Filtern
Erscheinungsjahr
Dokumenttyp
- Beitrag zu einem Tagungsband (26)
- Zeitschriftenartikel (17)
- Vortrag (14)
- Beitrag zu einem Sammelband (9)
- Forschungsbericht (4)
Schlagworte
- Ductile cast iron (5)
- Bruchmechanik (3)
- Dynamic crack resistance curve (3)
- Fracture mechanics (3)
- Key curve (3)
- Brüche (2)
- Crack resistance curve (2)
- Dampferzeuger (2)
- Duktile Schädigung (2)
- Dynamic fracture mechanics (2)
Eingeladener Vortrag
- nein (14)
Im Rahmen des Teilprojektes 1 "Analyse und Erweiterung der Einsatzgrenzen der elastisch- plastischen Bruchmechanik zu tiefen Temperaturen hin mit der Option der systematischen Nutzung von Zähigkeitspotentialen im Übergangsbereich" wurden an drei Versuchswerkstoffen bei jeweils mehreren Prüftemperaturen Serien von Risswiderstandskurven aufgenommen. Die Arbeiten dienten der quantitativen Erfassung des Übergangs vom Spaltbruch zum Gleitbruch im Bereich nahe Ta mit dem Ziel, Sicherheitsreserven des Werkstoffs in diesem Bereich ausnutzen zu können. Im Rahmen des Teilprojektes 2 "Methoden zur Extrapolation von JR-Kurven für die Vorhersage der Rissöffnungsgeometrie unter postulierten Belastungen" wurden experimentelle Untersuchungen an Rohren aus dem Werkstoff 15NiCuMoNb5 (WB36) durchgeführt. Für die Rohrbiegeversuche wurde ein Prüfstand aufgebaut, auf dem bei Raumtemperatur Vierpunkt- Biegeversuche an Rohren mit unterschiedlichen Außenumfangsfehlergeometrien durchgeführt wurden. Mit Hilfe der zerstörungsfreien Verfahren Ultraschallprüfung, Schallemissionsprüfung, Gleichstrompotentialsondenverfahren und optische Analyse wurde das Risswachstums- und Leckageverhalten unter den komplexen Bedingungen der Rohrversuche quantitativ untersucht und detailliert vergleichend ausgewertet.
Die bruchmechanische Beurteilung von Komponenten erfordert die Kenntnis von Risswiderstandskurven, die jedoch häufig aufgrund fehlenden Probenmaterials nicht zur Verfügung stehen. Vielmehr sind typische werkstofftechnische Kennwerte verfügbar, wie Streckgrenze, Zugfestigkeit, Gleichmaßdehnung, Bruchdehnung sowie die Kerbschlagarbeit in der Hochlage und die laterale Breitung von Kerbschlagbiegeproben. Ziel dieses Forschungsvorhabens ist die Erarbeitung einer Methode zur Konstruktion von Risswiderstandskurven, basierend auf diesen Daten.
Im Hinblick auf das Verhalten von Bauteilen aus ferritischen Stählen im Übergangsbereich vom Gleitbruch zum Spaltbruch wurden in einem vom BMBF geförderten Forschungsvorhaben, über das nachfolgend auszugsweise berichtet wird, u.a. folgende Ziele verfolgt:
• Abgrenzung des Anwendungsbereichs der elastisch-plastischen Bruchmechanik zu tiefen Temperaturen hin,
• Möglichkeiten und Grenzen der Nutzung von Zähigkeitspotentialen im Übergangsbereich,
• Ermittlung des Einflusses des Spannungszustandes auf das Bruchverhalten im Übergangsbereich und
• Übertragbarkeit auf Bauteile.
Die beiden Schwerpunkte dieses Berichtes liegen in der Entwicklung eines numerischen Verfahrens zur quantitativen Beurteilung rissbehafteter Proben oder Bauteile nach Warmvorbelastung und in der Durchführung von, auf die numerischen Arbeiten abgestimmten, experimentellen Untersuchungen zum Einfluss der Art der Vorbelastung und des Spannungszustands (Constraint) im Rissspitzenbereich auf die Bruchzähigkeitserhöhung.
Der vorliegende Bericht stellt die zugrunde liegende Theorie nachvollziehbar dar und beschreibt den numerischen Lösungsalgorithmus nach dem Mehrziel-Schießverfahren. Die Winkelfunktionen des Spannungs- und Verzerrungsfeldes werden graphisch und in zahlreichen Tabellen für verschiedene Verfestigungsexponenten dokumentiert.
