Nominal method

Thanks to its simplicity, this solution remains to be the most popular method for fatigue calculation. It operates over a S-N (Wöhler) curve, which relates level of loading (stress – S) with number of cycles to final damage (N).

The method often does not require knowledge of exact values of local loads in the critical place. Instead, a categorization of the notch and loading leads to a specified value of a notch factor. The level of loading is related to a nominal stress found in a near cross-section. In conjunction with knowledge of technological effects in the surface layer, effect of size of the component and other influences, a fatigue factor is set. The fatigue factor changes position of fatigue limit or slope of S-N (Wöhler) curve so that it corresponds to the special loading conditions achieved in the critical place.

To set a notch factor for complicated structures can be a very speculative task. If there are more places examined, the notch factor has to be specified separately in each of them. The accessibility of FE-methods made possible to enumerate exact local values of loads in the critical places. If the knowledge of the local loading is so detailed, why not to use it directly and pump the data into some fatigue software? A transition to local elastic stress analysis initiated like that.

It is very inconvenient for any user to enumerate the notch factor for each examined place and then relate it to that place. The local elastic stress analysis goes more straightforward. The nominal method as itself is not implemented in PragTic and its incorporation is not expected anytime later.

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