For high performance propulsion systems in efficient ships the demand for light and highly optimized propellers are a crucial point to consider. Nevertheless, those propellers with a weight of several tons made of aluminum bronze alloys are highly subjected to mechanical loading in terms of vibration and cyclic loads superimposed by the corrosive effect of the surrounding seawater resulting in a complex load. Understanding this overall situation and the resulting local stress concentration as well as their interaction with material parameters influenced from the production process is important for optimized light and safe propellers. Moreover, new developments in the aluminum bronze alloys and the effect of local material defects need to be assessed to conduct a proper and safe component design. While present standards and guidelines offer methods for a synthetic lifetime assessment for propellers under corrosion these methods do not imply present material developments and do not consider the effect of local microstructures and material defects on lifetime.
Within the research project "Bross" the Mecklenburger Metallguss GmbH and the Fraunhofer Institute for Structural Durability and System Reliability LBF work on the further development of bronze alloys and a corresponding fatigue and reliability design concept for large ship propellers. Therefore, in first steps different alloys based on standard aluminum bronze for propellers were cast in component-comparable cast blocks. Afterwards, axial fatigue specimens were removed from those cast blocks. Further on, the cyclic stress-based material behavior was derived from those specimens under ambient air and seawater to gain correct and serial process fatigue data to enable a proper lifetime assessment. The main objective of "Bross" is to check the influence of chemical composition, casting process parameters, wall thickness as well as the resulting local microstructure in combination with seawater on the fatigue strength of aluminum bronze and to compare those data with the given standards for a material certification.
In further steps of the project also the influence of local material defects as well as additional alloys will be investigated under air and seawater to enhance the possibilities for highly efficient ship propellers.
