The diffusion rate of a large range of solvents into carbon fibre reinforced PEEK (APC-2) has been measured to discover the effect of the physical characteristics of the solvent. Three dimensional graphs have been plotted which correlate four parameters (solvent size, shape, hydrogen bonding capacity and solubility parameter) to solvent uptake. In the composite the effects of: thickness, lay-up, background water content and strain level on solvent diffusion have been assessed. The effect of composite thickness can be predicted using the film thickness scaling law provided the diffusion is Fickian. The effect of background water content is small, tending to make the diffusion profile two-stage. The effect of lay-up has been shown to have a major Affect on diffusion rate, unidirectional lay-ups having a much slower diffusion rate. Several theories have been postulated to explain this behaviour. The effects of stress on diffusion rate can be predicted by free volume models, provided that the stress/strain is kept below a certain critical level. It has been shown that the damage caused by a solvent, provided the stress does not exceed a critical value, is dependent on the amount of solvent in the matrix. This is due to plasticisation effects. Attempts to model this behaviour using free volume models have proved successful. Stress has been shown to enhance environmental attack. With certain solvents, above a critical stress or strain, environmental stress cracking occurs, leading to a considerable reduction in mechanical properties. Photographic evidence shows that cracking is initiated at stress concentrators within the matrix. Crack propagation is entirely matrix related and independent of spherulite boundaries. Overall, APC-2 has been shown to possess excellent environmental resistance when used in aerospace applications.

The electrical resistivities of typical AWG20-Class3 and AWG18-Class27 Ni-coated Cu wires were monitored at 400 ºC for times up to 5500 hours; the resistivities increased by 6.9% and 2.3%, respectively. Microstructural analysis of the thermally aged wires revealed evidence of Ni-Cu interdiffusion. Diffusion experiments were performed on Ni-Cu metal foils in the range 400 to 600 ºC; Ni-Cu compositional profiles across the Ni-Cu interface were collected by energy dispersive X-ray spectrometry. Ni-Cu interdiffusivities determined by the Boltzmann-Matano method were typically 2.5×10-17 m2s-1;calculated activation energies for Ni-Cu interdiffusion were between 79.4 and 89.8 kJ•mol-1. Analysis of the available Ni-Cu interdiffusion data suggested a dependence on grain size of the Cu foils used. A concentric-circle, diffusion-resistivity model was developed. Using the experimentally determined Ni-Cu interdiffusion data, it was possible to accurately predict the resistivity of a Ni-coated Cu wire at 400 ºC as a function of time. It is predicted that the resistivity of the AWG20-Class3 wire would increase by 10% after annealing for 48,000 hours at 400 ºC; in contrast, heating an AWG18-Class27 wire for a much longer time of 140,000 hours would incur the same increase in its resistivity. Low temperature co-fired ceramics (LTCC) with a formulation of 11ZnO-10MoO3 (NSZM) were prepared with additions of 0.5 to 2.0 wt% B2O3 via the mixed oxide route. The NSZM samples were sintered at 850-950ºC to over 96% of theoretical density with co-existence of both ZnMoO4 and Zn3Mo2O9 phases. With increasing the addition of B2O3 to NSZM the relative permittivity, dielectric strength and thermal conductivity increased. NSZM prepared with 1.0 wt% B2O3 exhibited a relative permittivity of 11.1, dielectric strength of 17.6 kV•mm-1, linear thermal expansion of 4.7 ppm•K-1and thermal conductivity of 1.3 W•m-1•K-1. The LTCC material is a possible candidate for insulating applications because of its low dielectric constant and adequate dielectric strength. LTCC insulation films were applied to Ni disc substrates by dip coating; the suspensions contained 5 to 20 vol% NSZM ceramic powders, 1.0 wt% B2O3, a polyvinyl butyral (PVB) based binder system, plus solvents and organic additives. A microstructural study of the LTCC films revealed that the insulation thickness varied from 4.3 to 47.3 µm with the ceramic content of starting suspension. The dielectric strength of these films was in the range 24.2 to 43.7 kV•mm-1. These results showed that dip coating is a promising method for applying the LTCC insulation to Ni-based metal substrates. LTCC-insulated wires were manufactured by withdrawing Ni-coated Cu conductors from the suspension, containing 15 vol% ceramic powders, followed by co-firing at 500 ºC. The LTCC-coated wire exhibited an insulation thickness of 40.3 µm and a breakdown voltage of 798 V. These results suggest that the LTCC-coated wire is a possible candidate for use in high temperature machine windings.

