The introduction of gas-driven Otto engine and the corresponding usage of bio-fuels in heavy-duty engines will render the exhaust atmosphere more corrosive and bring a higher working temperature to exhaust manifolds. The current service material, a ferritic ductile cast iron called SiMo51, will soon meet its upper temperature limit set by the ferrite-austenite transformation at 860ºC. Three alternative materials, as well as SiMo51 serving as reference, are investigated in the present thesis emphasizing on high-temperature corrosion fatigue.  The first aim of this study is to obtain material data and give a quantitative ranking of the materials’ performance. Low-cycle fatigue (LCF) tests at 800ºC in a synthetic exhaust gas (5%O2-10%CO2-5%H2O-1ppmSO2-N2 bal.) are conducted to evaluate the materials’ performance in simulated real working scenarios, where high-temperature, corrosive atmosphere and fatigue conditions during testings are similar to the conditions experienced by the exhaust manifolds. To evaluate the individual effect from high-temperature fatigue and isolate the impact from corrosion, the materials are tested under the same settings but in an argon atmosphere. To evaluate the individual effect from high-temperature corrosion and isolate the impact from mechanical deformation, oxidation tests are carried out at 800ºC in the same synthetic exhaust gas. The second aim is to identify and understand different oxidation behavior and failure mechanisms in the materials, realized by considerable characterizations of the tested specimens. From the fatigue tests, it is found that the austenitic stainless steel HK30 has the highest fatigue resistance, followed by the austenitic cast iron Ni-resist D5S, and the ferritic ductile cast irons SiMo1000 and SiMo51, a ranking valid in both atmospheres. In the exhaust atmosphere, for instance, the improvement in fatigue strength at 15,000 cycles relative to SiMo51 are 260%, 194% and 26%, respectively. Different crack initiation and propagation mechanisms are found for the various combinations of materials and atmospheres. In the exhaust atmosphere, for instance, crack initiation is assisted by oxide intrusion in SiMo51 and crack propagation is affected by crack branching in HK30, mechanisms not observed in argon. By comparing the S-N fatigue curves in the two atmospheres, the influence of oxidation on fatigue life is evaluated. The fatigue life of the cast irons are surprisingly found to be higher in the exhaust atmosphere. Several explanations are suggested for this, considering their very different oxidation behaviors.  This study provides accurate test data that can be used to help industry avoid over-dimensioned design. The investigation of the failure mechanisms promotes better understanding of the correlation between microstructure and mechanical properties. Moreover, the combination of fatigue tests in argon, fatigue tests in exhaust and oxidation tests in exhaust, shows how corrosion and fatigue individually and synergistically affect the materials’ performance at high temperature. / <p>QC 20180917</p>

Ces travaux ont pour objectif l'identification et la compréhension de l'effet de la corrosion aqueuse sur la durée de vie en fatigue à grand nombre de cycles (HCF) d'un acier inoxydable martensitique utilisé dans des applications aéronautiques. Tout d'abord, l'effet géométrique des défauts de corrosion sur la limite de fatigue à 10^7 cycles à l'air a été étudié avec quatre tailles différentes de piqûres de corrosion. A partir de ces résultats, une nouvelle approche volumique non locale de modélisation numérique a été proposée pour prendre en compte une géométrie réelle d'un défaut de corrosion issu des analyses en microtomographie X. Ensuite, les phénomènes de couplage chargement cyclique/corrosion ont été identifiés par des essais de fatigue à grande durée de vie (entre 10^5 et 10^7 cycles) dans une solution aqueuse à 0,1 M NaCl (à pH = 6) pour deux rapports de charge (R = -1 et 0,1). Le comportement électrochimique du film passif a été étudié in situ au cours des essais de fatigue-corrosion par le suivi du potentiel libre de corrosion et des mesures d'impédance électrochimique. Les observations des mécanismes d'amorçage de fissures et des mesures électrochimiques in-situ ont permis d'identifier un scénario d'amorçage de fissures de fatigue. Ce scénario implique des processus de rupture locale du film passif (induite par le chargement cyclique) et de corrosion assistée par le chargement cyclique. Finalement un modèle analytiquede prévision de la durée de vie en fatigue dans un milieu aqueux corrosif a été proposé à partir des résultats expérimentaux. / This study addresses the effects of corrosion on the high cycle fatigue (HCF)strength of a martensitic stainless steel used in aeronautic applications. First, the geometry of corrosion pits on the fatigue strength in air at 10^7 cycles were studied with four different pit sizes. A new non-local fatigue criterion was proposed to simulate real shapes of pits as identified by X-ray microtomography. Corrosion fatigue synergy phenomena was studied by HCF tests (between 10^5 and 10^7 cycles) in a 0.1 M NaCl aqueous solution (pH = 6) with two load ratios (R = -1 and 0.1). Next, the electrochemical behavior of the passive film was investigated during in situ corrosion fatigue tests by free potential measurements and electrochemical impedance spectroscopy (EIS). Based on fractography analysis and electrochemical test results, corrosion fatigue crack initiation mechanisms were investigated. A scenario of fatigue crack initiation was proposed based on physical evidence. This scenario implied combined processes of local passive film rupture (induced by the cyclic loading), stress-assisted corrosion and enhanced pitting development. Local passive film ruptures were the main cause of the corrosion fatigue crack initiation. Finally, a analytical model for corrosion fatigue crack initiation was proposed based.

