Abstract
Keywords: fatigue analysis, stochastic simulation, scattering of data
A lot of input data and parameters such as material properties, structural stresses or temperatures are needed for the fatigue analysis of a component. The values of these parameters can be found in material tables or as results of measurements, tests and simulations. All those parameters have in common that they show statistical variations. Yet, only one deterministic value can be used for each parameter in common simulation methods. Therefore the calculated fatigue damages and fatigue safety factors derive directly from the deterministic values of the input data. This means that the result of a fatigue analysis shows only one possible result depending on specific input data.
In order to include the scatter of input data in the design and analysis of components, new methods and approaches have to be developed. Up to now, safety factors are used to deal with this problem. This method is common practice, mostly shows good results and has a good ratio of cost and value.
Yet, there are tasks where the use of safety factors does not eliminate the problematic influence of varying input data, since the scatter is either too large, too unpredictable or varies spatially. Quiet often that´s the case, if the system behaviour is greatly non-linear. It´s also problematic if a component optimization based on spatially constant probability of failure has to be achieved. The fatigue analysis of pistons is a typical example of such problematic behaviour. The failure mechanisms of pistons vary spatially. In high temperature areas, such as the piston top, the reduced structural loading capacity is mainly responsible for failures, whereas in cooler areas, such as the piston skirt, compressive stresses can lead to failures.
While the deterministic fatigue analysis shows good results in the cooler areas and usually correctly predicts cracks and failures, there are differences between simulation and real endurance tests in high temperature areas such as the borders of the piston bowl.