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Se presenta una guía completa del procedimiento experimental para la determinación del valor umbral ΔKth intrínseco y caracterización de curvas FCGR sobre el acero de alta resistencia S960QL empleando probetas de pequeña escala. El análisis se realiza comparando probetas SENB de diferentes geometrías, distinguiendo entre “convencionales” y “miniatura”. Las primeras con espesor (B) de 6 mm y anchura (W) de 19, y las segundas con B = 3 y dos valores distintos de W, 4 y 6 mm. A fin de identificar el valor umbral ΔKth intrínseco, la relación de cargas (R) aplicada en todos los ensayos fue 0,8. La técnica de caída de potencial (DCPD) se utilizó para la monitorización del tamaño de grieta a lo largo de los ensayos.
La evaluación de los resultados experimentales confirma la equiparación de resultados entre las distintas geometrías y configuraciones de ensayo practicadas, así como la viabilidad de emplear probetas de pequeña escala en la determinación del valor umbral ΔKth intrínseco. Esto abre nuevas perspectivas en lo relativo al empleo de probetas de reducidas dimensiones en la caracterización de materiales a fatiga. A su vez, permite ofrecer una serie de recomendaciones para reproducir con éxito ensayos semejantes utilizando probetas de pequeñas dimensiones de acero y otros materiales metálicos.
La tecnología de fabricación aditiva (AM) continúan progresando y permitiendo alcanzar diseños cada vez más complejos y optimizados. La industria química es uno de los sectores donde componentes AM han adquirido un gran interés. La falta hasta la fecha de una directiva europea que regule la inspección, certificación y aceptación de equipos sometidos a presión hace necesario progresar en esta línea. El objetivo que se persigue en este trabajo es el de desarrollar una metodología de diseño sobre componentes fabricados aditivamente basada en la estimación de vida a fatiga de las zonas más susceptibles de sufrir dicho tipo de fallo. El estudio comprende diversas facetas de análisis, simulaciones numéricas, análisis de la microestructura del material y una extensa campaña experimental. La evaluación de la integridad estructural se realiza aplicando mecánica de fractura. La historia térmica a lo largo del proceso de fabricación determina la microestructura del componente en cada región y, por ende, influye en las propiedades mecánicas en cada una. Se presentan los resultados preliminares de un proyecto de investigación en curso dirigido a la caracterización de propiedades mecánicas en recipientes de presión producidos por fusión láser en lecho de polvo (L-PBF, por sus siglas en inglés) de acero inoxidable 316L. Se detallan los resultados preliminares en términos de velocidad de crecimiento de grietas por fatiga (FCGR), y se comparan los resultados de probetas extraídas de diferentes regiones de los depósitos.
Design methodology of vessel produced by L PBF stainless steel using representative specimens
(2022)
This work presents the preliminary results of an ongoing project with a double objective: on the one hand, the characterisation of the mechanical properties against fatigue damage of an additively manufactured 316 stainless steel produced by laser powder bed-based (L-PBF) technology; on the other hand, the implementation of numerical simulation techniques able to predict the mechanical behaviour of the material in order to optimise and reduce the design costs of vessels used in the chemical sector. The current state of the work developed in this research framework allows showing the first batch of experimental results of crack propagation rate (FCGR) and high cycle fatigue (HCF) tests. The geometry of the vessels studied presents three clearly differentiated regions, either in terms of thickness (11-15 mm) or concerning the inclination of the walls to the direction of manufacturing (0º - 45º). The experimental campaign carried out so far allows identifying the differences in behaviour when comparing different extraction locations around the vessel. This is due to the variations in thermal cycles that the deposited material undergoes during the manufacturing process. Therefore, this causes variations in the microstructure which lead to changes in the response of the material. In this work, these differences are analysed qualitatively and quantitatively from the results of FCGR and HCF, thus allowing to locate the regions with the highest risk in terms of structural integrity against fatigue. This preliminary phase together with the numerical simulation of the additive manufacturing process are key to achieving a reliable description and modelling of the material. The latter will make it possible to address the priority aim of this project, involving the manufacture of independent samples whose properties are representative of the original material extracted from the reference vessels. It is, therefore, a comprehensive methodology for the design of additively manufactured components based on the localised fatigue mechanical properties of representative specimens.
This plenary lecture aims at showing the progress in the experimental determination of the fatigue propagation resistance in metals and its application to fatigue assessment. In particular, different new experimental techniques are introduced, which allow a better description of the crack propagation resistance from short to long crack regime. These data can be then applied in the fatigue assessment to predict the fatigue limit and the total life of metallic components.