A análise da vida em fadiga em amplitude variável associada à corrosão é bastante complexa, devido a combinações entre as interações eletroquímicas ocorridas, tendo em vista a exposição da frente da trinca à névoa salina somada aos efeitos deletérios do carregamento de amplitude variável. Esta combinação de efeitos ainda não é modelada matematicamente, e os dados experimentais observados em outros trabalhos ainda geram hipóteses inconclusivas. Neste contexto, este estudo tem como objetivo avaliar a vida em fadiga da liga 7475 T7351, utilizando carregamento de voos simulados TWIST e FALSTAFF e submetida a névoa salina a 3,5 e 5%. Foram executados ensaios eletroquímicos, para verificar o potencial de corrosão dessa liga e análise microestrutural para verificar a formação de pites. O crescimento de trinca por fadiga com carregamento de voo foi simulado, sem considerar os efeitos do meio, nos programas NASGRO 4.0, AFGROW e CRACK 2000, com parâmetros calibrados no NASGRO 4.0, no intuito de verificar quais modelos matemáticos e programas descrevem a curva experimental em ar com menor erro. Os resultados mostraram que os programas NASGRO 4.0 e AFGROW se aproximaram com menor erro à curva experimental em ar. Os ensaios de crescimento de trinca por fadiga foram executados com carregamento de voos simulados e os parâmetros para o sistema de geração de névoa salina foram determinados experimentalmente. As curvas de crescimento de trinca e taxa de propagação em névoa salina foram comparadas às curvas em ar, as quais também foram obtidas sob as mesmas condições de carregamento. Observou-se que para os ensaios em névoa salina, a trinca rompeu com maior número de voos em relação às curvas em ar. / The corrosion fatigue life analysis under variable amplitude loading is very complex due electrochemistry effects in crack tip exposed to salt spray matched to harmful effects of amplitude variable loading. This effects combination is not modeling mathematically actually, and the experimentally data observed in other research generates inconclusive theories. In this context, the main aim of this research to evaluate the corrosion fatigue life of 7475 T7351 under spectrum loading TWIST and FALSTAFF and exposed to 3.5% and 5% NaCl. Electrochemistry test were performed to verify the corrosion potential followed of microstructural analysis by SEM, to verify the pitting on 7475 T7351 alloy. The fatigue crack growth was simulated, without environmental effects, using the codes NASGRO, AFGROW and CRACK 2000, the simulation parameters was fit on NASGRO 4.0, to analyze whether the crack growth models and codes are able to represent properly the air experimental crack growth data. The results showed that NASGRO 4.0 and AFGROW codes described the fatigue crack growth in air with less error. The fatigue crack growth tests were performed under spectrum loadings and the parameters to obtain salt spray were determined experimentally. The corrosion fatigue crack growth and rate data were compared to air data, under the same loading conditions. The corrosion fatigue tests showed that the specimens cracked under a larger number of flights than specimens in air.