The purpose of this work is the development and numerical implementation of a non-local extension of existing Gurson-based modeling for isotropic ductile damage and attendant crack growth. In the current work, this extension is based on a generalization of Gurson-based ductile damage modeling formally analogous to that of Needleman and Tvergaard as based on a (local) effective damage parameter f* which accounts in their case in an effective fashion for the effect of void coalescence on yield behaviour. Here, the corresponding generalized effective damage parameter ? is introduced in order to delocalize the model damage process. To this end, ? is modeled in this work as a scalar-valued continuum microstructural field or generalized phase field via a recent thermodynamic approach to the modeling of such fields. In the last part of the work, the complete model for coupled damage-deformation is implemented numerically using the finite element method and utilized to investigate the damage behaviour of an inhomogeneous steel block in simple tension. The corresponding simulation results demonstrate in particular that delocalization of the model damage process also leads to minimization of mesh-dependence. For simplicity, the current formulation is restricted to the case of small strain and isothermal conditions.
Modern structural integrity assessment procedures in the field of nuclear related technology incorporate fracture
mechanical concepts. Therefore, they inevitably require the availability of both, loading parameters as well as material
characteristics in terms of fracture mechanical quantities. Especially in case of dynamic loading conditions, the methods
for the determination of the loading parameters need further improvement and there is a lack of material characteristics
as well. In Germany, ductile cast iron (DCI) is used for heavy-sectioned casks for radioactive materials. New developments
in cask design and efforts to extend the application limits require further investigations. The present study is part
of an ongoing fracture mechanics research programme of BAM which is focused on the systematic mechanical and
fracture mechanical material characterisation of DCI materials under dynamic loading conditions. In this study, results
of fracture mechanics investigations on ductile cast iron from an original DCI container with a wide variety of microstructure
under dynamic loading conditions in the temperature range from -50 °C to +22 °C are presented. Large scale
as well as small scale single edge crack bend specimens SE(B) with thicknesses of 140 mm and 15 mm, respectively
were tested. Furthermore, it is reported on the results of a finite element simulation of the dynamic large scale fracture
mechanics tests. Strength and deformation characteristics were determined in dynamic tensile tests. They are discussed
with respect to the influence of pearlite content and test temperature. The material specific experimental difficulties in
the determination of reliable dynamic crack initiation toughness values of DCI are outlined.
The purpose of this work is the formulation, numerical implementation and initial application of a non-local extension of existing Gurson-based modelling for isotropic ductile damage and attendant crack growth. It is being carried out under the premise that void coalescence results not only in accelerated damage development (e.g., Needleman and Tvergaard, 1984), but also in damage delocalisation (i.e., via interaction between neighbouring Gurson RVE's). To this end, we proceed by analogy with the approach of Needleman and Tvergaard (1984) who replaced the Gurson void volume fraction f with a (local) effective damage parameter f* in the Gurson yield condition to account for the effect of void coalescence on the material behaviour. In the current case, the role of f* is taken over and generalised by an effective continuum damage field ?. A field relation for ? is formulated here in the framework of continuum thermodynamics. In the simplest case, the resulting relation is formally analogous to the inhomogeneous temperature equation in which void nucleation and growth represent (local) sources for ? and in which void coalescence takes place in a process zone whose dimension is determined by a characteristic material lengthscale. Analogous to temperature, then, ? represents an additional continuum degree-of-freedom here, resulting in a coupled deformation-damage field model. In the last part of the work, the complete model for coupled damage-deformation is implemented numerically using the finite-element method on the basis of backward-Euler integration and consistent linearisation. Using this implementation, the behaviour of the current extended Gurson-based damage model is investigated for the case of simple tension of an inhomogeneous steel block. In particular, the corresponding simulation results document quantitatively the dependence of the delocalisation of the model damage process and minimisation of mesh-dependence on the characteristic dimension of the damage process zone.
A two-dimensional analysis of the Charpy V-notch specimen subjected to impact loading, according to the standard DIN EN 10045-1, is carried out, using a transient explicit dynamic finite element program. An elastic-viscoplastic, temperature dependent, constitutive relation for a porous plastic solid based on the Gurson damage model is developed. Ductile fracture of the matrix material will be described by the nucleation and subsequent growth of voids to coalescence. An updated Lagrange–Jaumann formulation is employed accounting for large strain and rotation. The discretization is based on four-node plane strain solid elements with one Gauss point. The equations of motion are integrated numerically by an explicit integration algorithm utilising a lumped mass matrix. The predictions of the numerical analysis in terms of force deflection response, crack resistance behaviour and deformation energy absorbtion are compared with results from Charpy tests which were carried out according to the low-blow technique.
