Stressed area effect
In: Notch effect
Notches influence local stress/strain fields by forming highly stressed areas. The size of such an area and the related stress/strain gradients are expected to relate to the final damage. The Weibull weakest link concept assumes that as the highly stressed area broadens, the probability, that some weak link in the microstructure initiates a micro-crack, increases.
If the S-N curve based methods are used, the local elastic stress at the examined place is usually derived from the FE-model. If we nevertheless compare fatigue limits under different axial load modes (tension, bending, rotating bending) we found that they differ even though valid for the same specimen. The local stress itself is not the only variable which should affect the final solution. The state around the examined point should be included in some way.
The stress gradient concept is the only solution up to now implemented in PragTic. It is today used within the LESA method only. Its extension to other methods is planned to be studied further in my work.
Note: There exists a number of other concepts that process the same part of analysis - theory of critical distance, volumetric methods, etc. I hope that some of them will appear in next versions of PragTic.
Because of the most of other fatigue calculation methods cannot involve this effect, look at the important remark concerning axial fatigue limit.
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