bamboo structure, third internode, over entire culm wall thickness.

Figure 3.

(a,b) SEM micrographs of inner and outer vascular bundles of internode 7. (c) Sclerenchyma fibres, r/a ∼ 0.5, internode 7. (d) Longitudinal section, depicting sclerenchyma fibres (centre) and surrounding parenchyma, r/a ∼ 0.5, internode 7.

Figure 4.

(a) Vascular bundle volume fraction plotted against normalized radial position, curves are best-fit equations (3.1), (3.3)–(3.5). (b) Vascular bundle solids fraction plotted against normalized radial position, line is best-fit equation (3.6). (Online version in colour.)

Figure 5.

Geometry of mechanical test specimens. (Online version in colour.)

Figure 6.

Typical load–deflection curve for a bending test (specimen: internode 14, r/a = 0.817, ρ^* = 888 kg m⁻³, width = 6.85 mm; thickness = 1.90 mm; span length = 85.73 mm). (Online version in colour.)

Figure 7.

(a) Young's modulus along the axial direction from bending plotted against density, lines represent the model. (b) Modulus of rupture along the axial direction plotted against density, lines represent the model. (Online version in colour.)

Figure 8.

(a) Young's modulus along the axial direction from bending plotted against normalized radial position, curves represent the model. (b) Modulus of rupture along the axial direction plotted against normalized radial position, curves represent the models. (Online version in colour.)

Figure 9.

Typical axial compression stress–strain curve (axial specimen: internode 11, r/a = 0.444, ρ* = 541 kg m⁻³, radial specimen: internode 7, r/a = 0.259, ρ* = 585 kg m⁻³, tangential specimen: internode 7, r/a = 0.357, ρ* = 556 kg m⁻³). (Online version in colour.)

Figure 10.

Compressive strength in the axial, radial and tangential directions, plotted against density, upper line represents the model and lower line is radial strength average. (Online version in colour.)

Figure 11.

Compressive strength in the axial direction plotted against normalized radial position, curves represent the models. (Online version in colour.)

Figure 12.

Stress–strain curve from deformation stage test, with micrographs of the specimen superimposed on the curve with arrows indicating stress for each image.

Figure 13.

Micrographs of gold-coated internode 7 specimen under compressive loading in the radial direction, with radial stress shown above each image. (a) The composite structure, same as images in figure 12. (b) The higher magnification micrographs of the same location showing only the parenchyma under loading.

Figure 14.

Calculated fractions of V_vb and V_vbS_f of outer flexural specimens plotted against measured density, lines are best-fit equations (6.4) and (6.5). (Online version in colour.)