Advanced Materials Letters

Bamboo is a unidirectional fibre-reinforced composite with radially graded and almost transversely isotropic elastic properties. The cracks originated in bamboo under bending due to wind loads propagate along the fibre direction. This process is controlled by interlaminar fracture toughness. In order to observe the spatial distribution of the fracture toughness in bamboo, energy release rate is theoretically deduced from the general equations for crack-tip stress fields in anisotropic bodies. The analysis shows that the fracture toughness has graded distribution and the trend is opposite to that of axial modulus. To verify this, the energy release rate (or fracture toughness) is experimentally calculated for double cantilever beam specimens (with a crack placed in different fibre density region) in mode-I i.e. crack opening mode. It is observed that the crack propagation parallel to fibres (splitting) develops easily and the energy release rate decreases with increased density of fibre bundles. The observed trend closely corroborates the results from theoretical analysis. From the results of real-time wind load simulations (reported elsewhere) on tapered bamboo-like structure it is concluded that with the help of radially graded fracture toughness bamboo converts flaws of all orientations into splitting mode.

Bamboo is a natural composite material consisting of unidirectional fibre bundles, oriented along axial direction, embedded in soft parenchymatous matrix. The bundles are arranged such that the fibre density (or fibre volume fraction) varies from outer to inner periphery of bamboo shoot. The gradation in volume fraction of unidirectional fibre bundles qualifies bamboo as a typical radially graded transversely isotropic material. Being largely a cellulosic material, the fibre bundles have high tensile strength. However, there is great dispersion of these properties. In this work, an attempt is made to model the progressive failure of fibre bundles to predict the failure strength of bulk bamboo in uniaxial tension. A two-parameter Weibull distribution is proposed to analyse the strengths of fibre bundles having different cross-section areas. Tension tests are performed on fibre bundles, selected from different fibre density regions in the transverse cross-section of bamboo, for determining statistical parameters. The results highlight the close resemblance between the Weibull probability distribution of the experimental results on fibre bundles and overall mechanical behaviour of the bulk bamboo. Thus, the use of Weibull parameters is established for predicting the strength of bulk bamboo from fibre bundle testing of different cross-section areas.  

In this work, we fabricated samples of titanium dioxide nanotube arrays via electrochemical anodization by using titanium foils as anode and cathode. A solution of water, ethyleneglycol, and ammonium fluoride (NH4F) at room temperature was used for the samples synthesis process. Different times and anodizing voltages were used during reaction. From X-ray diffraction (XRD) and micro-diffraction (mXRD) measurements, rutile and anatase phases were identified as function of deposition parameters. The Ti3O5 phase was observed by deconvolution of Debye-Scherrer rings of the microdiffraction spectra. Annealing processes were performed for all samples in the range between 273 and 723 K, without changes in the material’s morphological properties, while the crystalline structure was affected. Nanotube diameters varying between 30 and 42 nm were observed from SEM micrographs, when NH4F concentration was changed from 0.25 to 0.50 wt%. An alternating anodizing voltage generates the formation of nanotubes evenly spaced on the surface with nodes in bamboo-type form, while a smooth form for nanotubes was observed with constant applied voltage. From stereoscopic 3D micrographs, a reconstruction of the topographic surface of the TiO2 nanotubes was conducted. A correlation between synthesis parameters and morphological properties is presented.