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Elementary fibres isolated from mechanically processed technical hemp were axially sectioned and imaged with transmission electron microscopy (TEM) to reveal details of the axial morphology of dislocations in the fibre. The overall aim was to investigate the detailed axial structural changes that the fibres undergo during processing, to help better understand the alterations in the deformation behaviour the fibres undergo following processing. The images showed the structure and morphology of dislocations as well as the different forms of damage that processing produced in the fibre structure, such as misalignment of the microfibrils, delamination, and buckled cellulose microfibrils. Furthermore, the results of this work show the ability that axial sectioning of the fibre has to reveal new details of the cell wall structure of hemp to offer new insights in the study of the fibre structure. In turn, the results of this work may help explain the mechanical behaviour of the fibres when they are loaded, as well as help explain the greater chemical accessibility of dislocations, for example, when the fibre is acid hydrolysed.
This work investigated the impact that the processing of hemp (C. sativa L.) fibre has on the mechanical properties of unidirectional fibre-reinforced epoxy resin composites loaded in axial tension, and particleboard reinforced with aligned fibre bundles applied to one surface of the panel. For this purpose, mechanically processed (decorticated) and un-processed hemp fibre bundles, obtained from retted and un-retted hemp stems, were utilised. The results clearly show the impact of fibre reinforcement in both materials. Epoxy composites reinforced with processed hemp exhibited 3.3 times greater tensile strength when compared to the un-reinforced polymer, while for the particleboards, the bending strength obtained in those reinforced with processed hemp was 1.7 times greater than the un-reinforced particleboards. Moreover, whether the fibre bundles were processed or un-processed also affected the mechanical performance, especially in the epoxy composites. For example, the un-processed fibre-reinforced epoxy composites exhibited 49% greater work of fracture than the composites reinforced with processed hemp. In the wood-based particleboards, however, the difference was not significant. Additionally, observations of the fracture zone of the specimens showed different failure characteristics depending on whether the composites were reinforced with processed or un-processed hemp. Both epoxy composites and wood-based particleboards reinforced with un-processed hemp exhibited fibre reinforcement apparently able to retain structural integrity after the composite’s failure. On the other hand, when processed hemp was used as reinforcement, fibre bundles showed a clear cut across the specimen, with the fibre-reinforcement mainly failing at the composite's fracture zone.
Wood is an anisotropic material which exhibits different mechanical properties depending on the direction in which it is tested. In the longitudinal direction, Young's modulus is around 10 times higher than in the transverse one, but also different orientations in the transverse direction exhibit different mechanical properties. In this current research, two different sizes of Norway spruce specimens have been tested in compression in the transverse direction, radially and tangentially, to find how morphology affects the mechanical properties and how these properties differ from one direction to another. The important role of rays acting as beams keeping the integrity of the structure, as well as the differences between earlywood and latewood under external strains, has been elucidated. At the same time, some techniques have been developed in the production of small specimens, razor blade cutting being the most important one, being able to produce a flat surface and minimizing the damage in the cell walls, allowing the further possibility of imaging the surface at the mesoscale level with the microscope to obtain images of the cell's deformation. As a consequence of testing different specimen sizes, an unexpected effect associated with the specimen size arose that affected the values of mechanical properties. It was found that this effect was previously called size effect, and it is not only affecting wood but it is common in other cellular materials such aluminium or polyurethane foams, ceramics or sea ice.
The development of THz technology and communication systems is creating demand for devices that can modulate THz beams rapidly. Here we report the design and characterisation of high-performance, broadband THz modulators based on the photo-induced transparency of carbon nanotube films. Rather than operating in the standard modulation mode, where optical excitation lowers transmission, this new class of modulators exhibits an inverted modulation mode with an enhanced transmission. Under femtosecond pulsed illumination, modulation depths reaching +80% were obtained simultaneously with modulation speeds of 340 GHz. The influence of the film thickness on the insertion loss, modulation speed and modulation depth was explored over a frequency range from 400 GHz to 2.6 THz. The excellent modulation depth and high modulation speed demonstrated the significant potential of carbon nanotube thin films for ultrafast THz modulators.
The goal of this study is to investigate how processing of hemp fibre bundles affects their mechanical properties. The mechanical properties of interest are the ultimate tensile strength and the stiffness. Therefore, tensile tests of seven differently processed fibre bundle types have been conducted. The fibre bundle types varied in the applied method of retting (field retted, unretted, winter retted) and in the processing method (decortication, rollercarding, refining). The values obtained from the tensile tests and the diameter measurements conducted with the optical microscope were used to calculate the strength and stiffness of in total 226 fibre bundles (30 – 35 specimen per fibre type). It was expected that the mechanical properties (strength and stiffness) decrease with further processing of the fibre. However, the values obtained from the tensile tests did not show a clear trend. This could be accounted to the fact that the diameter measured under the optical microscope was incorrect due to tissue still adhering to the fibre bundles. Especially with the unprocessed fibres a distinction between the bark and the actual fibre was not possible with the optical microscope. Therefore the measured diameter was not that of the fibre bundle but of the fibre bundle plus the surrounding tissue which does not have a load bearing function. Due to the (wrong) high diameter the resulting strength and stiffness properties were very low. Other means were taken into account to describe the relationship between processing and fibre properties such as the load at which the fibre bundles failed and the share of non-linear stress-strain curves per fibre bundle type. It can be concluded that processing adds defects to the fibre cell wall and thus the mechanical properties strength and stiffness decrease with further processing. Nevertheless, processing is not solely responsible for defects in fibres which cause weak fibre properties. Considering only processing methods as a reason for fibre bundle properties is not sufficient. Growth conditions can also influence the occurrence of defects. Furthermore, inaccurate and varying means of determining fibre properties make it challenging to compare the results with findings from other authors. It is therefore concluded to standardize fibre bundle tensile testing and to not only focus on processing with regard to defects in fibres but also on upstream processing steps.