Low Cycle Fatigue (LCF) is characterized by repeated plastic deformation. Essential here is that we are talking about repeated plastic deformation, which is the case if the nominal stress level exceeds the yield strength in each cycle, meaning we have plastic deformation on macro scale, so in the full net section of the structure.

The situation is completely different when only the stress in a small area near a notch exceeds the yield strength (Kt x S in the image below). In this case when applying a load cycle, only in the small area near the notch will there be a plastic zone, as per the image below. In the remaining net section, you will still have elastic deformation because stresses are lower than yield.

This implies that you will have a redistribution of stresses over the net section. During unloading of the load, the complete distribution will be unloaded elastically, leading to a residual stress distribution after unloading (shown on the right in the image below). Here the region close to the notch will have a compressive residual stress.

With further cyclic loading in the case in the figure below, you will get elastic behaviour, i.e High Cycle Fatigue (HCF) and no repeated plastic deformation!

To conclude exceeding the yield strength locally at a notch will not lead to Low Cycle Fatigue (LCF) but will merely lead to a stress redistribution. Any fatigue failure related to this case will be due to HCF.

Check out our "Essentials of Metal Fatigue" and "Comprehensive Guide to Fatigue" learning paths to expand your theoretical knowledge on fatigue in metallic structures.

This blog was curated by Johannes Homan from Fatec Engineering.