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- Zeitschriftenartikel (23) (entfernen)
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- Non-metallic inclusions (4)
- Fatigue (2)
- Fatigue crack (2)
- Pores (2)
- Safe life design (2)
- Schadensanalyse (2)
- Undamped structural elements (2)
- Vibrationsprüfung (2)
- Accompanying measures (1)
- Austenitic steel (1)
Organisationseinheit der BAM
- 9 Komponentensicherheit (5)
- 9.0 Abteilungsleitung und andere (4)
- 9.4 Integrität von Schweißverbindungen (4)
- 5 Werkstofftechnik (3)
- 5.1 Mikrostruktur Design und Degradation (3)
- 7 Bauwerkssicherheit (1)
- 7.6 Korrosion und Korrosionsschutz (1)
- 8 Zerstörungsfreie Prüfung (1)
- 8.1 Sensorik, mess- und prüftechnische Verfahren (1)
Shortly after midnight on 15th November 1992, a severe train crash was caused by the break off of a buffer from a tool and gear wagon of a freight train. The buffer fell between the rails, tumbled, and then lifted one axle of a following freight wagon off the rails. Several freight wagons derailed in the station of Northeim, Germany, blocking the adjacent track. A night train travelling on that blocked track was already too close (only 180 m) to stop, despite emergency braking initiated immediately and automatically by the track signalling system. When the night train crashed into the freight wagons, some of its coaches were heavily damaged. Eleven people were killed, and 51 injured, some of them seriously.
The task for failure analysts was to find out why the buffer broke off from the front frame of the tool and gear wagon. Therefore, the material of the attachment, the front frame, and the welds were tested. Chemical composition, hardness, and microstructure were allowable. The welds showed imperfections which were determined not to be causal. The fracture surfaces had graded corrosion which again was not causal and beach marks that depicted fatigue cracks before final fracture.
The cause for the break off was the fact that the front frame had been repaired at this same buffer attachment some years before and that the repair patch used was thinner than the original front frame profile.
Additionally, the force characteristics of the buffer showed reduced stroke but increased forces which was due to several friction springs that had been broken for a long time. The buffer forces in service were assessed from standards, literature, and the deformations of the attachment. The in-service stresses at the welds were estimated using FEM analysis. High cyclic stresses at the welds resulted in fatigue cracking and finally break off of the attachment with the buffer.
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.
In July 2008 an ICE3 high speed train rated for 330 km/h service speed derailed during departure from Cologne, Central Station, Germany, due to fatigue failure of one of the driving axles. The train was emergency stopped immediately and, due to low travel speed at this point, no serious injuries occurred to passengers. Referring to public interest, the public attorneys office solicited the German Federal Institute for Materials Research and Testing (BAM) for the analysis of the root cause.
No deviations from specification were found in the geometries of the basic parts of the bogie or the wheelset assembly. Inspection of the axle fragments using standard acoustic non-destructive testing (NDT) techniques revealed no additional cracks and no indications of oversized discontinuities. Metallographic and chemical inspection of the axle material and its microstructure revealed all parameters to be acceptable except for an elevated impurity level.
The fracture surfaces of the axle fragments were heavily damaged due to some continued travel after final breakage on the high speed line before Cologne Central Station. Extensive visual inspection of the remaining beachmarks was carried out to find the origin of the fatigue crack. The region of the crack origin was located near the axle surface but could not be analysed in detail due to secondary damage. Fatigue was identified as the mechanism of crack growth until final fracture, but the reasons for crack initiation initially remained unclear.
Neither standard NDT techniques nor metallography according to the relevant axle specifications were able to identify inclusions in the material that could have served as crack initiation sites. However, discontinuities were detected near the crack origin in micro computer tomography and ultrasonic immersion testing. Subsequent metallographic sample preparation was targeted to specific areas based on the location coordinates of the flaws identified by these NDT techniques. These revealed non-metallic inclusions that were much larger than admissible for the relevant specifications. It is likely that the fatigue crack in the highly loaded axle volume initiated at those non-metallic inclusions.
Intramedullary hip screws (IMHSs) are implanted for the fixture of pertrochanteric femoral fractures (thigh bone). The present work investigates the failure of an IMHS implant after the rehabilitation period which required a revision surgery where the upper femur had to be replaced by a hip endoprosthesis. Due to litigation the corresponding failure analysis was conducted at BAM Federal Institute for Materials Research and Testing and is the subject of this article. By order of the customer failure causes due to material and fabrication failures should be particularly investigated. Therefore, thorough fractographic, chemical as well as metallographic investigations were carried out. In order to assess possible fabrication failures selected dimensions of the components were compared with the technical drawings and the surface topography was analysed.
