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In einem Gemeinschaftsprojekt der Stahl- und Automobilindustrie wurden an zwanzig Stählen für Feinblech elastische, plastische und zyklische Kennwerte für die FE-Simulation ermittelt. Die Untersuchungen mit dem Ziel, Eingabekennwerte für FE-Berechnungen zu erhalten, erbrachten einen erheblichen Erfahrungszuwachs im Bereich der Messtechnik sowie der Datenerfassung und -weiterverarbeitung. Der vorliegende Beitrag gibt einen Überblick über die Untersuchung und ihre Ergebnisse.
In corrosive environments such as CCS bore holes or geothermal power plants the materials loaded cyclically are also exposed constantly to the highly corrosive hot thermal water. The lifetime reduction of (X46Cr13, AISI 420C) is demonstrated in in-situ-laboratory experiments (T=60 °C, geothermal brine: Stuttgart Aquifer flow rate: 9 Nl/h, CO2). S-N plots, micrographic-, phase-, fractographic- and surface analysis were applied to obtain sustainable information on the corrosion fatigue behavior. Maximum number of cycles (here 12.5 x 106 cycles to failure) is reached at σa =173 MPa. No typical fatigue strength exists and passive corrosion fatigue is most likely failure cause.
The development of procedures to calculate the service life of railway rails requires test results which describe the damage of rails thus affording their validation. Within the DEUFRAKO integrated project NOVUM, ten rail specimens were tested under constant and variable amplitude loads to investigate the propagation of lateral cracks and to ascertain the service life. The test conditions were based on real rail loading and were used in the 3-point-bending test frame under alternating stress. An eccentric notch was applied as pre-arranged damage in the 60 E1 (260) rail specimens. The notched rails showed a similar behaviour like rails with strong head checks damaged by rolling contact fatigue.
On October 24th 1950 the 'Freedom Bell' rung for the first time in the tower of the former West Berlin town hall. One year after the Berlin blockade and at a first climax of the 'Cold War', the 'American National Committee for a Free Europe' donated the bell to the city of West Berlin. The bronze bell has a weight of about 10 tons. Its clapper has a length of 2.40 m and a weight of nearly 1 ton. In 1966 the hanging of the clapper and in 1979 the clapper of the bell broke down. Both failures were found to be caused by fatigue fracture. In 1974, a crack in the bell body was detected. After years of periodic monitoring using non-destructive testing, the crack was closed in 2001.
In July 1968 the Swedish motor vessel 'MV Stureholm' traveled from Sweden across the North Atlantic with destination to the American gulf harbours. MV Stureholm was a freighter with 9700 tdw build in 1957 by a German shipyard. About 100 nautical miles north of the peninsula Yucatan the 9,000 HP diesel engine failed as a consequence of fatigue failure of the crankshaft. The ship was disabled and had to be towed to Veracruz, Mexico. The Swedish shipowning company decided to perform a temporary repair in Veracruz by welding the fatigue crack. Towing the motor vessel back to Sweden would have been too expensive.
Back in Sweden the damaged but repaired crankshaft was dismounted from the engine and inspected by surveyors. The result was that a fatigue crack had initiated from a subsurface solidification (hot) crack. The latter was formed during the solidification of the casting process. Before delivering the casted crankshaft to the engine manufacturer this crack was repaired by a so called 'weld for fabrication', which is a common procedure for large casted components. Unfortunately the repair weld was not appropriate in such a manner that the hot crack was only welded on its surface for a depth of 10 mm, the entire crack depth however was about 30 mm.
The shipowning company evaluated its amount of financial loss to 266,000 USD and filed a complaint at the German regional court in Düsseldorf against the German engine's manufacturer. The defendant contradicted the thesis of the Swedish surveyor, and the court of law entrusted (Federal Institute for Material's Research an Testing, a non commercial sovereign institute) BAM to carry out an independent survey report.
BAM survey report, predominantly based on metallographic analyses, stated the Swedish conclusions as applicable and moreover confirmed the results of the fracture mechanics based crack propagation calculations which are explaining a very slow crack propagation and a fast final failure even after more than 10<sup
>8 load cycles.
