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In order to reduce CO2 emissions and improve fuel efficiency for the aerospace industry, a leading edge sheet metal forming technology, namely solution heat treatment, forming and in-die quenching (HFQ) was utilised to form lightweight, complex-shaped components, efficiently and cost-effectively. The work performed in this research project contains two major achievements. The first achievement is successfully forming a complex AA2060 (Al-Li alloy) wing stiffener demonstrator part, and an L-shape AA7075 demonstrator part, without necking or fracture, using HFQ forming technology. The feasibility of forming the aluminium alloys was based on a series of fundamental experimental tests including uniaxial tensile test, isothermal forming limit test and artificial aging test. The second achievement is the development of a novel forming limit prediction model, namely the viscoplastic-Hosford-MK model. This model enables the forming limit prediction of AA2060 and AA7075 alloys under hot stamping conditions, featuring non-isothermal and complex loading conditions. This prediction model fills a significant need in industry for accurately predicting the forming limit of aluminium alloys under such complex forming conditions. The effectiveness of the developed model was analytically verified for AA2060, demonstrating accurate material responses to cold die quenching, strain rate and loading path changes. By applying the developed model to the hot stamping of an AA2060 component, its accuracy was successfully validated. Furthermore, the viscoplastic-Hosford-MK model was also demonstrated for use in industry by determining the optimum initial blank shape of an L-shape AA7075 component. An iterative simulation procedure implementing the forming limit prediction model was used to arrive at an optimum blank shape by the minimisation of the failure criterion. The optimised initial blank shape design was applied in the experimental hot stamping of a demonstrator AA7075 component. The accuracy of the developed model was validated by the successful forming of the component, without necking or fracture.

To meet stringent control and emissions requirements, diesel fuel injectors need to be characterised accurately in terms of timing and rate of change of mass flow rate. Such characterisation is carried out with various rate metering devices which overwhelmingly utilise a liquid into liquid injection process. These devices have historically been hampered by unwanted 'noise' in the measurement signal whose source was poorly understood and mitigation relied on post processing filtering techniques. A model of a constant volume metering device with optical access was constructed and a hybrid schlieren laser imaging technique applied to the flow field external to the nozzle with simultaneous chamber pressure measurement. This technique is sensitive to the second derivative of density and so directly able to visualise pressure waves within the domain. LES simulations were also performed to extract relationships not available through experimental data. The experimental results show cloud cavitation in the shear layer of the jet whose vapour bubbles begin collapsing shortly after injection begins and persist more than 500 microseconds after the end of injection. Compression waves due to the collapse of cavitation bubbles are visualised directly and parameters such as their spatial origin and time are calculated. Compression wave diffraction, reflections and interaction between individual jets are demonstrated. The 'noise' in a constant volume chamber is therefore shown to actually be an accurate representation of the pressure field arising from the superposition of complex flow phenomena in the near field of the injector nozzle due to an interaction of pressure waves and cavitation. The novel use of a hybrid schlieren technique demonstrates the utility of this arrangement to fully three dimensional problems. Cavitation and its associated processes are shown to be the dominating force in liquid to liquid injection processes for the flows encountered in fuel injection metering.

Hot stamping and cold die quenching has been developed in forming complex shaped structural components of metals. This study is the first attempt to develop unified viscoplastic damage constitutive equations for the prediction of formability of metals under hot stamping conditions. In order to achieve the aim of this study, test facilities and methods need to be established to obtain experimental formability data of metals under hot stamping conditions. The research work is concerned with four aspects: thermo-mechanical properties of an alloy under hot stamping conditions, feasibility study of a novel biaxial testing system for hot stamping applications, formability tests by cruciform specimens under hot stamping conditions, and developed material models for formability evaluation and prediction. Hot tensile tests were performed at various temperatures and strain rates after heating and cooling processes to study the thermo-mechanical properties of AA6082 under hot stamping conditions. An error analysis of the proposed strain measurement method was carried out using an FE model coupled with thermo-electrical and thermo-mechanical conditions. The viscoplastic deformation behaviour of AA6082 was analysed in terms of temperature and strain rate dependence based on the experimental results. A viscoplastic damage constitutive model was developed to describe the thermo-mechanical response of the metal, material constants in which were calibrated from the hot tensile test results. A novel biaxial testing system was developed, patented and used for formability tests of AA6082 under hot stamping conditions after the feasibility study of this new testing method. Three heating and cooling strategies were proposed to investigate the temperature and strain distributions in a type of cruciform specimen. The dimensions of cruciform specimens adopted for the determination of forming limit under various strain paths were designed and optimised based on the selective heating and cooling method. Formability tests of AA6082 were conducted at various temperatures and strain rates after the heating and cooling processes. Two unified multi-axial viscoplastic constitutive models were developed and determined from the formability test results of AA6082 for the prediction of forming limit of alloys under hot stamping conditions. This research, for the first time, enabled formability data to be generated and forming limits to be predicted under hot stamping conditions. The technique has been verified for a particular aluminium alloy and can be applied to other metals under hot stamping conditions.

