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Microstructural Analysis and Mechanical Behaviour of Bamboo Fibres and Bamboo Fibre Composites

Book - Dissertation

Among natural fibres, bamboo fibres are an ideal option to reinforce polymers in the new era of sustainable materials, thanks to their availability and singular characteristics. Moreover, bamboo plants have always been present in many cultures around the world, and they have played an important role for the local economy where they grow.In order to make an adequate use of this new material as reinforcement for composites, it is indispensable to fully evaluate the fibre mechanical properties and get a complete understanding of its behavior in function of the microstructure, as well as to study the effect of severe conditions, like the presence of moisture, on the mechanical behaviour of bamboo fibres and bamboo fibre composites. These are the two main topics that will be presented in this doctoral thesis.In a first stage of the research, (tensile) mechanical properties have been studied at four different span lengths to determine the fibre quality after the extraction process and the mechanical performance of the material. The tensile strength has a small variation at the different span lengths and remains around 800 MPa. This indicates that the extraction process did not significantly affect the fibre quality. The Young's modulus is around 43 GPa. Bamboo fibre composites were also produced using untreated and chemically-treated fibres to evaluate the effectiveness of the reinforcing material. Good results were achieved with untreated fibres in epoxy resin.The morphology of technical bamboo fibre and the microstructure of the elementary fibre have been observed and examined by using different techniques applied in the study of wood anatomy. Observations have provided us with a vast knowledge of the complex microstructure of this natural fibre from the macro down to the micro scale level, where different features like the distribution of the elementary fibres within the fibre bundle, dimensions and the layering pattern of the elementary fibres and the main microfibril angles could be measured. This information allows further selection of fibres based on growing conditions, location in the culm, etc.The mechanical properties such as strength and Young's modulus of the elementary fibre were calculated using a novel approach that combines the micromechanics of composite materials, commonly used for unidirectional short fibre composites, and the fibre microstructure. The estimated Young's modulus of the elementary fibre is 50 GPa and the tensile strength 1150 MPa for an aspect ratio (length/diameter) larger than 38. Also, the failure modes of single fibres after tensile testing are analyzed by microscopic observations, to have an indication of the stress development in the elementary fibres and the different failure mechanisms.The impact of moisture on the mechanical properties of bamboo fibres was studied focusing on the effect of different humidity levels. Due to high variation in the obtained results, it was difficult to get statistically significant results. Nevertheless, the fibres are influenced by the humidity treatment. It was found that an increase in moisture level induces an increase in ultimate strain. Increase in humidity changes the fibre behaviour from a brittle behaviour to a more ductile behaviour. Remarkably, the fibre modulus hardly decreases at high humidity.To verify the effect of moisture on the mechanical properties of bamboo fibre composites, unidirectional composites were acclimatized to extreme humidity conditions. While the mechanical properties of the individual fibres did show little impact by the humidity, the bamboo fibre composites were more influenced when they were acclimatized. A high humidity level induces plasticization in the composite. Also there is a significant increase in strength due to the presence of moisture.During the preparation of bamboo fibres to be used as reinforcement in polymeric matrices, the fibres are exposed to different humidity states. It is therefore necessary to study the effect of these changes of humidity on the mechanical properties of the fibres. The effect of harsh wetting and drying cycles on bamboo technical fibres was investigated. Technical fibres were subjected to three, eight and thirteen cycles of wetting and drying. Tensile test results did not show a significant decrease of Young's modulus, irrespective to the amount of cycles. On the other hand, a slight decrease (-10%) in tensile strength was observed as the fibres were subjected to longer cycling conditions. This slight decrease has a linear tendency. It is thus advised to prevent the technical fibres from wetting/drying conditions as much as possible to prevent irreversible damages in the fibre structure.Five samples of cross-ply bamboo/epoxy composites were kept in environmental conditions and five samples were subjected to three wetting/drying cycles. No significant influence of moisture on mechanical properties was observed compared to untreated composites. SEM images of the cross-ply composites' surfaces show brittle fracture surfaces. This can be explained thanks to the good bonding between the epoxy resin and the bamboo fibres. It was observed that the cracks are more prone to propagate through the technical fibres, hence following the middle lamellae, rather than through the interface.The last part of the research was focused on the study of the transverse tensile and off-axis behaviour of unidirectional bamboo fibre composites, where the fibres were placed at a defined angle (30, 45, 60 and 90 degrees). The calculated value of the transverse tensile modulus of bamboo fibre is close to the modulus of the neat resin. As is the case for other unidirectional fibre composites, the composite Young's modulus was affected when the fibres were not placed in the load direction. Results indicated the high anisotropy of bamboo fibres which is a common characteristic for natural fibres. Also, the composite shear modulus was estimated based on the relationship between off-axis Young's modulus, and the principal properties of a 2-D composite ply.In conclusion, the results obtained during this doctoral research allowed us to confirm that bamboo fibres have adequate technical characteristics to be used as reinforcement in different composite applications, and are an actual alternative to other natural and synthetic fibers, currently used in the composites industry. In addition, the development of bamboo fibres can not only contribute to valorise the abundant natural resource but also, to stimulate the correct exploitation and management of bamboo plantations, which can have positive social, environmental and technological impacts.
Publication year:2022
Accessibility:Open