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Composite materials based on natural bast fibres offer potential commercial and environmental benefits due to the low cost, availiability, and biodegradability of the fibres. However, such benefits cannot be realised without a comprehensive evaluation of processing and properties. This thesis involved a comprehensive evaluation of composites based on two types of natural bast fibre (hemp and flax), and two types of matrix - synthetic (epoxy), and biodegradable (Novamont Mater-Si). The experimental work involved four strands: the effects of growing conditions and fibre processing on the properties of raw bast fibres; the optimisation of a pultrusion process for epoxy-matrix composites; development of a film stacking process for Mater-Bi composites, and a detailed evaluation of the mechanical properties of the composites themselves. Fibre bundles and individual fibre cells were investigated to characterise their structure, with particular focus on compressive kink defects. The kink bands were sectioned using a novel technique of focused ion beam milling, and kinking was found to induce delamination and voiding of the lamellar fibre structure. The defect concentration per unit length was assessed for conventionally-processed fibres and for hemp fibres from plants grown under controlled conditions to assess the effect of wind shear and stem flexure on fibre defect concentration. No effect was found for plant flexure, while industrially processed fibre was found to have increased defect concentration. The loading behaviour of both types of composite was seen to be initially linear with a yield point at 20 - 30 MPa and a transition to nonlinear deformation dominated by damage mechanisms as a result of fibre kinks. Epoxy composites possessed an inital modulus of 30 GPa with a 30 - 60% reduction in modulus after yield. Flax reinforcement was found to increase the modulus of Mater-Bi from 0.1 to 20 GPa and strength from 24 to 169 MPa. Fibre addition was also found to significantly embrittle the polymers.

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A new interferometric method which indirectly measures the total chromatic dispersion of a single mode fiber is demonstrated. The technique utilizes a short length of fiber, an unmodulated broadband source, simple low frequency electronics, and a standard interferometer. The concept of this measurement is based on the behavior of the uncorrelated individual bursts of light from the elemental emitters that constitute a thermal source. Their propagation through a dispersive media, e.g., silica fiber, which is placed in one arm of the interferometer, is delayed and broadened. They will interfere with their counterpart from the other arm, generating a train of time-varying fringes as one mirror of the interferometer is uniformly translated. The local frequency of the fringes at a given position of the moving mirror is a direct measure of the instantaneous wavelength, while the mirror position itself demarks the corresponding relative delay. A colinearly launched HeNe laser beam is used as a reference to calibrate the other source's fringe width and location of the mirror. In this experiment, an edge-emitting LED of λo = 830 nm and Δλ = 60 nm was used. The tested fibers had a length of 27.9 cm and 38.3 cm, which made the width of the crosscorrelation function approximately 100 times greater than the source's coherence length. The speed of the mechanically driven mirror set the frequency of the HeNe fringes to approximately 800 Hz with an r.m.s. fluctuation around the mean of 0.2%. The SNR of the HeNe fringes was four times larger than the LED's. Ten different runs for each fiber were executed. Data from the two sets of simultaneous measurements of delay versus wavelength were used to fit the best linear and quadratic polynomials with a minimum residual mean error square. The derivative of this function with respect to wavelength gave the dispersion relation. The accuracy of measured delay and wavelength were 0.1 ps and 6 nm, respectively. The dispersion value and its standard error for the best linear fit was approximately 117 ∓ 2 ps/km nm. The standard error for the quadratic fit was much larger due to the high noise level accompanying signal. A thorough investigation of the noise sources, accuracies, standard error of the polynomial's coefficient, and SNR analysis is conducted. This measurement is simple and has the potential of achieving substantially higher accuracy--especially for the longer wavelength region where dispersion is minute.

A Research Report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfillment of the requirements for the degree of Master of Science in Engineering(Mechanical Engineering 30 January 2017 / An extensive variety of experimental techniques exist to determining residual stresses, but few of these techniques is suitable, however, for finding the residual stresses that exist in orthotropic or anisotropic layered materials, such as carbon-fibre reinforced polymers (CFRP). Among these techniques, particularly among the relaxation techniques, the incremental hole-drilling technique (IHD) has shown to be a suitable technique to be developed for this purpose. This technique was standardized for the case of linear elastic isotropic materials, such as the metallic alloys in general. However, its reliable application to anisotropic and layered materials, such as CFRP materials, needs to be better studied. In particular, accurate calculation methods to determine the residual stresses in these materials based on the measured in-depth strain relaxation curves need to be developed. In this work, existing calculation methods and already proposed theoretical approaches to determine residual stresses in composite laminates by the incremental hole-drilling technique are reviewed. The selected residual stress calculation method is implemented using MATLAB. For these calculations, specific calibration coefficients have to be numerically determined by the finite element method, using the ANSYS software. The developed MATLAB scripts are then validated using an experimental procedure previously developed. This experimental procedure was performed using CFRP specimens, with the stacking sequence [0o, 90o]5s and, therefore, this composite laminate was selected as case study in this work. Some discrepancies between the calculated stresses using the MATLAB scripts and those imposed during the experimental calibration procedure are observed. The errors found could be explained considering the limitations inherent to the incremental hole-drilling technique and the theoretical approach followed. However, the obtained results showed that the incremental hole-drilling can be considered a promising technique for residual stress measurement in composite laminates. / MT2017