Structural Reliability Methods for Improved Designs Against Fatigue

In the design of fatigue sensitive components, attention is often focused on defining a representative load spectrum that can be used with the fatigue properties of the material to assess the relative merits of a particular design. In practice, these techniques often employ conservative assumptions on the loads and large safety factors on the fatigue properties (e.g. S-N curve) to evaluate successful performance over a design lifetime. While such approaches are usually sufficient there are many examples where successful design against fatigue failure has proven to be a challenging and costly endeavor. In these cases the fatigue problem is complicated by variability in the underlying load environment, the stress response, or shortcomings with the computational techniques employed for fatigue. Successful design against fatigue failure can only be achieved by considering all of these contributions to the fatigue problem and properly modeling the uncertainty in their definition. In this paper a generalized fatigue formulation is presented that includes the effects of the load environment, the stress response given the load environment, and a fatigue failure criterion. Structural reliability techniques are used to determine the relative impact that each underlying variable has on fatigue life and to compute the failure probability. With this information the designer is more informed about critical deficiencies in his design and can take the necessary steps to address them. Proper use of the proposed methodology is demonstrated with suitable examples.