Publicación: Análisis de la influencia de concentradores de esfuerzo en la propagación de grietas por fatiga
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2024-10-22
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info:eu-repo/semantics/openAccess
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Universidad Nacional de Educación a Distancia (España), Universidad de Concepción - Chile. Departamento de Ingeniería Mecánica
Resumen
En la fabricación de prótesis se utilizan diferentes materiales con alta resistencia mecánica y desempeño biocompatible, entre estos, el AISI 316L es uno de los más utilizados debido a su alta disponibilidad y bajo costo en comparación con otros materiales. Cuando se diseñan estos dispositivos dirigidos a la población joven cuya actividad física diaria implica imponer cargas de mayor magnitud y frecuencia que la población de edad avanzada [1], el sometimiento de estos dispositivos a cargas fluctuantes provoca que con el tiempo se presente la aparición de grietas en la estructura del componente, generadas por el fenómeno de la fatiga. El comportamiento de propagación de grieta por fatiga depende de diversos factores, entre los cuales se encuentran los concentradores de esfuerzos directamente relacionados con la geometría del componente, como agujeros, entallas, ranuras y cambios de sección, que, a diferencia de aspectos microestructurales más complejos, son un factor bajo la capacidad de control del diseñador. En consecuencia, este trabajo propone analizar del comportamiento de la trayectoria de propagación de grieta y la vida útil en placas agujeradas, pre entalladas de AISI 316L mediante ensayos experimentales y simulación computacional. Dichos ensayos experimentales se realizaron para 24 probetas cortadas por chorro de agua, cuya dirección de laminación se aseguró paralela al eje de la carga aplicada. Los especímenes fueron dimensionados en base a la norma ASTM E399 para flexión tres puntos [2], se ensayaron tres de estos con geometría normalizada y grieta centrada para la caracterización de la tasa de crecimiento de grieta en el material, por otro lado, los especímenes restantes se diseñaron con la grieta desfasada del centro con el objetivo de tener modo mixto durante el ensayo. Para las probetas de grieta desfasada, se incorporaron diferentes agujeros como concentradores de esfuerzo, cuya localización y dimensiones se definieron a partir de simulación computacional por medio del método dual de elementos de contorno, mismo método con el cual se contrastan todos los resultados experimentales obtenidos. Para las corridas experimentales se utilizó la máquina universal de ensayos MTS Bionix, y a lo largo de cada ensayo se capturaron periódicamente diferentes fotografías macro en la zona de propagación de grieta para cada probeta, dichas fotografías que fueron debidamente escaladas y procesadas con el objetivo de determinar el comportamiento del tamaño de grieta contra número de ciclos, así como la trayectoria de propagación en cada prueba. Para lograr la captura de fotografías limpias, se programó la disminución periódica de la frecuencia de la onda de fatiga de 20 Hz a 0.5 Hz y se ubicó una escala milimétrica sobre cada probeta en la fotografía inicial. En cuanto a los resultados, se observó como la presencia de un agujero influye en la curvatura de la grieta, haciendo que esta se desvíe o colapse directamente hacia él, lo cual depende principalmente del tamaño del agujero y de la proximidad entre este y la grieta. Este comportamiento se da por la variación de los factores de intensidad de esfuerzo 𝐾𝐼 y 𝐾𝐼𝐼, ya que el factor de intensidad de esfuerzos correspondiente al modo II aumenta cuando la trayectoria se ve influenciada por el agujero, en contraste a la disminución del factor de intensidad de esfuerzos para el modo I [3]. Lo cual demuestra que se puede aumentar la vida útil de un componente sometido a cargas de fatiga, si se colocan agujeros de manera estratégica, de manera que la grieta no colapse de manera prematura, sino que lo haga directamente contra el agujero al final de la trayectoria de propagación.
