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TR2

Technical Review 2 - Fatigue Calculations - Two Approaches

Most fatigue life calculations could be considered to fall into two broad categories:

1. Stress based (S-N) methods are common for high lives. In comparison with strain based approaches, the stress-life (S-N) methods are easier to understand and require fewer calculations.

The S-N method requires fatigue data in the form of curves of stress S, vs. cycles to failure N. Normally, smooth specimens are used to provide these curves. An FE model would be used to give the peak stresses in a component, and a life obtained by comparing this with S-N data from a series of smooth specimen tests.

2. Strain based fatigue methods are used especially for lives below about 10⁴ or 10⁵ cycles. Below this level, stresses can exceed the elastic limit. Using stresses to assess damage at these critical regions will hence be non-conservative since they no longer accurately represent the deformation a region undergoes if the yield point is exceeded. Strains are thus the more appropriate measure of the fatigue damage sustained. Chalice Engineering has extensive experience of implementing strain based fatigue life calculations, using FE results. Applications we have worked on include software coding of these techniques for aerospace components and ratchetting and shakedown analysis for pressure vessel equipment.