Failure of a motor vessel’s crankshaft 1968

• Freighter MV Stureholm was put in service in October 1957.
• Disability to maneuver on open sea due to breakdown of the diesel engine in July 1968.
• Fatigue failure after more than 108 load cycles of the engine’s crankshaft.
• Inappropriate repair weld of a hot solidification crack.
• Micrographic preparation, analyses and graphic rendition of results according to the methods of A. Martens.

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 108 load cycles.