Evaluación de tenacidad a la fractura e integridad estructural de tubos de generadores de vapor nucleares. / Evaluation of fracture toughness and structural integrity of nuclear steam generators tubes.

Bergant, Marcos A. (2016) Evaluación de tenacidad a la fractura e integridad estructural de tubos de generadores de vapor nucleares. / Evaluation of fracture toughness and structural integrity of nuclear steam generators tubes. PhD Thesis in Engineering Sciences, Universidad Nacional de Cuyo, Instituto Balseiro.

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Abstract in Spanish

Dado el impacto negativo asociado a la ocurrencia de fallas en tubos de generadores de vapor (TGVs) en centrales nucleares, el estudio de la integridad estructural de éstos ha comenzado a recibir mayor atención recientemente. Diversas metodologías basadas en análisis de carga límite han sido propuestas para asegurar la integridad estructural de los tubos, según los requerimientos establecidos por las autoridades regulatorias. Éstas han conducido, sin embargo, a la definición de criterios de reparación o taponado de TGVs excesivamente conservativos. Por lo tanto, con el objetivo de reducir la cantidad de tubos innecesariamente removidos de servicio, nuevos criterios de evaluación de integridad han sido propuestos recientemente en la literatura. En este contexto, la mecánica de fractura elastoplástica se presenta como una alternativa para la evaluación de la integridad de TGVs, requiriéndose dos elementos para su aplicación: la estimación de la fuerza impulsora en términos del parámetro elastoplástico (por ejemplo, la integral J) y la medición experimental de la tenacidad a la fractura del material de los tubos (por ejemplo, a través de la curva de resistencia J-R). Este trabajo presenta el desarrollo de técnicas experimentales no normalizadas para la determinación de curvas J-R para TGVs con fisuras pasantes circunferenciales y longitudinales. Debido a las dimensiones reducidas de los TGVs, diferentes probetas no normalizadas fueron propuestas. Además, en los ensayos se utilizaron condiciones de carga de tracción y flexión con el objetivo de modelar más adecuadamente los estados tensionales y las condiciones de constraint reales en TGVs. Los valores de la integral J fueron estimados utilizando el método del factor η. La aptitud del método fue evaluada a partir de simulaciones numéricas de los ensayos propuestos mediante análisis elastoplásticos con la técnica de elementos finitos. Se encontró que condiciones de mayor constraint asociadas con fisuras profundas y cargas de flexión favorecen la validez del método del factor η, mientras que configuraciones de menor constraint dan como resultado factores η que exhiben una mayor dependencia con el nivel de carga aplicada. También se observó que los factores η basados en la apertura de la boca de la fisura (Crack Mouth Opening Displacement o CMOD) presentan una dependencia mucho menor con el nivel de carga respecto a los factores η definidos a partir del desplazamiento del punto de aplicación de la carga (Load Line Displacement o LLD). Se presentan los valores del factor η para las probetas estudiadas con fisuras profundas (a/W ≥ 0,40). Se realizaron ensayos de fractura a temperatura ambiente y 300 °C con probetas obtenidas de TGVs nucleares fabricados a partir de las aleaciones 690 (Ni: 61; Cr: 29; Fe: 8,95, % en peso) y 800 (Ni: 33; Cr: 21,6; Fe: 42,2, % en peso). Durante los ensayos de fractura a temperatura ambiente, la extensión estable de fisura fue medida mediante una técnica óptica utilizando un microscopio digital. Para estos ensayos también se aplicó el método de normalización que propone la norma ASTM E1820-15 en el Anexo 15, encontrándose una buena coincidencia entre las longitudes estimadas por éste y las medidas ópticamente. De esta manera, el método de normalización fue utilizado para los ensayos a alta temperatura. Los resultados experimentales mostraron que ambos materiales tienen elevadas tenacidades a la fractura, siendo la aleación 800 la que presentó curvas J-R más elevadas que la aleación 690 tanto para fisuras circunferenciales como longitudinales. Las curvas J-R para ambas aleaciones mostraron un efecto marcado con la orientación de la fisura, es decir que existe una importante anisotropía en las propiedades de fractura: las fisuras circunferenciales presentaron curvas J-R más elevadas que las fisuras longitudinales. El nivel de constraint desarrollado en los ensayos, dado por las condiciones de carga de tracción y flexión, evidenció poco efecto sobre las curvas J-R para probetas con fisuras profundas (a/W ~ 0,50). A su vez, la temperatura de ensayo (temperatura ambiente y 300 °C) presentó un efecto prácticamente nulo para ambas aleaciones. Usando las propiedades de fractura obtenidas en este trabajo, la metodología FAD (Failure Assessment Diagram) fue propuesta y utilizada para la predicción de las condiciones de falla de TGVs fisurados para diferentes geometrías de fisura y condiciones de carga. La comparación entre análisis teóricos y datos experimentales muestra la potencialidad del FAD como una metodología capaz de predecir adecuadamente las fallas de estos componentes.