Schwerpunkte der BAM aus Forschungsvorhaben zur Komponentensicherheit sowie zukünftige Entwicklungen
(2002)
This paper deals with life prediction by observation and simulation of short fatigue cracks in cyclically loaded smooth and notched specimens made of 0.15 wt-% carbon steel SAE1017. The measured crack behaviour was simulated in a microstructural environment. Reference tests under constant amplitude loading allowed to determine model parameters and hence a reasonable life prediction by simulation resulted even for service loading. Short crack behaviour was a measurable property for damage also to compare smooth and notched specimens.
End of November 2005 strong south-west wind and heavy snowfall were predominant in the region Münsterland, north-western part of Germany. This led to accretion of a considerable quantity of wet snow to overhead electrical lines in form of snow rolls on the conductors. Eighty-two transmission towers failed catastrophically, most of them by buckling, however some by brittle fracture. As a consequence nearly 250,000 people have been cut off from electrical power supply for several days with major media attention.
This paper describes the forensic analysis in order to investigate the failure cause. Therefore extensive materials investigations, mechanical testing of original components and specimens thereof, estimations for the real wind and snow loads and their combinations, structural analyses as well as detailed evaluations on the basis of previous investigations, literature and regulations were conducted. It was revealed that some of the examined components were manufactured from Thomas steel which was partially in embrittled condition. The investigated towers fulfilled the design codes valid at the time of erection. However the present line loads of the wet snow rolls on the conductors exceeded by far the ones given in the design codes valid at that time.
The load case leading to failure was reconstructed by the derived positions of loads mainly caused by unequal and asymmetric distribution of snow rolls on left and right electrical system. The loads and corresponding stresses acting on the structure before failure were estimated. By comparison with the fracture forces from mechanical testing of original members of the collapsed tower the component that primarily failed was localised. The primary fracture occurred on a diagonal member under tension made of Thomas steel which was weakened by embrittlement. The failure cause was a combination of heavy weather conditions (storm, approx. 0 °C and wet snowfall leading to heavy snow rolls on conductors), asymmetric loading conditions and the usage of Thomas steel which was partially embrittled. Finally, recommendations for avoiding future failures are given.
Fatigue testing in the VHCF ränge requires testing machines with increased frequencies. However, application of energy-saving resonance testing leads to a linkage between test frequency and specimen Parameters. The frequency ränge and the available forces of the testing machine are limiting possible specimen dimensions.
Electrodynamic shakers combine test frequencies up to 4000 Hz with forces in the kN range. Thus, a relatively wide range of tests is possible which allows studying the influence of specimen size on VHCF behaviour.
Electrodynamic shakers as well as the respective Computer control Systems and Vibration measurement techniques are well established in scientific and technical applications. We present results of a first study of VHCF testing on electrodynamic shakers conducted at BAM. Test frequency, specimen size and stress levels have been varied. One goal is to balance the needs of materials testing with the Standard test Systems of environmental Simulation to provide a powerful and flexible tool for very high cycle fatigue testing. Further investigation to optimize the technique is required.
Entwicklung einer Key Curve-Methode für Gusseisen mit Kugelgraphit bei dynamischer Beanspruchung
(2011)
Am 9. Juli 2008 entgleiste ein Hochgeschwindigkeits-Triebzug der Baureihe 403, ICE3, bei der Ausfahrt aus dem Kölner Hauptbahnhof. Ursache dafür war der Schwingbruch einer Radsatzwelle. Im Auftrag der Staatsanwaltschaft Köln untersuchte die BAM die Schadensteile, insbesondere die gebrochene Treibradsatz-Hohlwelle.
Zunächst wurden das Drehgestell, der Antriebsstrang, die Räder, die Bremsen und die gebrochene Radsatzwelle visuell untersucht und deren Zustand dokumentiert.
Anschließend erfolgte die zerstörungsfreie Prüfung der beiden Bruchstücke der Hohlwelle mit teilmechanisierter Ultraschall-Handprüfung.
Vor der Zerlegung der Radsatzwelle wurden ausgewählte Maße kontrolliert und die Oberfläche eingehend untersucht. Nach der Probenahme und Präparation wurden die relevanten spezifizierten Eigenschaften des Wellenwerkstoffs überprüft.
