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- Fatigue failure (4) (entfernen)
Organisationseinheit der BAM
Concrete is a complex material and can be modeled on various spatial and temporal scales. While simulations on coarse scales are practical for engineering applications, a deeper understanding of the material is gained on finer scales. This is at the cost of an increased numerical effort that can be reduced by the three methods developed and used in this work, each corresponding to one publication.
The coarse spatial scale is related to fully homogenized models. The material is described in a phenomenological approach and the numerous parameters sometimes lack a physical meaning. Resolving the three-phase mesoscopic structure consisting of aggregates, the mortar matrix and the interfaces between them allow to describe similar effects with simpler models.
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.
Investigations on the breakdown of a heat recovery steam generator during the initial operation run
(2014)
Several years ago, in 2003, an industrial heat recovery steam generator in charge of generating process steam in a petrochemical refinery was installed and prepared for initial operation. The steam generator enclosed an evaporator section and a superheater section consisting mainly of bundles of tubes with the longest up to several metres in length. During initial operation test runs severe leakages in the evaporator and superheater modules became noticeable. The test runs were stopped and after disassembly, through-wall cracks in several tube bends were found. BAM was commissioned to carry out the investigations in order to find out the reasons for the failure of the tubes. During on-site inspection a number of relevant damaged components and parts were selected and taken away to the laboratories for detailed inspection. Planned analyses were to comprise metallographic as well as fractographic investigations, mechanical fatigue testing and experimental as well as finite element vibration analyses on specimens and components. Soon, the fracture mechanism was found to be mechanical fatigue due to the fact that the examined fracture surfaces showed the very characteristic beachmarks and colouring patterns. To identify the particular loading and time at which crack initiation and crack propagation took place, experimental and numerical vibration analyses of specific tubes as well as mechanical fatigue tests on tube bends were carried out. Thus it was possible to identify the eigenfrequencies of individual tubes, to estimate the dynamic response as well as the nominal stresses and, hence, experimentally characterise the in-service fatigue strength of the components. Mechanical and thermal comparative tests on tube bends were performed simulating the conditions during the initial test run in order to get crack surfaces comparable to that of the originally damaged components. Thereby it became obvious that the fatigue cracks were initiated by vibrations the tube bundles were exposed to during rail transport from the manufacturers site to the place of installation. Based on these results, the damaged components could be repaired or exchanged without modification of the construction, but the rules relating packaging and securing for shipping had to be revised.