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The strength of carbon fibres in unidirectional hybrid composites of glass and carbon fibres have been investigated. It has been shown that the strength distributions of bundles of carbon fibres impregnated with resin, both unsupported and in a hybrid, may be described by the Weibull model confirming earlier reports. The primary objective of this work has been to investigate the strength and the state of dispersion of the carbon component. Hybrid composites consisting of accurately aligned arrays of bundles in two and three dimensions have been fabricated and tested. It has been shown that the strength decreases when the bundle spacing is less than a critical distance but at very low spacings the strength begins to increase again. This observation is explained with reference to the fracture behaviour and the implications for practical composites are discussed. The hybrid composites exhibit a greater strength over impregnated bundles. An hypothesis is proposed to explain this phenomena which combines thermal effects with the constraining influence of the glass and the differences in the severity of flaws in hybrid bundles. The last two arguments result in a larger critical group of failed fibres being required in the hybrid before catastrophic failure occurs. The type of hybrid specimen tested in this work enables the fracture process in the composite to be followed closely. The investigation has in principle supported the model for composite strength proposed by Batdorf, the "critical i-plet" model. However, experimental evidence indicated that a slightly different fracture process to that proposed by Batdorf was operating. The significance of this fracture process with respect to the strength and the size effect in composites is discussed.

This study investigated the application of fibre reinforced composite materials to spacecraft sandwich structures. In particular, aspects of the manufacture, analysis and design optimisation of components fabricated using the co-cure process were studied. The manufacturing process was developed to ultimately enable a full size thrust tube structure to be built using a single step cure, the design of which was verified by a modal survey test. Techniques for the analysis of stiffness, strength., vibration frequencies and local instability were established and found to correlate well with tests on co-cured sandwich specimens. The current wrinkling theory for composite faced sandwich was extended to the more general case to allow facesheet constitutive matrix coupling and multiaxial loding to be accomodated. The analytical methods were incorporated within simple optimisation schemes, amenable to employment at the preliminary design stage, to allow alternative feasible designs for panel and thrust tube structures to be generated. These illustrated the benefits of the use of composite materials and the co-cure manufacturing technique for spacecraft sandwich components.

The classical problem of pullout of a long elastic rectilinear round bar (fiber) embedded in an elastic half-space (matrix) is considered before and while local debonding occurs. An approximate analytical solution derived from the elasticity theory, the intuitive Saint Venant's principle, the idea of boundary layer in hydrodynamics, and invariant Γ-integrals is presented. The problem is analyzed numerically by means of the finite element method using the ANSYS program. The cases of loading before and after the initiation of the debonding are studied. Both approaches, analytical and numerical, are compared in order to establish the concidence between them. The discrepancy is very small in the global sense though substantial differences appear at particular points.

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Previous work on tensile and compressive failure of both monofibre and hybrid unidirectional composites is reviewed, together with a summary of some of the more important works on the variation of elastic modulus.

A morphing aircraft can be defined as an aircraft that changes configuration to maximize its performance at radically different flight conditions. Morphing structures require a large aspect ratio and area change during flight in order to optimise operational performance. Morphing wings are being developed to mimic bird’s wing movements. Birds have different wing profiles at different points in their flight, where swept wings reduce the drag at higher speeds at flight lift-off and long straight wing profile is better for performance at low loitering speed. Hyper-extensible braided cords have been developed to be used within morphing ‘skin’ materials. The cords use a low-modulus elastomeric core braided around with high-modulus yarns. These cords can be produced with various braid angles, which influence the extensibility of the cords. The higher the braid angle, the greater the extension The braid angle is controlled by the precision pre-tension of the elastomeric component. A computational model for predicting the load-strain behaviour of these hyper-extensible cords has been developed. Opposite to hyper-extensible cords are inextensible cord reinforcement composites, such as toothed timing belts used in car engines, which utilise a combination of reinforcement techniques to guarantee a high quality high strength product. Braiding is an alternate technology for producing cords with potentially superior performance in terms of improved ability to resist unravelling as well as superior interface due to ‘Chinese finger-trap effect.’ Carbon core with varying glass fibre braid have been developed. This led to various braid patterns being formed. A system for mapping braid pattern/topology has been developed. Aswel as the braid pattern, the braid colour patterns can also be produced. This mathematical model involves basic matrix manipulations, which have been proved using the MatLab program. The predicted braid patterns have been compared with actual samples. Being able to model braid patterns is a time and cost effective compared to previous trial and error methods.

A thesis submitted in compliance with the requirements for the Master's Degree in Technology: Mechanical Engineering, Durban Institution of Technology, 2003. / Composites have become important engineering materials, especially in the fields of automotive, aerospace and marine engineering. This is due to the high specific strength and stiffness properties they offer. At present, fibre-reinforced plastic (FRP) laminates are some of the most common types of composite used. They are produced in various forms with different structural properties. As with all engineering materials, there is the existence of both advantages and disadvantages. One of the main disadvantages is the expense involved in producing both the material and the finished product. The design time is also costly as the material has to be designed concurrently with the structure. / M

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