Turbines encounter blade failures due to fatigue and creep. It has been shown in the literature that the primary cause of steam turbine blade failures worldwide can be ascribed to fatigue in low pressure (LP) turbine blades. The failure and damage to these blades can lead to catastrophic consequences. Some utilities use empirical methods to determine the forces experienced by turbine blades but desire more accurate methods. The inaccurate prediction of high-cycle fatigue (HCF), thermal durability and stage performance is introduced when one does not consider blade row interaction. Blade row interactions can, however, be accounted for by means of computational fluid dynamics (CFD). Furthermore, modern high- fidelity CFD tools would be able to contribute greatly in predicting the forces experienced by turbine blades. Numerical tools such as CFD and nite element analysis (FEA) can greatly contribute to the estimation of the remaining useful life (RUL) of turbine blades. However, in this estimation process, there are various uncertainties and aspects that affect the estimated RUL. Understanding the sensitivity of the estimated RUL to these various uncertainties and aspects is of great importance if RUL is to be estimated as accurately as possible. In this dissertation, a sensitivity analysis is performed with the purpose of establishing the sensitivity of the estimated RUL of the last stage rotor of an LP steam turbine, to the number of harmonics used in a nonlinear harmonic (NLH) CFD simulation. The sensitivity of the estimated RUL is evaluated in the HCF regime, where the cyclic stresses occur below the yield strength of the turbine blade. A CFD model, FE model, and fatigue model were therefore developed in such a manner that would suffice, regarding the purpose of the sensitivity analysis. The CFD model is validated by comparing the predicted CFD power to that of actual generated power of a dual 100MW LP steam turbine. The sensitivity analysis is performed for 3 operation conditions, and for each operational condition the aerodynamic forces were computed using 1, 2, and 3 harmonics in an NLH simulation. The estimation process considers a weak coupling between the CFD model and FE model. NLH simulations are firstly performed to calculate the unsteady static surface pressure distributions on the last stage rotor. This is followed by the mapping thereof to the FE model, for which a transient structural analysis is performed. Finally, the RUL is estimated by performing a fatigue analysis on the stress history obtained from the transient structural analysis. Based on the results of the sensitivity analysis, the following recommendations were made, from a conservative point of view. Firstly, in general, if the RUL is to be estimated with reasonable accuracy, just using 1 harmonic in an NLH simulation will not be sufficient and 2 harmonics should be used. Secondly, if the RUL has to be estimated with high accuracy, 3 harmonics should be used. / Dissertation (MEng)--University of Pretoria, 2018. / National Research Foundation (NRF) / Mechanical and Aeronautical Engineering / MEng / Unrestricted

No presente trabalho se estuda a correlação de diferentes parâmetros de sinais de Ruído Magnético de Barkhausen (RMB) e de fluxo magnético com o comportamento elastoplástico em chapas de aço carbono, visando à aplicação dos resultados como técnica de Ensaio Não-Destrutivo. Comprovou-se que o comportamento de parâmetros como a energia (RMBenegia), o valor quadrático médio(RMBrms) e a envolvente dos sinais de RMB com os diferentes estados elastoplásticos do material resultou fortemente dependente da anisotropia de laminação, quantidade de Carbono e fases, gerados no processo de produção das chapas. Como novidade introduziu-se o parâmetro chamado de Área em baixo da curva da distribuição de pulsos do sinal de Barkhausen. A evolução deste parâmetro com a deformação identificou com boa aproximação a região de transição elasto-plástica do material. Harmônicos não lineares (HNL) do fluxo magnético apresentaram uma melhor correlação com a anisotropia do que com os estados elasto-plásticos do material, mas com alta dependência ao acoplamento sonda-amostra. Enquanto que o RMB apresentou elevados níveis de reprodutibilidade. Propriedades mecânicas e comportamentos característicos de ensaios de tração em regime discreto e contínuo foram reproduzidos e identificados com um alto grau de aproximação pelo RMB e os HNL. Outros métodos magnéticos e não magnéticos usados neste trabalho não apresentaram níveis altos de sensibilidade às mudanças microestruturais devido às deformações elasto-plásticas no material, no entanto