This thesis has presented the results and findings of a study carried out into the effects of fibre pre-stressing on the impact performance of composite laminates. Fibre prestress has been explained as a way of mechanically altering the internal residual stress state of a composite, which typically is a result of thermal, moisture and chemical expansions. It has been suggested that pre-stressing can offer potential benefits to composites by reducing or reversing the hygro-thermal stresses in a composite. It has also been suggested that the impact performance could be improved through fibre prestressing, which has given rise to this study. In this study panels have been made with various levels of pre-stress. A special system was developed to apply pre-stress to the laminates and the produced laminates were tested under low- and high-velocity impact regimes. To apply these regimes, an instrumented falling weight and a gas gun were used respectively. A short finite element study was carried out to supplement the experimental study and offer further insight into the failure mechanics. The main findings of the study were that although pre-stressing had no discernable effect on the high-velocity impact performance of the composite laminate considered, there was a noted effect on the low-velocity impact performance. Under low-velocity impacts the laminate showed an improved impact performance for increase levels of pre-stress, except at one critical pre-stress level (60 MPa), where the laminate absorbed less energy per damage area compared with unpre-stressed laminates.

The formation of poly (ethylene terephthalate) (PET)/organo-montmorillonite and poly (vinyl alcohol)(PVOH)/ montmorillonite nanocomposites and the diffusion behaviour of water and acetone/water mixtures into the latter have been investigated. Nanocomposites of PET and various, commercially available, organoclays have been prepared by solution intercalation and the structure of the resulting composite investigated in dependence of surfactant on the organoclay, clay loading, solvent, stirring time, polymer concentration, polymer type and drying temperature. All samples prepared had an intercalated structure with layer spacing depending mainly on the type of surfactant present on the clay. Thermal stability of the samples was similar to that of PET, yet decomposition was found to start at temperatures up to 40 °C lower than for the pristine polymer. Formation of nanocomposites of PVOH and montmorillonite has been achieved by solution intercalation from aqueous solutions. For these samples the influence of molecular weight of the PVOH, clay loading, clay structure and interlayer cations has been investigated. PVOH/ clay (nano-) composites have been prepared over the full range of compositions, from "true" nanocomposites to PVOH adsorbed on clay. For clay loadings up to 10 wt% XRD silent nanocomposites have been obtained. Clay loadings between 20 and 40 wt% resulted in intercalated nanocomposites with wide ranges of layer spacings, while clay loadings of 45 - 75 wt% resulted in intercalated composites with a narrower distribution of spacings. Above this loading adsorption of PVOH onto clay with two distinct layer spacings could be observed. Results were similar for higher molecular weight PVOH. Thermal stability of these samples was also found to depend on the clay loading. An increase of the degradation onset temperatures by 10-20 °C was measured for nanocomposite samples. Lithium and sodium montmorillonites showed similar dispersion patterns. Charge reduction of the lithium clay had a strong influence on the dispersion of the clay. Lower charged layers resulted in poorer dispersion. Li+ MCBP fired at 210 °C did not form nanocomposites with PVOH independent of the clay loading. Diffusion measurements of water into PVOH showed strong influences of swelling, gelling and dissolution of the samples. Generally, diffusion into the nanocomposites showed shorter time delays before it became measurable, yet the diffusion coefficient decreased with increasing clay content. Diffusion was found to be dependent on the dispersion of the clay with microcomposite structures resulting in better barrier properties than their nanocomposite counterparts. Higher temperatures resulted in faster diffusion rates. During the diffusion of water crystalline regions of the PVOH were dissolved and the clay remained dispersed in the swollen PVOH.Diffusion of acetone/ water mixtures was found to be strongly dependent on the concentration of water in the diffusant. In mixtures with an excess of water or a molar ratio of acetone: water of 1:1 the diffusion of acetone and water proceeded at the same time in PVOH and its nanocomposites. This has been attributed to formation of acetone/ water complexes. At excess levels of acetone in the diffusant acetone diffusion is delayed and occurs at a slower rate. Presence of clay in these samples leads to longer delay times before diffusion can be measured and slower diffusion rates. Microcomposite samples were again found to have better barrier properties than the nanocomposites and it is assumed that partial delamination of the clay layers in these samples increases the aspect ratio of the clay. Swelling is found to decrease with decreasing water content as well as increasing clay content. Crystallinity of the polymer is initially decreased, yet some crystallinity is recovered over the course of the experiment in the neat polymer. Presence of clay reduced the extent to which crystallinity was recovered. Analysis of the hydrogen bonding of the water within the polymer in the equilibrium spectra showed decrease of strongly and weakly hydrogen bonded water with increasing acetone/ contents in the diffusant. In the nanocomposites only decrease of the weakly hydrogen bonded water could be observed. Following the changes in hydrogen bonding over the course of the experiments showed increases in weakly hydrogen bonded water while strongly hydrogen water decreased due to break down of the hydrogen bonding network. Hydrogen bonding of the polymer also decreased due to swelling of the polymer.

