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- Non-metallic inclusions (4)
- Railway axles (3)
- Safe life design (3)
- Corrosion (2)
- Damage tolerance design (2)
- Fatigue strength (2)
- Flying ballast impact (2)
- Korrosion (2)
- Nichtmetallische Einschlüsse (2)
- Pores (2)
Organisationseinheit der BAM
Die Sicherheit von Radsatzwellen beruht gegenwärtig auf zwei Säulen: der Schwingfestigkeitsauslegung und periodischen Inspektionen. Ausgehend von einem Schadensfall, der gebrochenen ICE 3-Welle von Köln, 2008, werden ausgewählte Aspekte dieser beiden Säulen diskutiert und Vorschläge zur weiteren Verbesserung gemacht. Diese betreffen u. a. Fragen einer Betriebszeitbegrenzung, Veränderungen der Schwingfestigkeit während des Betriebs infolge von Korrosion und Steinschlag, den Einfluss von Korrosion auf Schwingrissinitiierung und -fortschritt, den potenziellen Einfluss nichtmetallischer Einschlüsse auf die Schwingfestigkeit, die Möglichkeiten, sie im Rahmen der Qualitätssicherung zu detektieren, sowie Fragen der Bruchmechanik und der Zuverlässigkeit zerstörungsfreier Schwingriss-Prüfverfahren im Rahmen eines Konzepts regelmäßiger Inspektionen.
Die Sicherheit von Radsatzwellen beruht gegenwärtig auf zwei Säulen: der Schwingfestigkeitsauslegung und periodischen Inspektionen. Ausgehend von einem Schadensfall, der gebrochenen ICE 3-Welle von Köln, 2008, sollen ausgewählte Aspekte beider Säulen diskutiert und Vorschläge zur weiteren Verbesserung gemacht werden. Diese betreffen u.a. Fragen einer Betriebszeitbegrenzung, Veränderungen der Schwingfestigkeit während des Betriebs infolge von Korrosion und Steinschlag, den Einfluss von Korrosion auf Schwingrissinitiierung und -fortschritt, den potentiellen Einfluss nichtmetallischer Einschlüsse auf die Schwingfestigkeit, die Möglichkeiten sie im Rahmen der Qualitätssicherung zu detektieren sowie Fragen der Bruchmechanik und der Zuverlässigkeit zerstörungsfreier Schwingriss-Prüfverfahren im Rahmen eines Konzepts regelmäßiger Inspektionen.
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.
The safety assessment of railway axles is based on a two-stage approach: fatigue strength design and regular inspections which, in terms of a general safety philosophy refer to safe-life and damage tolerance concepts. Starting with a recent failure case, a broken axle of a German high speed train, a discussion is presented on issues of both safety levels. These include ideas for finite life design, the treatment of in-service effects on the fatigue strength due to flying ballast damage and corrosion pits, the effect of corrosion on fatigue crack initiation and propagation, potential effects of non-metallic inclusions in steels, the way to detect them by quality control measures and reliability aspects of non-destructive testing with respect to the detection of fatigue cracks. Proposals are made how the safety level could be further improved.
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.
According to the definition of the ASM handbook [1,3], a defect is "an imperfection. that can be shown to cause failure by a quantitative analysis and that would not have occurred in the absence of the imperfection". The topic of the present three-part review is a discussion of defects which can cause failure in cyclically loaded structures. The features discussed comprise material defects such as non-metallic inclusions, pores or micro-shrinkages, etc. and geometric defects such as surface roughness and secondary notches which have their origin in manufacturing, and defects such as surface damage due to scratches, impact events or contact fatigue as well as corrosion pits which arise in service. In this first part, the discussion is prefaced by an introduction to basic aspects which are essential for a deeper understanding of the characteristics and mechanisms how the defects influence fatigue crack initiation and propagation. These include the life cycle of a fatigue crack from initiation up to fracture, crack arrest, multiple crack initiation and coalescence, and the material and geometrical properties affecting these.
This third part of the review on defects as root cause of fatigue failure addresses cavities (pores, micro-shrinkages, unmelted regions), defective microstructures and microcracks as material defects and defects due to local damage during manufacturing, service and maintenance such as dents, scratches and localized corrosion. In addition, damage due to contact fatigue and the effect of surface roughness are discussed in the context of fatigue failure. Also addressed is the competition between different kinds of defects in controlling the initiation and early growth of fatigue cracks.
Defects as a root cause of fatigue failure of metallic components. II: Non-metallic inclusions
(2019)
This second part of the review on defects as root cause of fatigue failure comprises the origin, the nature and the effects of non-metallic inclusions. Topics addressed are the different kinds of inclusions formed during the manufacturing process, various types of mis-match causing local stresses and, as a consequence, fatigue crack initiation, and effects of characteristics such as size, morphology, localization, spatial distribution and orientation of the defects on the fatigue behavior. Methods for inclusion counting and sizing are discussed along with statistical aspects necessary to be considered when evaluating structural components.
Although fatigue crack propagation and fracture cause a large part of failure events in industrial practice, fracture mechanics in failure analysis seems to be still a side issue. Starting from an introduction into important basic questions of failure analysis and fracture mechanics, the authors specify what kind of questions in failure analysis can be effectively solved by fracture mechanics (and which can't). They illustrate their discussion with a number of 13 case studies from the literature. Much more pronounced than in the design stage the benefit of fracture mechanics in failure analysis depends on its accuracy. This is limited by both, intrinsic factors of the method and the availability and quality of the input information. The authors discuss the various aspects and provide the reader with some background information which, as they believe, will be helpful for better understanding the prospects and limitations of fracture mechanics in failure analysis and the conditions of its application.
The paper provides an overview on material defects which may serve as fatigue crack initiation sites and can cause final fatigue failure of a component. These comprise nonmetallic inclusions in Steel and aluminum alloys, cavities such as pores, micro-shrinkages and un-welded regions in cast, sinter and additively manufactured alloys, graphite nodules, shrinkages and other items in modular cast iron, regions of defective microstructure, microcracks and secondary notches such as undercuts and surface roughness. Besides their origin, the effect and mechanisms on fatigue crack initiation and propagation are discussed.
The considerations are proceeded and accompanied by a Brief discussion of some Basic aspects such as the stages of crack propagation along their length scale, the overcoming of crack arrest and the question when a secondary notch can be treated as a crack.