Nenhuma / A corrosão-fadiga é provavelmente um dos mais complexos de todos os fenômenos, sendo influenciada por fatores ambientais, mecânicos, metalúrgicos, somados a uma componente cíclica de tensões. A corrosão-fadiga pode ser definida como a ação combinada de um ambiente agressivo e uma tensão cíclica que conduzem à falha prematura dos metais por trincamento. A maior parte da vida em fadiga está relacionada a uma sucessão de processos durante os quais planos de escorregamento, deformações localizadas, danos e micro trincas se iniciam e desenvolvem até a nucleação de trinca macroscópica. Neste estudo, a influência da solução de NaCl 3,5% nas propriedades de fadiga do aço foi avaliada. Foram analisadas as curvas de S-N-P obtidas sob ambiente corrosivo e comparadas com as curvas de S-N-P obtidas em água destilada. Em geral, a corrosão-fadiga em soluções aquosas é um processo eletroquímico. O mecanismo de iniciação de trincas por corrosão-fadiga proposto para explicar a redução da vida em fadiga de metais em ambientes corrosivos, foi analisado neste trabalho

The environmentally-assisted fatigue crack growth behaviour of austenitic stainless steels in deoxygenated high temperature water at low strain rates has been reported to be greatly affected by the sulphur (S) content of the specimen, with high S specimens exhibiting significant reduced crack growth rates (retardation) when compared to low S specimens. To further the understanding of the mechanistic behaviour, fatigue crack growth experiments have been performed on high and low sulphur Type 316 austenitic stainless steel specimens tested in high temperature water and evaluated via microstructural characterisation techniques. At high strain rates the enhanced crack growth for both specimens appeared to be crystallographic and associated with slip localization. Furthermore, matching fracture surface analysis indicated discrepancy of the slip steps and micro-cleavage cracks between the matching surfaces, suggesting that slip steps and micro-cleavage cracking occurred after the crack-tip had advanced. It was also postulated that their formation may involve cathodically-produced hydrogen and shear deformation on the fracture surface. However, when the loading frequency was decreased, the high S specimens retarded the crack growth and the path was no longer crystallographic. Significant differences in the crack-tip opening displacements were observed in both materials, with blunt crack-tips in the high sulphur specimen and sharp tips in the low sulphur specimen when the strain rate was low. EBSD analysis at the crack-tips of both specimens showed that the strain was more localised at the crack-tip of the low sulphur specimen whist the strain ahead of the high sulphur specimen was more homogenous. It is thus postulated that retardation occurs when slip localisation is no longer the dominant factor. The localised deformation during enhancement is believed to have been caused by hydrogen enhanced localised plasticity (HELP) which causes the crack-tip to sharpen. The diffused strain distribution during crack growth retardation is believed to have been caused by hydrogen enhanced creep (HEC) which causes the crack-tip to blunt. It is also believed that both enhancement and retardation mechanisms are associated with contrasting effects deriving from hydrogen enhanced plasticity. Oxide induced crack closure was excluded as a mechanism responsible for retardation of fatigue crack growth when the stress ratio is high. Effects of hydrogen induced alpha' and ε martensite phases on oxidation behaviour of austenitic stainless steels in deoxygenated high temperature water have also been studied. Microstructural characterisation shows that hydrogen induced alpha' martensite enhances oxidation of austenitic stainless steels in deoxygenated high temperature water. The implications of this finding on environmentally assisted cracking of austenitic stainless steels in deoxygenated high temperature water is discussed.