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Components found within many industries contain crack like defects. The work detailed here considers such a component under the combined influence of primary and secondary stresses; where primary stresses contribute to plastic collapse and secondary stresses are redistributed under plastic deformation. A number of approaches are available to detail the combined loading on the crack tip parameter J, or KJ, which is used to assess proximity to failure from crack extension. However, these approaches are recognised to be conservative and can lead to the unnecessary replacement of components, stricter surveillance and inspection regulations, and further costs associated with downtime.The aim of the work presented is to investigate these conservatisms and develop a further approach to quantify the interaction of primary and secondary stresses on fracture. A large matrix of cracked body finite element analyses of a circumferentially cracked cylinder has been performed under a range of loadings. This is then used to detail the interaction of primary and secondary stresses on fracture by providing a function to describe a scaling term, g, that multiplies the secondary crack driving force contribution. This term has been shown to be relatively independent on the magnitude of secondary stresses and is also dependent on the material stress strain relation. This relation for g has also been shown to be compatible with the R6 defect assessment procedures V factor approach, through the Vg plasticity interaction term, that provides a scaling term to the secondary contribution in R6. A review of experiments considering combined loading has indicated that the number of tests that cover a range of primary stress induced plasticity levels is limited. Further experiments were therefore considered within this research to provide added experimental fracture toughness data by which to compare the R6 V factor and Vg approaches. These experiments introduced a compressive pre-load to the ends of three-point bend specimens so that a tensile residual stress resulted on unloading. A crack was introduced and the specimens tested at one of three temperatures so that changes in the materials fracture toughness with temperature ensured different levels of plasticity at failure; so that crack growth occurred over three sets of load normalised to the load for plastic collapse. Tests were also conducted that did not include the residual stress so that the effect of residual stress could be shown under different levels of plastic redistribution. The Vg Approach and the existing Complex R6 V Approach have then been applied to all available experimental data for validation. The results show that both approaches conservatively predict the failure of all tests and that the Vg Approach can reduce the level of conservatism.

The interpretation and modelling of the dielectric response of thermosetting materials during cure was the main focus of this study. The equivalence of complex permittivity and complex impedance in terms of information content was outlined in a series of case studies covering the separate effects of dipolar movements and charge migration as well as the combined effect of the two polarisation mechanisms. Equivalent electrical circuits were used in order to model the evolution of the complex impedance during cure. A numerical method that can model consecutive spectra throughout the cure was developed. The method is based on Genetic Algorithms and requires only input from the modelling of the initial spectra. Complex impedance spectra were collected during the cure of a commercial epoxy resin formulation under isothermal and dynamic heating conditions. The spectra were analysed and modelled. The modelling was successful over the whole frequency range of the measurements (1 Hz – 1 MHz). The analysis of the estimated model parameters showed that charge migration dominates the dielectric response in a wide frequency range. In addition, the modelling algorithm also distinguished between the effects of electrode polarisation and dipolar movements in the signal. A new equivalent circuit was used in order to map the frequency regions where the each one of the three phenomena that together comprise the dielectric signal can be monitored most effectively. A chemical cure kinetics model was developed for the studied system. A correlation between the maximum point of the imaginary impedance spectrum and the reaction conversion was established. A mathematical model, based on a simple linear dependence of the dielectric signal on conversion and temperature, was built. The model predictions agreed well with the experimental data. The aim of simplifying the interpretation of the dielectric signals led to the development of a new experimental technique. Temperature Modulated Dielectric Analysis employs temperature modulations superimposed on an underlying thermal profile in order to separate the influence on the signal of the temperature alone from that of the cure reaction. The early study carried out here shows that such measurements are feasible and reveals important issues for its further development.