The concept of More Electric Aircraft, where is to utilize the electrical power to drive more or all aircraft subsystem instead of conventional combination of pneumatic, hydraulic, mechanical and electrical power, can be recalled to World War II. It has been proven to have more advantages for decades in terms of energy efficiency, environmental issues, logistics and operational maintenance. It can also enhance aircraft weight, volume and battle damage reconfigurability.Thanks to the new electronics technologies and the emergence of new materials, It becomes feasible for high power density electrical power components to drive the majority of aircraft subsystem. However, sustaining the transmission of hundreds of kilowatts of electrical power at low voltages is not feasible owing to the penalties incurred due to high cable weights and voltage drop may become critical. It is very easy to come up with the solution of the increase of voltage. However, higher voltage will introduce other problems such as the reliability of insulation coordination on the aircraft due to the increased probability of electrical discharge. For aircraft designers, it is very important to understand the rules of insulation coordination on the aircraft including determining clearance and creepage distances, and also have a clear investigation of the phenomena and mechanism of electrical discharges. Past research has identified a number of the concerns of operating electrical systems at higher voltages in an aerospace environment, especially for dimensioning of clearances. However, there is little study on dimensioning of creepage distances and relevantly flashover and electrical tracking on solid insulating material surfaces. This thesis firstly discusses the rules for determining clearances and creepage distances. The experimental validation work was done for breakdown in air gap and on the solid insulating material surfaces under dry condition so that some standard recommended values can be evaluated not only with theoretical values such Paschen's law. Suggestions of application of those standards were provided. Secondly, the complex electrical discharge under wet condition on solid insulating material surfaces was discussed. A mathematical model to predict this type of electrical failure -electrical tracking (the electrical discharges on solid insulation materials which will lead to physical damage in the materials) with the consideration of different environmental conditions including air pressure, ambient temperature, and pollution degrees was developed. A series of electrical tracking tests were carried out on organic materials to find out the mechanism of electrical tracking and validate the finding by the mathematic model. Finite element analysis simulations were also conducted to find out the background thermal transfer mechanism to support our explanation of those phenomena of electrical tracking. Different test techniques have ben developed for specific impact factors. Finally, the suggestions for utilization of the standards and feasible test techniques for electrical tracking under wet conditions were provided.

No description available.

Titanium alloys are important engineering alloys that are extensively used in various industries. This is due to their unique combination of mechanical and physical properties such as low density combined with high strength and toughness as well as outstanding corrosion resistance. An additional benefit associated with Ti alloys, in general, is that their properties are relatively temperature-insensitive between cryogenic temperature and ~500 °C. Amongst the Ti alloys, Ti-6Al-4V (referred as Ti64) is a widely used alloy. Conventionally cast Ti64 possesses classical Widmanstätten microstructure of (hcp) α and (bcc) β phases. However this microstructure suffers from large prior β grain size, which tends be in the order of a few mm. Such large grain sizes are associated with poor processability as well as inferior mechanical performance. The necessity to break this coarse as-cast microstructure down, through several successive thermo-mechanical processing steps, adds considerably to the cost of finished Ti alloy products, making them expensive vis-à-vis other competing alloys. The addition of small amount of B (~0.1%) to Ti64 alloys, on the other hand reduces the cast grain size from couple of mm to ~200 µm. Moreover, addition of B to Ti alloys produces the intermetallic TiB needles during solidification by an in situ chemical reaction. The overall objective of this work is to gain insights into the role of microstructural modifications, induced by B addition to Ti64, on the mechanical performance of the alloys, in particular the room temperature damage tolerance (fracture toughness and fatigue crack growth) characteristics. The key questions we seek to answer through this study are the following: (a) What role does the microstructural refinement plays on the quasistatic as well as fracture and fatigue behavior and high temperature deformability of the alloys? (c) A hierarchy of microstructural length scales exist in Ti alloys. These are the lath, colony and grain sizes. Which of these microstructural parameters control the mechanical performance of the alloy? (b) What (possibly detrimental) role, if any, do the TiB needles play in influencing the mechanical performance of Ti64 alloys? This is because TiB being much stiffer, strain incompatibility between the matrix and the TiB phase could lead to easy nucleation of cracks during cyclic loading as well as can pose problems during dynamic deformation. (d) What is the optimum amount of B that can be added to Ti64 such that the most desirable combination of properties can be achieved? Five B-modified Ti64 alloys with B content varying from 0.0 to 0.55 wt.% were utilised to answer the above questions. Marked prior β grain size reduction was noted with up to 0.1 wt.% B addition. Simultaneous refinement of α/β colony size has also been observed. The addition of B to Ti64, on the other hand increases the α lath size. The TiB needles that form in-situ during casting are arranged in a necklace like structure surrounding the grain boundaries for higher B added Ti64 alloys. An anomalous enhancement in elastic modulus, E, of the alloy with only 0.04 wt.% B to Ti64 was found. E has been found to follow the same trend of variation with B content at higher temperatures (up to 600 °C) as well. Nanoindentation experiments were conducted to evaluate the moduli of the various phases present in the microstructure and then rationalize the experimental trends within the framework of approximate models. Marginal but continuous enhancement in strength of the alloys with B addition was observed. It correlates well with the grain size refinement according to Hall-Petch relationship. Ductility on the other hand increases initially with up to 0.1 wt.% B addition followed by a reduction. While the former is due to the microstructural refinement, the latter is due to the presence of significant amount of brittle TiB phase. Room temperature fracture toughness decreases with B addition to Ti64. Such reduction in fracture toughness with the refinement of prior β grain size has been justified with Ritchie-Knott-Rice model. Contradictory roles of microstructural refinement have been observed for notched and un-notched fatigue. While reduction in length scale has a negative role in crack propagation, it enhances the fatigue strength of the alloy owing to better resistance to fatigue crack initiation. TiB needles on the other hand act as sites for crack initiation and hence limit the enhancement in fatigue strength of alloys with 0.30 and 0.55 wt.% B. An investigation of the high temperature deformability of the alloys has been performed over a wide range of temperature (within the two phase α+β regime) and strain rate windows. Results show that microstructural refinement does not alter the high temperature deformation characteristics as well as optimum processing conditions of the alloys. TiB needles, however act as sites for instability owing to differences in compressibility between the matrix and the whisker phase. In summary, this study suggests that the addition of ~0.1 wt.% B to Ti64 can lead to the elimination of certain thermo-mechanical processing steps that are otherwise necessary for breaking the as-cast structure down and hence make finished Ti components more affordable. In addition, it leads to marginal enhancement in the quasi-static properties and significant benefits in terms of high cycle fatigue performance.