Abstract in English

The assessment of the structural integrity of steam generator tubes (SGTs) in nuclear power plants has been receiving increasing attention in the literature in the last years due to the negative impact related to their failures. Diverse failure prediction methodologies based on limit load analyses were proposed in the past to ensure tube integrity by fulfilling regulatory authorities’ requirements. They have lead however to overly conservative plugging or repairing criteria. Therefore, with the purpose of reducing the number of tubes that are unnecessarily removed from service, new revised fitness for service criteria was recently proposed in the open literature. In this context, elastic plastic fracture mechanics (EPFM) methodology appears as a potential tool for the analysis. The application of EPFM requires, necessarily, knowledge of two aspects, i.e., the driving force estimation in terms of an elastic plastic toughness parameter (e.g., J-integral) and the experimental measurement of the fracture toughness of the material (e.g., the material J-R resistance curve). The present work describes the development of non standardized experimental techniques aimed to determine J-R curves for SGTs with circumferential and longitudinal through wall cracks. Due to the reduced dimensions of SGTs, different candidate non-standard specimens´ geometries were proposed. Also, tensile and bending loading conditions were used in the tests in order to modeling the actual stress and constraint conditions in SGTs. The J-integral values were estimated through the η-factor method. The suitability of this method was evaluated from the numerical simulation of the proposed tests using elastoplastic finite element analysis. It was shown how higher constraint conditions associated with deep cracks geometries and prevailing bending loads favor the η-factor validity while lower constraint configurations resulted in a η-factors exhibiting a higher dependence on the applied load. It was also verified that η-factors based on the crack mouth opening displacement (CMOD) showed much less dependency on the loading level than η-factors derived from load line displacement (LLD). The η-factors for the studied specimens with deep cracks (a/W ≥ 0.40) were reported. Fracture tests at room temperature and 300 °C were performed with specimens obtained from nuclear SGTs made of alloy 690 (Ni: 61; Cr: 29; Fe: 8.95, % in weight) and 800 (Ni: 33; Cr: 21.6; Fe: 42.2, % in weight). During the fracture tests at room temperature, the stable crack extension was measured by means of an optical technique using a digital microscope. The normalization method proposed in Annex 15 of ASTM E1820-15 was also applied to estimate the crack length during these tests, showing a general good coincidence with the experimental optical data. Therefore, the normalization method was used for high temperature tests. The experimental results exhibited that both materials have high fracture toughness, while alloy 800 showed higher J-R curves than alloy 690 for both circumferential and longitudinal cracks. The J-R curves for both alloys presented a strong effect on the crack orientation, i.e., there is an important anisotropy in fracture properties: circumferential cracks showed higher J-R curves than longitudinal cracks. The constraint level in the tests, due to tensile and bending loading conditions, showed a reduced effect for specimens with deep cracks (a/W ~ 0.50). Furthermore, the testing temperature (i.e., room temperature and 300 °C) showed a negligible effect for both materials. Using fracture properties obtained in this research, the Failure Assessment Diagram (FAD) methodology was proposed and used for predicting the failure conditions of defective SGTs for varied crack geometries and loading conditions. The comparison between theoretical analyses and experimental data results indicates the potentiality of the FAD as a comprehensive methodology for predicting the failure of flawed SGTs.

Item Type:Thesis (PhD Thesis in Engineering Sciences)
Additional Information:Área temática: Mecánica de fractura e integridad estructural.
Keywords:Steam generator; Generadores de vapor de agua ; Fracture; Fracturas; Crack propagation; Propagación de fracturas; Fracture mechanics; Mecánica de fracturas;[ Resistance cruves; Curvas de resistencia]
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Subjects:Engineering > Materials defects
Physics > Physics of materials
Engineering
Divisions:Investigación y aplicaciones no nucleares > Física > Física de metales
ID Code:549
Deposited By:INVALID USER
Deposited On:25 Aug 2016 12:35
Last Modified:25 Aug 2016 12:48

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