Fatigue life prediction based on variable amplitude tests - methodology

The reliability of mechanical systems depends in part on the fatigue strength of the material, whose properties need to be determined by experiments. In a standard fatigue test a Wöhler curve is established, i.e. the relation between the life in the number of cycles and the load amplitude. Based on a damage accumulation rule, the life can be predicted for service loads. However, most service loads are far from constant amplitude loads, often resulting in large systematic errors in life predictions. The method in this paper is based on an estimation of the Wöhler curve directly from variable amplitude tests. Consequently, both the life prediction and estimation are performed at similar load situations. In the Wöhler curve the constant amplitude is replaced by an equivalent amplitude given by the load spectrum. An important part of this paper is the analysis of the statistical uncertainties in life estimation and prediction. The features and usefulness of our method are demonstrated by data from Agerskov. Our method can be seen as an extension of the Gassner line and the relative Miner rule.