o RMB e os HNL identificaram tais mudanças microestruturais dando margem a novas soluções nos campos de ensaio e inspeção não-destrutivos. / The present work studies the relationship between different parameters of the Magnetic Barkhausen Noise (MBN) and non-linear harmonics of the magnetic flux signals (NLH) with the elastic-plastic behavior in carbon steel plates, in an effort to use the results as a non-destructive testing technique. MBN parameters such as the envelope of the MBN signal, and the scalar parameters MBNenergy, MBNrms were found to be highly dependent on the lamination processes, carbon content and phases originated during the fabrication of the samples. A new parameter entitled area under the MBN jump distribution was defined. The behavior of this parameter as strain evolved provided a good approximation of the elastic-plastic transition region. Non-linear harmonics offered a good correlation with both anisotropy and the elasticplastic states of the material, nevertheless depending heavily on the sensor coupling with the sample surface, while at the same time MBN provided good reproducibility. The characteristic behavior as well as the mechanical properties obtained from discrete and continuous tensile strength tests were reproduced by both MBN and HNL. Other magnetic and non-magnetic methods did not show the same level of sensibility to microstructure changes due to elastic-plastic strains when compared to the results obtained using MBN and HNL, giving room to new solutions in the nondestructive inspection and testing fields.

No presente trabalho se estuda a correlação de diferentes parâmetros de sinais de Ruído Magnético de Barkhausen (RMB) e de fluxo magnético com o comportamento elastoplástico em chapas de aço carbono, visando à aplicação dos resultados como técnica de Ensaio Não-Destrutivo. Comprovou-se que o comportamento de parâmetros como a energia (RMBenegia), o valor quadrático médio(RMBrms) e a envolvente dos sinais de RMB com os diferentes estados elastoplásticos do material resultou fortemente dependente da anisotropia de laminação, quantidade de Carbono e fases, gerados no processo de produção das chapas. Como novidade introduziu-se o parâmetro chamado de Área em baixo da curva da distribuição de pulsos do sinal de Barkhausen. A evolução deste parâmetro com a deformação identificou com boa aproximação a região de transição elasto-plástica do material. Harmônicos não lineares (HNL) do fluxo magnético apresentaram uma melhor correlação com a anisotropia do que com os estados elasto-plásticos do material, mas com alta dependência ao acoplamento sonda-amostra. Enquanto que o RMB apresentou elevados níveis de reprodutibilidade. Propriedades mecânicas e comportamentos característicos de ensaios de tração em regime discreto e contínuo foram reproduzidos e identificados com um alto grau de aproximação pelo RMB e os HNL. Outros métodos magnéticos e não magnéticos usados neste trabalho não apresentaram níveis altos de sensibilidade às mudanças microestruturais devido às deformações elasto-plásticas no material, no entanto o RMB e os HNL identificaram tais mudanças microestruturais dando margem a novas soluções nos campos de ensaio e inspeção não-destrutivos. / The present work studies the relationship between different parameters of the Magnetic Barkhausen Noise (MBN) and non-linear harmonics of the magnetic flux signals (NLH) with the elastic-plastic behavior in carbon steel plates, in an effort to use the results as a non-destructive testing technique. MBN parameters such as the envelope of the MBN signal, and the scalar parameters MBNenergy, MBNrms were found to be highly dependent on the lamination processes, carbon content and phases originated during the fabrication of the samples. A new parameter entitled area under the MBN jump distribution was defined. The behavior of this parameter as strain evolved provided a good approximation of the elastic-plastic transition region. Non-linear harmonics offered a good correlation with both anisotropy and the elasticplastic states of the material, nevertheless depending heavily on the sensor coupling with the sample surface, while at the same time MBN provided good reproducibility. The characteristic behavior as well as the mechanical properties obtained from discrete and continuous tensile strength tests were reproduced by both MBN and HNL. Other magnetic and non-magnetic methods did not show the same level of sensibility to microstructure changes due to elastic-plastic strains when compared to the results obtained using MBN and HNL, giving room to new solutions in the nondestructive inspection and testing fields.