A indústria aeronáutica tem buscado melhorias no conceito de integridade estrutural, a partir do desenvolvimento de projetos mais otimizados com a produção de aeronaves mais leves e seguras. Tais projetos garantem aeronaves operacionalmente mais viáveis e redução no consumo de combustível, beneficiando o fabricante, os operadores e a população em geral, pois aplica o conceito de aeronave verde. Estruturas aeronáuticas em operação recebem solicitações de cargas típicas de vôo, como pressurização, sustentação, que por sua vez, geram esforços dos mais variados tipos e suas combinações, como tração, compressão, fadiga, torção, flexão, flambagem, cargas aeroelásticas, vibrações e flutter, associados a ambientes críticos. O conhecimento sobre o comportamento do material estrutural perante ambientes corrosivo ou de baixa temperatura, é importante para avaliação da vida útil das aeronaves. Neste contexto, o objetivo deste trabalho foi avaliar o comportamento da liga AA2050-T84 sob ponto de vista de corrosão-fadiga e tenacidade a fratura em criogenia e, correlacionar com a microestrutura e fractografia através de ensaios mecânicos e de tenacidade a fratura em temperatura ambiente e criogênica. As condições de ensaios foram determinadas próximas a de serviço a que o material será inserido sendo, para criogenia -54 ºC e para os ensaios de propagação de trinca por fadiga em atmosfera ambiente e névoa salina com concentração de solução de 3,5% e 5% NaCl, frequência de 15Hz e razões de carga R = 0,1 e 0,5. Os resultados em criogenia mostraram que há melhora nas propriedades trativas e estabilidade na tenacidade, pois o material retém dutilidade com o decréscimo da temperatura. Os ensaios de propagação de trinca por fadiga foram executados com &Delta;K decrescente e os parâmetros para geração de névoa salina foram determinados experimentalmente. As curvas de crescimento de trinca e determinação do fator limite foram comparadas quanto a razão de carga e atmosfera de ensaio. Observou-se que para os ensaios de propagação de trinca por fadiga a presença da névoa salina acelera a taxa de propagação e resultam em um fator limite menor. / The aviation industry has sought improvements with the concept of structural integrity, from the development of more optimized designs with production aircraft with low weight and secure. These projects ensure an aircraft more operationally doable and reduced fuel consumption, benefiting the manufacturer, operators and the people, applying the concept of green aircraft. Aerospace structures in operation receive requests loads typical of flight, how pressurization support and lift that produce several types and combinations of requests, such as tensile, compression, fatigue, torsion, bending, buckling , aeroelastic loads, vibration and flutter associated with critical environments. Knowledge about the behavior of the structural material into corrosive environments or cryogenic temperature, it is important for evaluation to the lifetime of the aircraft. Accordingly , the aim of this study was to estimate the behavior of AA2050 - T84 alloy, about corrosion-fatigue and fracture toughness at cryogenic situations and correlate with microstructure and fractography, by mechanical tests and fracture toughness at room temperature and cryogenic. The test conditions has been determined near of the service that the material will be inserted, for cryogenic temperature -54 ºC and testing of fatigue crack propagation in air and salt spray with concentration of 3.5% and 5% NaCl, frequency 15Hz and load ratios R = 0.1 and 0.5. The cryogenic results showed that there improvement in tensile properties and stability in toughness due the restraint of ductility with decreasing temperature. Tests of fatigue crack propagation were carried out with &Delta;K decreasing and the parameters to obtain salt spray were determined experimentally. The crack growth curves and determination of the threshold were compared the according to load ratio and test atmosphere. In the fatigue crack propagation it was observed that presence of salt spray accelerates the rate of propagation and result in a lower threshold.

As recentes descobertas na camada pré-sal da costa brasileira geraram uma série de oportunidades e desafios tecnológicos que deverão ser enfrentados para se garantir o sucesso da exploração e produção de petróleo offshore em águas profundas e ultra-profundas. A alta presença de gás carbônico (CO2) e de gás sulfídrico (H2S) encontrada nas jazidas produzem potenciais efeitos de corrosão e fragilização nos componentes de aço utilizados em sistemas submarinos de produção de petróleo, dentre eles os dutos flexíveis. Esses dutos são submetidos à elevadas cargas geradas devido à lâmina da água profunda, sendo os arames da armadura de tração extremamente solicitados por sustentar os carregamentos. Tradicionalmente, arames da armadura de tração de dutos que irão operar em ambientes contendo H2S (sour) apresentam limite de resistência à tração máximo de 850 MPa, enquanto para ambientes onde esse gás não está presente (sweet), os limites de resistência são consideravelmente maiores podendo chegar a 1400 MPa. Amostras de arames da armadura de tração com limite de resistência à tração de 1400 MPa foram revestidas com alumínio através do processo de aspersão térmica a arco elétrico e a eficiência do revestimento em ambiente corrosivo contendo CO2 e H2S foi avaliada. Essa avaliação envolveu ensaios mecânicos e de caracterização, como medições da rugosidade superficial, análise da