Thermoacoustic instabilities arise from the coupling of acoustic waves and unsteady heat release rate from combustion. This coupling can generate large pressure oscillations which may significantly reduce the life of aero-engine and ground-based gas turbines, or even lead to failure of the whole combustion system. Acoustic dampers, such as Helmholtz resonators, perforated liners and perforated plates, are widely used to absorb acoustic energy and thus damp these thermoacoustic oscillations. Such acoustic dampers typically comprise a circular hole with mean flow passing through, to convert acoustic energy into vortical energy, and finally into heat by viscous dissipation. A widely used analytical model [M.S. Howe. On the theory of unsteady high Reynolds number flow through a circular aperture, Proc. of the Royal Soc. A. 366, 1725 (1979), 205-223], which assumes an infinitesimally short hole, was recently shown to be insufficient for predicting the acoustics of holes with a finite length. In this thesis, an analytical model based on the Green's function method is developed to take the hole length into consideration. The importance of capturing the modified vortex noise accurately is shown. The vortices shed at the hole inlet edge are convected to the hole outlet and further downstream to form a vortex sheet. This couples with the acoustics, and the coupling may generate as well as absorb acoustic energy at low frequencies. Predictions from this model reveal the importance of capturing the path of the shed vortex. When this is captured accurately, predictions agree well with previous experimental and CFD results, for example predicting the potential for generation of acoustic energy at some frequencies. The expansion ratios either side of a short hole affect the vortex-sound interaction of it, effects captured by the model developed in this thesis. These hole models are then incorporated into a Helmholtz resonator (HR) model, allowing a systematic investigation into the effect of neck-to-cavity expansion ratio and neck length. The HR models are then incorporated into a low-order network model which applies to both longitudinal and annular combustors. It is firstly shown that previous methods for accounting for a temperature difference between the HR cavity and the combustor are inadequate; improved models are developed, implemented and investigated. Finally, location optimisation for multiple HRs is performed, for both longitudinal and annular geometries, with the latter including both location and geometry optimisations.

Self-heating is the tendency of certain porous solid fuels to undergo spontaneous exothermic reactions in oxidative atmospheres at low temperatures. Self-heating can cause accidental ignition of reactive porous media leading to wildfires, ecosystem damage, property damage, loss of industrial facilities and even loss of life. Traditional self-heating ignition literature studies have been dominated by coal. However, there are many other materials prone to self-heating ignition like biomass and other natural occurring materials. Using oven basket experiments coupled with the Frank-Kamenetskii theory of ignition, this thesis aims at quantifying self-heating ignition risk and properties, and quantifying the chemical kinetics and thermal properties of natural fuels such as shale, carbon-rich soils such as peatlands, and biochars produced from wheat, rice husks, and softwood upscaling the results to real systems. This method requires extensive laboratory time, but gives the most accurate self-heating results. The thesis focuses on the effect of three main physical parameters: carbon content, inorganic content, and particle size of the fuel. I show that shale rock can self-heat in normal ambient conditions. Using soil biomass samples with inorganic content between 3% and 86% of dry weight, I quantify the effect of inorganic content on self-heating ignition, and show that self-heating can initiate wildfires in some soil types and conditions. I quantify the reactivity of softwood biochar produced as a function of the pyrolysis reactor temperature. I show that the reactivity of softwood is not a monotonic function of pyrolysis reactor temperature. I present an experimental comparison of wheat, rice husk, and softwood biomass and biochar, quantifying the differences between different feedstock sources and what effect this has on the biochar's relative fire risk. Finally, I quantify the effect of particle size diameter on self-heating ignition for wheat, showing that the critical temperature for self-heating ignition decreases with particle size.

Environmental stress cracking (ESC) is a slow crack growth process which affects thermoplastics. It occurs under the combined effect of applied stress and an ingressing environment, without large-scale diffusion or chemical reaction. This thesis focusses on the development of a fracture mechanics test method, which is applied to characterise the ESC behaviour of several polymer-environment combinations of industrial interest. The materials investigated were PE in Igepal solution, HIPS in sunflower oil, PMMA in methanol, and PEEK in acetone. A rig was built for testing using the single edge notched bend (SENB) configuration, with crack growth measured via specimen compliance. Tests were conducted in both air and the liquid environment, with the effects of temperature and initial applied G (energy release rate) also investigated. Results were obtained in the form of crack initiation (G versus time) and propagation (G versus crack speed) plots. Standard material tests were also performed, as well as scanning electron microscopy (SEM) of the fracture surfaces to describe the different failure processes observed. For most of the materials tested, the models of relaxation- and flow-controlled crack growth were considered to hold, yielding values of critical crack opening displacement (COD) and characteristic crack length (L0) respectively. The ESC results were also used to perform component life predictions (tf) for the different materials, using either known operating conditions or 'critical' values, the latter leading to a proposed parameter λ. Overall, PEEK was judged as having the highest ESC resistance, and PMMA the lowest. The developed fracture mechanics method was found to successfully describe the ESC behaviour of several different polymer-environment combinations. It produced geometry-independent, inherent material information compared to standard industry tests, which typically output failure times under conditions that may not represent the situation in service.