Mittels Bruchflächenanalyse konnte die Startstelle des Schwingrisses lokalisiert, aufgrund der starken Sekundärschäden auf der Bruchfläche und an den Bruchkanten jedoch nicht direkt ausgewertet werden. Durch die Kombination von Mikro-Computertomographie und Ultraschall-Tauchtechnik wurden u. a. nahe der Schwingriss-Startstelle Inhomogenitäten erkannt, die durch metallographische Zielpräparation als nichtmetallische Einschlüsse identifiziert wurden. Die Größe dieser Einschlüsse ist gemäß gefordertem Reinheitsgrad unzulässig. Vergleichbare Einschlüsse in diesem hochbeanspruchten Bereich der Radsatzwelle haben vermutlich den Schwingriss initiiert.
In the approval procedure of transport packages for radioactive materials, the mechanical and thermal safety assessment is carried out in Germany by competent authority BAM. In recent years BAM was involved in several licensing procedures of new spent fuel and HLW package designs, where the cask body was made of Ductile Cast Iron (DCI). According to IAEA regulations package designs have to fulfill requirements for specific conditions of transport. Type B(U) packages must withstand the defined accident conditions of transport. The temperature range from -40°C up to the operational temperature has to be considered. For the cask material DCI, it is necessary to determine the brittle fracture behavior. The German guideline BAM-GGR 007 defines requirements for the fracture mechanics of DCI. Due to complex structure of the cask body and the dynamic loading a fracture mechanical assessment in an analytical kind is not always possible. Experience of recent design approval procedures show that the application of numerical calculations are applicable to determine the fracture mechanical load in the cask body. At the first step a numerical analysis has to be done to identify the loading state at the whole cask body. Secondly an analysis of a detail of the cask body is made considering the displacement boundary conditions of the global model. An artificial flaw is considered in this detailed model to calculate the fracture mechanical loading state. The finite element mesh was strongly refined in the area of flaw. The size of the artificial flaw is characterized by the ultrasonic inspection used for the quality assurance of the package. The applicant developed additional analysis tools for calculation of stress intensity factor and/or J-Integral. The assessment approach by BAM led to the decision to develop own tools to the possibility for independent check of the results.
The paper describes the authority assessment approach for the DCI fracture mechanics analysis. The validation procedure incl. the development of own tools is explained. BAM developed a post-processor called JINFEM to determine the fracture mechanical loads. The regulatory 1 m puncture bar drop test is used to give an example of the assessment procedure.
The paper gives an overview on safe life and damage tolerance methods applied to railway axles. It describes failure scenarios due to fatigue crack initiation and propagation. Besides common aspects of design, specific features such as corrosion and impact damage from flying ballast are discussed which may reduce the fatigue strength of axles during service. Potential effects of non-metallic inclusions from the steel manufacturing process are addressed in the context of the very high number of loading cycles railway axles are designed for. With respect to damage tolerance general lines of fracture mechanics residual lifetime analyses are introduced. More specific discussion is provided on aspects such as the threshold value of fatigue crack propagation and reliability aspects of non-destructive inspection.
Advances in the application of non-local damage models in the simulation of ductile crack-extension
(2004)
Local damage models usually have the disadvantage that results are strongly mesh dependent. The reason is that the type of the underlying partial differential equations changes under quasi-static conditions from elliptic to hyperbolic. Non-local damage models do not exhibit such behaviour under certain conditions. The usage of such non-local damage models in finite element analyses opens the possibility for preserving the ellipticity of the partial differential equations and thus avoiding mesh dependence of numerically obtained results. The loss of ellipticity for local models and its preservation for non-local models are demonstrated for a wide variety of examples enclosing ductile damage. In the present investigation, the non-local damage model is applied to the simulation of ductile crack extension in fracture mechanics specimens. The type of the underlying differential equations is permanently analysed and controlled.
Nicht-lokale, dehnratenabhängige Modellierung adiabatischer Scherbänder an Proben und Bauteilen
(2007)
Fracture mechanics model for fatigue strength of metallic alloys with large second phase particles
(2013)
On July 9, 2008 a high speed train derailed in Cologne main Station, Germany at a low speed because an axle was broken. Fortunately, the derailment happened at a low speed so that nobody was injured. The reason for the broken axle was investigated and it turned out that most likely large inclusions located shortly undemeath the surface in a T-transition were the origin of the final crack. Basing on that result, a systematic investigation on existing safety assessments of railway axles was performed. This results in an analysis of the production process of axles and in a critical review of existing of existing assessments. Improvements and future developments are outlined.