Testing for corrosion in the petrochemical industry has always been a significant challenge which takes up a large portion of the operating expenditure. Whereas major advancements have been made for the detection of general corrosion, inspection at inaccessible locations, such as at pipe supports, remains a demanding prospect; this signifies the need for an alternative technique, capable of dealing with various surface conditions encountered when testing at such locations including weld patches, T-joints, surface roughness and coatings. Long range guided waves are commonly used to detect relatively severe defects in plain sections of pipe but are less suited to inspection at supports because the support itself gives significant reflection. The reflection coefficient at the support reduces with frequency so it would be beneficial to test at higher frequencies, which can also improve the sensitivity of the test to smaller, pitting-type defects. Following the attractive properties of the Higher Order Mode Cluster (HOMC) proposed by Balasubramaniam et al. (IIT Madras), this research starts by investigating the nature of the mode cluster and shows that the features of this method are essentially those of the A1 mode in the high frequency-thickness regime. The study then goes on to investigate the possibility of exciting a single mode Lamb wave with low dispersion at a frequency-thickness of around 20 MHz-mm. Excitation of the A1 mode was considered because of its relationship with HOMC and due to its non-dispersive nature and low surface motion at such frequency-thickness products; this makes it attractive for inspection at supports since it will be unaffected by the support itself and also by surface roughness and attenuative coatings. The thesis then explores the relative ability of different transducer types for single mode excitation in the medium and high frequency-thickness regimes; here the practical feasibility of exciting the A1 mode at around 20 MHz-mm, in spite of its low surface motion, is investigated. Next, a systematic performance analysis of the A1 mode compared to the existing inspection techniques is carried out and, finally the sensitivity of this technique to realistic 3-D pitting-type holes is established. The thesis shows that the A1 mode is an attractive tool for the detection of localized, sharp, severe defects that will be missed by standard, lower frequency guided wave testing.

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Direct powder forging (DPF) is an innovative powder metallurgy (PM) process that aims to manufacture nickel-based superalloy components within a very short time, compared with current hot isostatic pressing (HIPing) and other processes, by applying high temperature and pressure to metal powders. DPF process has been proposed to reduce the microstructural defects, e.g. prior particle boundaries (PPBs), possessed by the conventional HIPing process and achieve considerably high production efficiency with low cost and energy saving. The aim of this project is to study the powder consolidation and microstructure evolution of a nickel-based superalloy, FGH96, during DPF process. The work in this thesis is divided into two parts: experimental studies and numerical modelling. The experimental studies reported in this thesis concentrate on the material characterisation of FGH96 and investigate the products of DPF process under different process conditions. Firstly, hot compression tests were conducted using Gleeble 3800 test station to obtain the mechanical properties of FGH96. Secondly, small size powder forging tests were designed to study the powder consolidation. Samples acquired from small size powder forging tests were examined with material density. Tensile tests were then carried out to evaluate the mechanical properties of FGH96 superalloy produced by DPF process, and microstructure observation was used to identify the microstructure features. Lastly, the DPF process was tested under both laboratory and industrial environment. Tests were conducted on a laboratory-based 250 kN hydraulic press machine and an industrial 20,000 kN hydraulic press machine to consolidate FGH96 powder which was encapsulated with a stainless steel container. Material density, hardness and microstructure of the FGH96 components were then examined to evaluate the feasibility of implementing DPF process to manufacture fully dense FGH96 superalloys with desired material properties and microstructure features. The numerical modelling was used to model the material behaviours of FGH96 and investigate the powder density evolution during DPF process. Firstly, a unified viscoplastic constitutive model was developed for fully dense FGH96 based on the results obtained from the hot compression tests, and calibration was carried out to achieve good agreement between numerical integrations and experimental results. Secondly, the constitutive model was modified with powder densification parameters and variables, providing a method to numerically describe the powder density evolution during DPF process. Thirdly, the modified constitutive model, i.e. the powder material model, was implemented into the commercial software DEFORM 2D/3D via user defined subroutines. Results were analysed in terms of stress state and powder density distribution, and the correlation between the material properties of DPFed components and process parameters were discussed. Lastly, future work suggestions were proposed to improve the DPF process and modelling technique.

A class-based framework for generation of heterogeneous reaction mechanisms has been proposed. The framework consists of a transition state theory method for estimating adsorption and desorption rate parameters, two-dimensional collision theory for homogeneous surface reactions and the unity bond index–quadratic exponential potential (UBI–QEP) for estimation of barrier heights. The framework has been developed to address the reliance of customary approaches on experimental sticking coefficients, and provide a fully self-consistent method for mechanism development on novel metals. Two different implementations of transition state theory have been considered, with the variational approach (VTST) showing improved quantitative results and higher robustness at negligible additional computational expense. Mechanisms prepared using this framework have been validated against a wide range of experimental data. On platinum, the VTST-based mechanism was applied for hydrogen, syngas, ethane and methane combustion. A new mechanism has been also developed for methane combustion on rhodium. The mechanisms have been applied under fuel-lean and fuel-rich conditions, various geometries, pressures and inlet velocities. The performance is generally better or as good as the optimised collision theory based determinations. The compatibility of the framework with high-accuracy density functional theory (DFT) data has been established using a case study of ethane adsorption on oxygen-covered platinum. At temperatures below 1500 K, the DFT study predicted considerably slower rate constants than the VTST approach. However, for the studied cases, the mol fraction profiles calculated with the VTST based rate determinations were in acceptable agreement with the experimental data. The developed method reproduces experimental data without the reliance on sticking coefficients, and facilitates the efficient generation of novel heterogeneous reaction mechanisms.