Natural fibre composites (NFCs) possess relatively good specific strength and stiffness properties. However, natural fibres (NFs) often show relatively poor interfacial adhesion with respect to polymeric matrices, may contain relatively high levels of moisture and have variable mechanical properties due to the route by which they have been harvested and manufactured. These aspects may result in inconsistent mechanical properties of such composites, especially evident in a poor interlaminar fracture toughness. Thus, the present work investigates the mode I interlaminar fracture toughness, of NFCs based upon an anhydride-cured diglycidyl ether of bisphenol-A (DGEBA) epoxy matrix. Further, this matrix was used as a ‘control’ or modified with silica nanoparticles and/or rubbery microparticles. Two types of natural fibres were employed: unidirectional flax fibre (FF) and plain-woven regenerated cellulose fibre (CeF). Two very different routes were explored for the production of the NFCs based upon these materials. One route was via a resin infusion under flexible tooling (RIFT) process and a second route employed a resin transfer moulding (RTM) process. A very low value of the interlaminar fracture energy of about 20 J/m2 was measured for the flax fibre-reinforced plastics (FFRPs), using the ‘control epoxy matrix, produced by the RIFT manufacturing process which was initially employed. However, such composite manufactured via the RTM process possessed fracture energy of about 963 J/m2. Further, this value was found to increase to 1264 J/m2 when the epoxy matrix was modified using a combination of silica nanoparticles and rubbery microparticles. Hence, optimization studies using the RIFT manufacturing process were undertaken which led to a simple modification of this manufacturing route whereby the natural fibres were first oven-dried. This resulted in the final RIFT process giving values of the fracture toughness of the same order as those obtained from the RTM process. Also of note was the observation that the FFRPs manufactured via the RTM or the final RIFT process had similar values of toughness as those measured for glass fibre-reinforced plastics (GFRPs) made using the equivalent type of epoxy matrix. Similar observations were recorded in the case of the cellulose fibre-reinforced plastics (CeFFRPs). The present study has also considered the underlying mechanisms for the above observations and used analytical models to predict the toughening mechanisms and a good agreement between the predictions and the experimental data for the NFCs was obtained.

Components in advanced gas cooled reactor (AGR) operating at elevated temperatures in the range of 500-650°C are typically susceptible to the initiation and growth of cracks due to creep. Type 316H stainless steel steam headers after a long term service are susceptible to reheat cracking in the vicinity of the weld driven by the presence of welding residual stresses. For this reason, this research has focused on developing pragmatic numerical methods for predicting creep crack growth behaviour of welded components containing residual stresses, using a simplified continuum damage and fracture mechanics method. The work presented had three main aims. The first was to derive a comprehensive set of plastic η factors for standard fracture mechanics geometries containing welds. The impetus for this was to improve material crack growth characterisation for welds by improving the creep C* solutions for these geometries that are presently recommended in standard codes of practice for creep crack growth testing. The second part was to experimentally examine appropriate creep material properties for as-received and service-exposed 316H stainless steels containing welds for use in numerical modelling and predictive methods for creep crack growth in a real component. The third was to develop and validate a simplified method of simulating residual stresses and creep crack growth behaviour in an ex-service AISI 316H weld header with reheat cracking. This approach simulates the presence of residual stresses using appropriate loading and boundary conditions in actual components that undergo reheat cracking without the need to develop full weld simulations to quantify them. The creep crack growth behaviour was studied using two methods based on the theories of fracture mechanics and continuum damage mechanics. Fracture mechanics parameter C* was firstly used to examine the approximate crack growth rate using the reference stress approach and approximate NSW model. The second method was to predict long term cracking by using a simplified continuum damage mechanics model, with a consideration of stress relaxation. For this purpose, a simplified multi-axial ductility exhaustion model was developed and implemented in an Abaqus user subroutine, taking into account the changes in the ex-service creep properties and the effect of reduction in creep ductility under low loads and long term operation at service temperatures. Resulting from the findings, the task was to identify the geometric and the material reasons of how and why the crack growth follows a path of least resistance and higher constraint which did not necessarily mean growing through the welds or the heat affected zone region.