The investigation revealed several causes for the fracture of the lag screw which is the implant component intended to fix the position of the femoral head. These are in descending order of relevance: (1) the screw was placed incorrectly and therefore had a smaller bending stiffness, (2) the femoral fracture zone was not stable, i.e., the lag screw 'moved out' of its proper position and (3) the laser marking on the lag screw was relatively large and led to a metallurgical notch at the now, because of the above mentioned points, severely stressed region. A material- or fabrication failure was not the root cause for the fracture of the lag screw. It was mainly due to its incorrect placement during surgery and the instability of the bone fracture. Furthermore, the implant geometry with 130° was not well-suited for the patient's anatomy. A 135°-IMHS might have been biomechanically preferable. Based on the determined failure causes and influences a relative lifetime estimation led to about 10% of the lifetime of a correctly placed 135°-IMHS assuming a sufficiently stable bone fracture zone.
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.
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.
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.
Die Empfindlichkeit von Beschleunigungssensoren hängt von der Temperatur ab, bei der sie eingesetzt werden. Aus diesem Grund fordert der Entwurf der Neuausgabe von IEC 60068-2-53, dass Beschleunigungssensoren bei allen Prüftemperaturen kalibriert sein müssen.
Darüber hinaus müssen die Grenzabweichungen der entsprechenden dynamischen Prüfverfahrensnormen eingehalten werden.
Das Messunsicherheitsbudget für die Beschleunigungsmessung bei einer kombinierten Vibrations- und Klimaprüfung wird aufgestellt. Einflussgrößen sind die Kalibrierunsicherheit, der Frequenz- und Temperaturgang des Sensors, sowie die Messunsicherheit der elektrischen Messkette.
Anhand typischer Prüfbedingungen werden Kriterien aufgestellt, wann der Temperaturgang für die Prüfung relevant wird.
Eigene Untersuchungen und Hinweise aus der Literatur zeigen, dass dies eher selten der Fall ist. Gegebenenfalls muss auf geeignete Maßnahmen zur Verringerung der Messunsicherheit zurückgegriffen werden. Eine Kompensation des Temperaturganges ist möglich, aber derzeit noch aufwändig.
The 100th anniversary of the death of Adolf Martens will be commemorated on July 24th, 2014. He is eponymously remembered today through the term martensite, which was first used by Floris Osmond as a name for the metastable phase that results from rapid quenching of carbon steels. Born in 1850 near to Hagenow in the region Mecklenburg, Germany, Martens was one of the pioneers of materials engineering in 19th century Europe.
Martens began his career working for the Prussian Eastern Railway before joining the Royal Industrial Academy in Berlin in 1880. In 1884, he was appointed director of the Royal Mechanical Experimental Station, a small institution associated to the academy. Failure analysis was continuously practiced at this institution, which became later the nucleus of the German Federal Institute for Materials Research and Testing (BAM), for the 110 years since. The history of Martens will be dealt with in an in-depth article in this special issue.
Since the 19th century, failure analysis techniques have been refined, and new methods of chemical analysis and non-destructive testing have been developed; however, the basic approach to failure analysis has not changed much since Martens' days. The basic tenets of failure analysis remain things like on-site inspection, extensive visual 'non-destructive' inspection, developing an understanding of the background story, performing materials testing, and 'connecting the dots.' Martens introduced and developed experimental techniques like macro photography, fractography, metallography, hardness measurements, and mechanical testing. Modern failure analysts continue adding even more techniques to this list, leading to a more interdisciplinary approach which many would say is the only way to find the root causes of complex failure events.
The present special issue of EFA presents an overview of more than 100 years of failure analysis at BAM and its predecessors, closing the circle from the beginnings of modern failure analysis done by Martens himself in the 1890s to its present-day application.
This special issue starts with an excursion back to Martens' work and innovations and presents a newly translated original manuscript of Martens from 1890. Whereas some papers of Martens and his co-workers are well documented, only little can be found about failure analysis in the period from 1914 to the 1950s. Most documents of this period have not survived until today. Beginning in the 1960s more and more significant works are preserved, which were using the interdisciplinary approach of Martens. Since the beginning of the digital age in the 1980s almost all text documents are accessible, whereas digital images were stored since the 1990s. Since then the problem is no longer accessibility but copyright issues that prevent many interesting case studies from being published. Maybe the next generation of failure scientists can reveal some of them later on.