On the radial variation of the transverse mechanical properties of bamboo

Bamboo is a functionally graded and axially reinforced material displaying high longitudinal mechanical properties. However, it is prone to longitudinal splitting, mainly due to tensile and shear stress components. Therefore, this experimental study was focused at determining the transverse mechanical properties of five species of bamboo using digital image correlation (DIC). Thus, two tests with parts of rings were used to independently determine the inner and outer circumferential tensile strength as well as the variation of the circumferential Young's modulus through the culm wall. In the first set-up, a semi-ring (SR test) was radially loaded at the middle section and supported on rollers at both ends such that the inner surface of the specimen was under tensile stress. In the second protocol, a specimen bigger than a semi-ring (C test) was loaded by two radially applied compression loads to generate tensile circumferential stresses on the outer surface. A closed-form solution assuming a linear variation of the Young's modulus through the culm wall was developed to analyze the experimental results. The bamboo species Phyllostachys edulis (EDU), Phyllostachys bambusoides (BAM), Phyllostachys Iridescens (IRI), Phyllostachys Violascens (VIO) grown in North of Italy, and Guadua angustifolia (GA) grown in Colombia were tested. The inner tensile failure strain (range of means for all species: 0.014–0.035) was generally higher than the outer (range of means for all species: 0.008–0.0019). The effective modulus of GA (931 MPa–1148 MPa) was lower than those of the European species (range of means for all European species: 1209–2983 MPa). The inner Young's moduli for all species except EDU (796–1694 MPa) was lower than the outer (range of means for all species: 1976–4694 MPa). The trend was different for EDU that showed an outer Young's modulus about 30% lower than the inner modulus. The outer tensile strength of BAM, IRI and GA (range of means for the three species:16.5–30.5 MPa) was significantly higher than the inner (range of means for three species: 9.9–17.6 MPa), while no signficant differences were found for VIO. A different trend was obtained for EDU, for which the inner strength was about three times higher than the outer.