porosidade do revestimento, ensaios de dobramento, avaliação da susceptibilidade ao trincamento induzido por hidrogênio e sulfeto, ensaios de fadiga ao ar e corrosão-fadiga. Foi observado que o grau de porosidade do revestimento variou entre 5 e 10% e que a rugosidade superficial do substrato aumentou devido ao jateamento prévio ao processo de revestimento. Esse fato mostrou ser uma das causas principais para a deterioração da vida em fadiga ao ar dos arames revestidos quando comparados com arames sem revestimento, sendo comprovado através de análise por Microscopia Eletrônica de Varredura. Entretanto, para ensaios de corrosão-fadiga em ambiente contendo CO2 e H2S, o desempenho de arames revestidos mostrou-se notadamente superior, onde o acabamento superficial do substrato teve importância secundária. Através de ensaios de dobramento, foi possível verificar a alta capacidade do revestimento em suportar extensa deformação plástica. Através dos ensaios estáticos para se avaliar a susceptibilidade ao trincamento por hidrogênio e sulfeto foi possível verificar uma considerável corrosão da superfície de arames não revestidos devido ao ambiente agressivo. Ademais, uma trinca induzida por hidrogênio foi identificada em uma das amostras de arame sem revestimento, enquanto as amostras revestidas não apresentaram sinal de degradação nem trincamento. / Recent discoveries in Brazil pre-salt layer generated a series of technological opportunities and challenges that must be met to ensure the success of offshore oil exploration and production in deep and ultra-deepwater. The presence of high levels of carbon dioxide (CO2) and eventually hydrogen sulfide (H2S) found in the deposits, produce potential corrosion effects and embrittlement of steel components of subsea systems used in petroleum production, including flexible pipes. These equipments are subjected to high loads generated by deep water condition where tensile armor wires are extremely required for sustaining the loads. Traditionally, armor wires that will operate in environments containing H2S (sour) have maximum tensile strength of 850 MPa, while for environments where this gas is not present (sweet), the tensile strength is considerably larger reaching up to 1400 MPa. Armor wire samples with tensile strength of 1400 MPa were coated with pure aluminum by thermal arc spray process and the effectiveness of the coating in corrosive environment containing CO2 and H2S was evaluated. This evaluation included mechanical and characterization tests, such as surface roughness measurements, coating porosity analysis, bending tests, evaluation of susceptibility to cracking induced by hydrogen and sulfide and fatigue test in air and in corrosive environments. It was found that the coating porosity ranged between 5 and 10% and the substrate roughness increased due to grit blasting prior to the coating process. This fact proved to be one of the main causes for the deterioration of the fatigue life in air of the coated wires when compared with uncoated wires, being proven through analysis by Scanning Electron Microscopy. However, for corrosion fatigue tests in an environment containing CO2 and H2S, the performance of coated wires was found to be markedly higher, where the integrity of the substrate surface had secondary importance. Through bending tests, it was found the high capacity of the coating to support extensive plastic deformation. Static tests to evaluate the susceptibility to cracking by hydrogen and sulfides showed that uncoated armor wire surface corroded considerably due to the aggressive environment. In addition, a crack induced by hydrogen was identified in one of uncoated wire samples, while the coated samples showed no sign of degradation or cracking.

In recent years, the demand for alternatives to fossil fuels has risen dramatically, and ethanol fuel has become an important liquid fuel alternative globally. The most efficient mode of transportation of petroleum-based fuel is via pipelines, and due to the 300% increase in ethanol use in the U.S. in the past decade, a similar method of conveyance must be adopted for ethanol. Low-carbon, low-alloy pipeline steels like X52, X60, and X65 comprise the existing fuel transmission pipeline infrastructure. However, similar carbon steels, used in the ethanol processing and production industry, were found to exhibit stress corrosion cracking (SCC) in ethanol service. Prior work has shown that contaminants absorbed by the ethanol during distillation, processing or transport could be the possible determinants of SCC susceptibility; 200 proof ethanol alone was shown not to cause SCC in laboratory studies. To ensure the safety and integrity of the pipeline system, it was necessary to perform a mechanistic study of SCC of pipeline steel in fuel grade ethanol (FGE). The objective of this work was to determine the environmental factors relating to SCC of X65 steel in fuel grade ethanol (FGE) environments. To accomplish this, a systematic study was done to test effects of FGE feedstock and common contaminants and constituents such as water, chloride, dissolved oxygen, and organic acids on SCC behavior of an X65 pipeline steel. Slow strain rate tests (SSRT) were employed to evaluate and compare specific constituents' effects on crack density, morphology, and severity of SCC of X65 in FGE. SCC did not occur in commercial FGE environments, regardless of the ethanol feedstock. In both FGE and simulated fuel grade ethanol (SFGE), SCC of carbon steel was found to occur at low water contents (below 5 vol%) when chloride was present above a specific threshold quantity. Cl- threshold for SCC varied from 10ppm in FGE to approximately 1 ppm in SFGE. SCC of carbon steel was inhibited when oxygen was removed from solution via N2 purge or pHe was increased by addition of NaOH. During SSRT, in-situ¬ electrochemical measurements showed a significant role of film rupture in the SCC mechanism. Analysis of repassivation kinetics in mechanical scratch tests revealed a large initial anodic dissolution current spike in SCC-causing environments, followed by repassivation indicated by current transient decay. In the deaerated environments, repassivation did not occur, while in alkaline SFGE repassivation was significantly more rapid than in SCC-inducing SFGE. Composition and morphology of the passive film on X65 during static exposure tests was studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results showed stability of an air-formed native oxide under static immersion in neutral (pHe = 5.4) SFGE, and dissolution of the film when pHe was decreased to 4.3. XPS spectra indicated changes in film composition at high pHe (near 13) and in environments lacking sufficient water. In light of all results, a film-rupture anodic-dissolution mechanism is proposed in which local plastic strains facilitates local breakdown of the air-formed oxide film, causing iron to dissolve anodically. During crack propagation anodic dissolution occurs at the crack tip while crack walls repassivate preserving crack geometry and local stress concentration at the tip. It is also proposed that SCC can be mitigated by use of alkaline inhibitors that speed repassivation and promotes formation of a more protective Fe(OH)3 film.

Cold rolled carbon steel 1018C is widely used in pressurized fuel pipelines. For those structures, stress corrosion cracking (SCC) can pose a significant problem because cracks initiate late in the lifetime and often unexpectedly, but grow fast once they get started. To ensure a safe operation, it is crucial that any damage can be detected before the structural stability is reduced by large cracks. In the early stages of SCC, microstructural changes occur which increase the acoustic nonlinearity of the material. Therefore, an initially monochromatic Rayleigh wave is distorted and measurable higher harmonics are generated. Different levels of stress corrosion cracking is induced in five specimens. For each specimen, nonlinear ultrasonic measurements are performed before and after inducing the damage. For the measurements, oil coupled wedge transducers are used to generate and detect tone burst Rayleigh wave signals. The amplitudes of the received fundamental and second harmonic waves are measured at varying propagation distances to obtain a measure for the acoustic nonlinearity of the material. The results show a damage-dependent increase in nonlinearity for early stages of damage, indicating the suitability for this nonlinear ultrasonic method to detect stress corrosion cracking before structural failure.

Abstract Recently, the expansion in power system capacities leads to the development of large-scale steam turbine generator units. As a result, a fault on the power system may induce large fault current and give rise to serious torque vibrations on turbine shafts and blades, which ought to be improved in order for the reliable operation of a turbine-generator system. In the thesis, countermeasures are proposed from electrical viewing-point and from mechanical viewing-point respectively. Based on electrical viewing-point, the apparatus in the generator stator side and in the rotor side respectively is applied to suppress the induced disturbing source. The high temperature superconductive fault current limiter bank introduces a large normal-state resistance to restrict the dc component of stator fault current. The choke coil acts as a low pass filter to restrict the system-frequency component of field fault current. Both of them lead to the reduction in electromagnetic torque of system-frequency and effectively improve the vibrating behavior of blades. Based on mechanical viewing-point, it is found from the electromechanical analysis that the Generator/LP-Turbine shaft stiffness and the Generator rotor inertia constant determine the responses of all turbine blades. Once the stiffness on this shaft section is reduced by replacing the rigid shaft coupling with a flexible one or the inertia constant is augmented by a system-frequency mechanical filter, the blades become intrinsically less responsive to electrical disturbances. As a result, the blades will bear less stress impact and can be designed with smaller safety factor. On the other hand, LP-turbine long blades operated in corrosive environment and underwent the statistical stress impact due to randomly distributed negative sequence current is studied also. In such situation, the blades may be subjected to corrosion fatigue and the long term effects of power system unbalance may become the cause of fatigue damage on blades though the negative sequence current is still within the limitation of generator thermo-rating. As a result, turbine blades are possibly unprotected by traditional system unbalance protection scheme. Therefore, it will depend on the operating environments and the blade materials whether such long-term stress can be neglected or not. If there is the potential of blade damage, one has to reconsider the I2 protection settings and rearrange the load distribution to limit the system unbalance.