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<p>NR 20140804</p>

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The effect of orientation and crystallinity of certain polymers, polyethylene, polypropylene, polyethylene terephthalate (PET) and poly (ether ether ketone) (PEEK) , upon the extent and nature of plasma oxidation was studied. It was found that increasing the extent of surface ordering lessened susceptibility to plasma oxidation and reduced the subsequent decay of surface treatment. The surface ageing of plasma oxidised PEEK was extensively studied with regards to the transient increase in hydrophilicity that has been observed after plasma modification. The decay and transient increase in hydrophilicity were found to be dependent upon crystallinity and storage temperature. An estimate of the activation energies for processes leading to the increase in contact angle after plasma modification were calculated and found to suggest that these processes were rotational reorganisations at the surface as opposed to migrational reorganisations. The decay of other plasma modified surfaces revealed that plasma oxidised PET and plasma fluorinated PEEK both underwent transitional reorganisations at the surface, however no such change was observed for ammonia plasma treated PEEK. Plasma modification of carbon fibres was investigated with regards to improving composite performance. Microwave plasma treatments were found to be as good as standard commercial treatments. Graphitic carbon was investigated as a model for carbon fibre surfaces, however, the plasma modified surface was found to age more readily and to be too labile for useful comparison.

Manufacturing of textile preforms involve preform compaction which influences the fibre volume fraction and level of crimp in the final laminates affecting the laminate properties. The preform compaction behaviour is highly non-linear and depends on a number of tow-level factors which in turn is guided by filament-level interactions. Hence experimentally predicting the compaction behaviour of a preform, made of large fibre bundles, remains as an obstacle to the understanding of the compaction mechanics due to the stochastic effects of filament-level interactions. This thesis proposes a novel multi-scale modelling technique which predicts the compaction behaviour of large fibre bundles or tows. The model considers real inter-fibre frictional interactions; the friction coefficients are obtained by carrying out friction tests on carbon fibres. Since the inter-fibre friction varies with the inter-fibre orientation, experiments are done to study the effects of fibre orientation on friction. The tests have shown a significant increase in coefficient of friction (from 0.2 to 0.45) for parallel tows due to bedding and entanglement of fibres in comparison to the friction between perpendicular tows. Modelling of the filament-level compaction behaviour requires inter filament friction coefficient which is not equal to the tow friction. In addition, the filaments within a tow can slip relative to each other. Therefore, inter filament friction can influence tow friction. Hence filament friction is determined from tow friction and used in the compaction models. Numerical models of compaction of large fibre bundles are developed which use this experimentally-obtained fibre friction coefficient as input. The solid model requires extensive computational effort. A two-dimensional (2D) model has been developed where the bending and torsional behaviour are incorporated with the help of springs. This 2D model has resulted in improved computational efficiency compared to the solid model (that is, a 99% improvement in CPU time for a 37 filament assembly). The model is then extended to tow- and fabric-levels. The tow-scale results are in close agreement (~5%) with validation tests. A further 3D modelling technique using beam elements has been presented as a further scope which is able to use the level of compaction obtained from the 2D model and also overcomes the limitations of the 2D model. This 3D modelling technique has shown 88% reduction in CPU time compared to that of solid model of same fibre bundle.

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Laser Raman Spectroscopy (LRS) has been employed to characterise the structure of carbon fibres, the effect of surface treatment and the response of the material to externally applied loads. The strain sensitivity provided a unique relationship between the applied strain and the Raman frequency for each type of fibre, termed as the Raman Frequency Gauge Factor. After examining a wide range of fibres, of various Young's moduli and various manufacturing routes, it was concluded that both tensile and compressive properties of carbon fibres can be improved by controlling the fibre morphology during manufacture. This morphological control seems to achieve its objectives by reducing the skin-core effect in the fibre structure. The result of such an alteration can be detected in tension by the increase of the initial fibre modulus and in compression, by the absence of premature catastrophic type of failure. Nevertheless, non-linear stress-strain phenomena seem to be a permanent feature of all carbon fibres and the significant modulus softening in compression appears to determine the limits of the fibre compressive strength. The load transfer mechanism at the carbon fibre/epoxy resin interface has been subsequently investigated during the fibre fragmentation process in a single fibre model composite. The fibre strain distribution along the fibre fragments has been derived through the Raman spectrum of the fibre and its Raman Frequency Gauge Factor. In turn, the interfacial shear stress distribution has been evaluated using a simple balance of forces model. The maximum shear stress, allowed to develop at the f ibre/matrix interface, has been considered as a reasonable estimate of its interfacial strength. It was concluded that both the fibre surface treatment and the use of a lower modulus filament can increase the system's interfacial strength, reduce debonding propagation and withhold the interfacial yielding in the vicinity of the fibre discontinuities.

Textile process and textile structures that are suitable for composites are carefully studied and chosen to have weft knitted fabrics. The aim of this research is to knit the carbon and Dyneema® fibres in circular weft knitting to fit contour shapes. Carbon/Dyneema® can also be knitted in warp knitting machines to get properties in multi axial direction. But the fabric was flat and can be used only for 2D shape products which are having less drapabiity. According to previous research, weft knitting is the best suitable for complex preforms. Before knitting these fibres properties were studied in order to avoid the damage to the carbon fibres. The carbon fibres have high bending rigidity, low resistance to friction and are very brittle. A small damage to the carbon fibre in knitting subsequently affect directly on the composite properties. The strongest manmade fibre manufactured till date is Dyneema® and these fibres could be used in composites due to its performance, properties and light weight. But, the Dyneema® fibres are expensive when compared to common polyester, so polyester fibres are used to compare the properties and cost performance ratio. The critical bending of the carbon fibres causes friction between the fibres and also between fibre and machine. This was considered carefully during the knitting of carbon fibres and the idea chosen is mentioned in this thesis. Between the two layers of Dyneema®/polyester, carbon fibres are laid circularly in unidirectional and in un-crimped condition. This makes the carbon yarn to possess good mechanical properties. The 2 layers of Dyneema®/polyester fibres exchange the loops at certain points to increase the inter-laminar strength and decrease the carbon fibre distortion. This structure helps to withstand external load. It is also lighter than the carbon composite with additional properties. This makes much more space in the future for the Dyneema® fibres in the 3D carbon composite manufacturing. The internal carbon fibres are fully covered by the Dyneema® fibres to withstand the external impact load and not to damage the carbon fibres. So the loop length, stitch density, fibre volume fractions are considered before knitting.