Fatigue Due to Random Loadings

Fatigue damage is traditionally determined from time signals of stress or strain using the Rainflow method to extract fatigue cycles. This approach is satisfactory when the damaging events are deterministic and are represented by short data samples of extreme transient loads such as aircraft landing, or automobile potholes. In many cases however, fatigue damage is attributable to long periods of random vibration loading which is stochastic in nature. In these cases it is impractical and statistically inappropriate to perform time signal analyses and a more efficient and appropriate method is to use a PSD (Power Spectral Density) representation of the loading. An example with a bracket will be given.

We will see how a spectral method can give an almost exact approximation of the Rainflow counting approach and offer more robust, efficient and statistically appropriate solutions.


The Author

Frédéric Kihm
Application Engineer
HBM United Kingdom Limited