In the manufacturing of prostheses, different materials with high mechanical strength and biocompatible performance are used. Among these, AISI 316L is one of the most utilized due to its high availability and low cost compared to other materials. When designing these devices for the young population whose daily physical activity involves imposing loads of greater magnitude and frequency than the older population [1], the subjection of these devices to fluctuating loads causes the appearance of cracks in the structure of the component over time, generated by the phenomenon of fatigue. The crack propagation behavior depends on various factors, among which are the stress concentrators directly related to the component's geometry, such as holes, notches, grooves, and section changes. Unlike more complex microstructural aspects, these factors are under the designer's control. Consequently, this work proposes to analyze the behavior of the crack propagation path and the service life in pre-notched and holed AISI 316L plates through experimental tests and computational simulation. These experimental tests were conducted on 24 waterjet-cut specimens, with their rolling direction ensured parallel to the axis of the applied load. The specimens were dimensioned based on the ASTM E399 standard for three-point bending [2]. Three of these were tested with standardized geometry and centered cracks for characterizing the crack growth rate in the material. On the other hand, the remaining specimens were designed with an off-center crack to achieve mixed-mode during the test. For the off-center crack specimens, different holes were incorporated as stress concentrators, whose location and dimensions were defined based on computational simulation using the dual boundary element method, the same method used to contrast all obtained experimental results. For the experimental runs, the MTS Bionix universal testing machine was used. Throughout each test, different macro photographs were periodically captured in the crack propagation area for each specimen. These photographs were properly scaled and processed to determine the behavior of crack size against the number of cycles, as well as the propagation path in each test. To achieve clear photographs, the frequency of the fatigue wave was periodically reduced from 20 Hz to 0.5 Hz, and a millimeter scale was placed on each specimen in the initial photograph. As for the results, it was observed how the presence of a hole influences the curvature of the crack, causing it to deviate or collapse directly towards it. This depends primarily on the size of the hole and the proximity between it and the crack. This behavior is due to the variation in the stress intensity factors K_I and K_II, as the stress intensity factor corresponding to mode II increases when the path is influenced by the hole, in contrast to the decrease in the stress intensity factor for mode I [3]. This demonstrates that the service life of a component subjected to fatigue loads can be increased if holes are strategically placed so that the crack does not collapse prematurely but rather collapses directly into the hole at the end of the propagation path.
In the manufacturing of prostheses, different materials with high mechanical strength and biocompatible performance are used. Among these, AISI 316L is one of the most utilized due to its high availability and low cost compared to other materials. When designing these devices for the young population whose daily physical activity involves imposing loads of greater magnitude and frequency than the older population [1], the subjection of these devices to fluctuating loads causes the appearance of cracks in the structure of the component over time, generated by the phenomenon of fatigue. The crack propagation behavior depends on various factors, among which are the stress concentrators directly related to the component's geometry, such as holes, notches, grooves, and section changes. Unlike more complex microstructural aspects, these factors are under the designer's control. Consequently, this work proposes to analyze the behavior of the crack propagation path and the service life in pre-notched and holed AISI 316L plates through experimental tests and computational simulation. These experimental tests were conducted on 24 waterjet-cut specimens, with their rolling direction ensured parallel to the axis of the applied load. The specimens were dimensioned based on the ASTM E399 standard for three-point bending [2]. Three of these were tested with standardized geometry and centered cracks for characterizing the crack growth rate in the material. On the other hand, the remaining specimens were designed with an off-center crack to achieve mixed-mode during the test. For the off-center crack specimens, different holes were incorporated as stress concentrators, whose location and dimensions were defined based on computational simulation using the dual boundary element method, the same method used to contrast all obtained experimental results. For the experimental runs, the MTS Bionix universal testing machine was used. Throughout each test, different macro photographs were periodically captured in the crack propagation area for each specimen. These photographs were properly scaled and processed to determine the behavior of crack size against the number of cycles, as well as the propagation path in each test. To achieve clear photographs, the frequency of the fatigue wave was periodically reduced from 20 Hz to 0.5 Hz, and a millimeter scale was placed on each specimen in the initial photograph. As for the results, it was observed how the presence of a hole influences the curvature of the crack, causing it to deviate or collapse directly towards it. This depends primarily on the size of the hole and the proximity between it and the crack. This behavior is due to the variation in the stress intensity factors K_I and K_II, as the stress intensity factor corresponding to mode II increases when the path is influenced by the hole, in contrast to the decrease in the stress intensity factor for mode I [3]. This demonstrates that the service life of a component subjected to fatigue loads can be increased if holes are strategically placed so that the crack does not collapse prematurely but rather collapses directly into the hole at the end of the propagation path.
Descripción
Organizado y patrocinado por: Federación iberoamericana de Ingeniería Mecánica y Universidad de Concepción - Chile. Departamento de Mecánica, FeIbIm – FeIbEM
Categorías UNESCO
Palabras clave
Crecimiento de grieta, Trayectoria de propagación, AISI 316L, Método dual de elementos de contorno, Crack growth, Crack propagation path, AISI 316L, Dual boundary element method
Citación
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Centro
E.T.S. de Ingenieros Industriales
Departamento
Mecánica