In geothermal power plants materials e.g. pumps are exposed to extremely corrosive thermal water. This results in corrosion fatigue and so inevitably the reduction of the lifetime of these components. Also in the field of the carbon dioxide storage technology (carbon capture and storage CCS) components are exposed to a corrosive environment and mechanical stress. In order to gain knowledge upon the corrosion fatigue strength of materials a corrosion chamber for "in situ" conditions was designed and successfully applied.Two different steels X46Cr13 and X5CrNiCuNb16-4 have been tested and their corrosion fatigue behavior was compared. To simulate the frequency of operating pumps (30 – 40 Hz) a resonant testing machine was used. In addition technical CO2 was introduced into the closed system at a rate close to 9 L/h to keep stable environmental conditions. The samples have a surface roughness of Rz = 4 to simulate technical machined surfaces. The calculated tensile strength of X46Cr13 with soft annealed microstructure (coagulated cementite in ferrite-perlite matrix) is about 680 MPa and the yield strength is about 345 MPa. The tensile strength of X5CrNiCuNb16-4 is about 1078 MPa and the yield strength about 928 MPa. Testing parameters are: corrosion media: saline aquifer water (Stuttgart Aquifer) temperature of the brine at 60 °C and load ratio of R=-1. For X46Cr13 a stress amplitude between 160 MPa to 270 MPa and for X5CrNiCuNb16-4 a stress amplitude between 150 to 500 was chosen. Cycles until crack initiation differ strongly and were found to start at 5 x 104 up to 12.5 x 106 cycles.X46Cr13 has reached a maximum number of cycles (12.5 x 106) at a stress amplitude of 173 MPa. X5CrNiCuNb16-4 has reached the maximum number of cycles (10 x 106) at a stress amplitude of 150 MPa. The range of scatter for X5CrNiCuNb16-4 is very high (1:34) in comparison the range of scatter for X46Cr13 (1:3.5).
Design of a high pressure system for in-situ tests on the corrosion fatigue of metallic materials
(2014)
In geothermal power plants pumps are exposed to corrosive and oscillating loads. This leads to corrosion fatigue and reduces the lifetime of certain components. Usually underwater power pumps are placed inside the borehole. In this case an installation depth of 600 to 800 meters below ground level is typical. In these depths an additional load due to high pressure and high temperature is introduced which is based on geothermal brine. Therefore the material which is used for the pump is steel.For fatigue testing at loads existing in deep geological layers an appropriate system for fatigue tests under in-situ conditions was set up. The requirements of the design are: temperature of the corrosion medium of 200 °C pressure of 200 bars and the complete immersion of the specimen in flowing corrosion medium containing mixtures of corrosive gasses during the entire test period.The test system has three main components the corrosion chamber the heating module and the reservoir. It is important that the corrosion chamber is only fixed onto the specimen which allows for highest flexibility of the test system operational in many different test machines. The heating module heats the corrosion medium up to 200 °C. The reservoir make sure that the volume of the corrosion medium satisfies requirements of the DIN 50905 Part 1 is conformed providing minimum required corrosive medium of 10 ml/cm² in relation to the sample surface.This innovative fatigue test system aims at performing in- situ fatigue corrosion tests simultaneously at high pressure and high temperature in liquid and gaseous corrosive environment. Results from the tests will allow determining the fatigue life of several materials influenced by pressure and temperature under corrosive conditions.
Die Verwendung von höherfesten, vergüteten Stählen für Radsatzwellen wird zurzeit kontrovers diskutiert. Für die zuverlässige Auslegung mit diesen Werkstoffen werden verlässliche mechanische und bruchmechanische Kennwerte benötigt. Zudem müssen die aus Prüfungen an Kleinproben und Bauteilen gewonnenen Werte konsistent sein. Das war bisher nicht immer der Fall.
In einem von der Bundesanstalt für Materialforschung und -prüfung (BAM) Berlin im Auftrag des Verbandes der Bahnindustrie in Deutschland (VDB) durchgeführten Forschungsvor- haben wurden Proben aus dem Werkstoff A5T (42CrMo4+QT), entnommen aus Original-Radsatzwellen und aus 1:1-Proben eines anderen Forschungsvorhabens, untersucht. Alle Proben haben für den Werkstoff A5T und seinem Wärmebehandlungszustand den gültigen Normen entsprechende Kennwerte im Rahmen der geforderten Eigenschaften gezeigt. Gefügefehler konnten an den vorliegenden Prüfstücken nicht nachgewiesen werden. Die vorgenommenen Wärmebehandlungen beeinflussen das Gefüge und die mechanischen Eigenschaften in der erwarteten Weise.
Im Rahmen eines gemeinschaftlichen Projektes der Stahl- und Automobilindustrie wurden für 17 Stahlfeinbleche des Automobilbaus in verschiedenen Anlieferungs- und Verformungszuständen unter -40°C, Raumtemperatur und +100°C mit 37 Versuchsreihen Anrisswöhlerlinien und zügige bzw. zyklische Spannung-Dehnung-Kurven bestimmt. Sämtliche Versuchspunkte, Hysteresen und ermittelte zyklische Kennwerte liegen in einer Datenbank vor. Eine Korrelationen zwischen den Kennwerten aus dem Zugversuch und den Kennwerten aus den zyklischen, dehnungsgeregelten Versuchen ist grundsätzlich möglich.