The Kitagawa-Takahashi (KT) diagram is a proven concept for describing the fatigue limit in presence of a defect or crack. It can be determined empirically with great experimental effort. It can also be estimated by means of the El Haddad relationship if the endurance limit and the long fatigue crack propagation threshold are available in reasonable accuracy. A third option is the determination using the cyclic R-curve, which describes the dependency of the fatigue crack propagation threshold on the crack growth at the short crack propagation stage. This can be experimentally determined using a closure-free initial pre-crack. It can then be applied to the determination of crack arrest for a given applied load and a given defect or crack size. Compared to the other two methods mentioned above, this option has considerable advantages: It can be applied to any component and any stress ratio. It allows the treatment of multiple cracks and provides estimations of the S-N curve in the finite life regime as well as at the endurance limit. Compared to the empirical determination of the KT diagram, the experimental effort is significantly lower and compared to the El Haddad approach it avoids problems such as the use of non-conservative long fatigue crack propagation thresholds (when the conventional load reduction method is applied to materials prone to corrosion) and the mathematical predetermination of the curve shape. The work introduces the method and provides a critical discussion as well as quantitative comparison between the different methods.
IBESS steht für „Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“ und ist das Ergebnis eines DFG-AiF-Clusters, an dem acht Partner beteiligt waren. Kernstück ist eine analytische Methode zur bruchmechanischen Beschreibung der Rissausbreitung unter Schwingbelastung, die im IBESS-Teilprojekt A3 entwickelt wurde. Da man es im Bereich der Schwingfes-tigkeit mit kurzen Rissen zu tun hat, auf die die etablierten Methoden der Langrissbruchmechanik nicht anwendbar sind, wurden Lösungen entwickelt, die es erlauben, sowohl die rissnahen Plastizitätsverhältnisse als auch die mit wachsendem Riss graduell zunehmenden Rissschließeffekte quantitativ zu beschreiben. Unter Einbeziehung der sogenannten „zyklischen R-Kurve“, deren experimentelle Methodik ebenfalls im Rahmen des Projekts weiterentwickelt wurde, ist es zudem möglich, Ausgangsrissgrößen vorherzusagen, die auf Dauerfestigkeitsniveau gerade so noch arretieren würden. Dies erlaubt umgekehrt eine Abschätzung der Dauerfestigkeit an den Schweißverbindungen sowie die statistisch basierte, analytische Vorhersagen des Zeitfestigkeitsastes der Wöhlerkurve. In diesem Aufsatz wird die Methodik vorgestellt und ihre Validierung an einem ausgewählten Beispiel diskutiert.
In diesem Aufsatz wird eine analytische Prozedur zur bruchmechanischen Beschreibung des Schwingverhaltens von Schweißverbindungen beschrieben, die im Rahmen des DFG-AiF-Clusters IBESS entwickelt wurde. Es wurden drei Verbindungen untersucht (Stumpfstöße, Kreuzstöße und Längssteifen), die aus warmgewalzten Stahlplatten produziert wurden. Die analytische Modellierung basierte auf drei wichtigen Aspekten: Quantitative Beschreibung der Rissausbreitung mechanisch-kurzer Risse; Statistische Effekte infolge der Variabilität der lokalen Schweißnahtgeometrie; Modellierung der Mehrfachrissinitiierung und des Mehrfachrisswachstums. Im Folgenden wird die Methodik vorgestellt und ihre Validierung an einem ausgewählten Beispiel diskutiert.
The 21st century brought new and complex technological challenges, which need to be solved. Of primary importance is the global energy transition that pushes forward the research and innovation in order to achieve the goal of replacing the existing non-renewable energy sources with new renewable and efficient ones, with positive effects on the world climate. In many countries worldwide mid- and long-term goals have been set out in order to reduce the greenhouse gas emissions. Germany, among others, intends to reduce the emissions by 80 to 95% within 2050, compared with 1990 levels. The achievement of this goal is aimed to be realized by the development of new and more efficient energy sources, but also by substantial investments in electromobility (the goal is to bring one million electrically driven vehicles onto German streets by 2020).
Another important challenge came into play following the global financial crisis, which pushed many industries to reduce their operational and maintenance costs. In particular, the life-cycle management of a component has become of primary importance. In some cases it has been shown that the underestimation or the lack of awareness in ageing of plants led to incidents due to the loss of technical integrity. Other studies demonstrated that the component life of many of the long-life components could be extended up to 50% without compromising safety.
The third, not less important, challenge is driven by the development of new technologies and materials. The trends show a large investment in additive manufactured metal components and new metallic materials which have to assure longer in-service life, lightweighting and efficient recycling.
This works aims at addressing part of these problems by proposing a fracture mechanics-based analytical procedure for the determination of the fatigue lives of engineering components and particularly of welded joints. In fact, an analytical tool, which is able to simulate the main failure mechanisms of weldments under fatigue loading, brings many advantages: i) drastic reduction of long and expensive experimental tests; ii) optimization of the geometry and production processes (possible weight reduction); iii) better estimate of the safety margins used in the design of the components (life extension without safety issues).
The results show that the model can fairly well predict the lives of the welded joints investigated experimentally, irrespective of the geometry and loading conditions.