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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)
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.
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.
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.
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.
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.
An einer Eisenbahnbrücke über die Elbe bei Lutherstadt Wittenberg führten windinduzierte Schwingungen zu einer unplanmäßigen und frühzeitigen Ermüdung ausgewählter Bauteile und Verbindungen des Stahltragwerks.
Für die zyklisch bereits vorbelasteten und noch im Bauwerk befindlichen Bauteile konnte rechnerisch eine ausreichende Restnutzungsdauer nicht ermittelt werden. Auf Basis von experimentellen Untersuchungen wurde daher ein ingenieurmäßiges und baustellengeeignetes Sanierungskonzept für die vorbelasteten Bauteile entwickelt, deren Hauptbestandteil das Beschleifen der betroffenen Oberflächen darstellt.
Weiterhin wurden für die ermüdeten Schweißverbindungen bruchmechanische Berechnungen angestellt, um auch deren Restnutzungsdauer abschätzen zu können. Hierfür wurden den im Bereich der Deutschen Bahn AG geltenden Rechenansätzen vergleichend weitere Ansätze aus dem Maschinenbau gegenübergestellt.
Durch die angestellten Berechnungen und Untersuchungen konnte nachgewiesen werden, dass für die betroffenen Bauteile nach deren Sanierung ausreichende Tragsicherheit und Ermüdungssicherheit vorschriftengemäß eingehalten werden.
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.