What is the Difference between Low & High Cycle Fatigue?

The difference between low cycle fatigue (LCF) and high cycle fatigue (HCF) has to do with the deformations. LCF is characterized by repeated plastic deformation (i.e. in each cycle), whereas HCF is characterized by elastic deformation. The number of cycles to failure is low for LCF and high for HCF, hence the terms low and high cycle fatigue.
Transition between LCF and HCF is determined by the stress level, i.e. transition between plastic and elastic deformations. That implies that there is no fixed transition life, e.g. 103, but that transition life depends on the ductility of the material.

S-N Curve - Low Cycle Fatigue and High Cycle Fatigue

Large numbers of small cycles

Small cycles (i.e. cycles with a small amplitude) lead to longer fatigue lives than large cycles (i.e. cycles with a large amplitude). This fact may lead to the incorrect conclusion that small cycles can be neglected in a fatigue life analysis. However, in a spectrum the number of small cycles is often much larger than the number of large cycles. If so, the small cycles do give a significant contribution to the damage accumulation.

It is sometimes even thought that small cycles can be neglected, resulting in a small number of large cycles, which situation then erroneously is interpreted as LCF. Note that LCF corresponds with cyclic plastic deformations, not with small number of occurrences of large (elastic) cycles.

Stress peaks exceeding the yield limit

In some stress spectra a peak stress may incidentally exceed the yield limit. Does that make the fatigue process LCF? As long as the cycles are dominated by elastic strains, the fatigue mechanism will be typical for the HCF process. An incidental cycle with increased contribution of plastic strain does not change this.

If stresses close to a notch exceed the yield limit, some local plastic deformation will occur. Subsequent elastic unloading leads to an inhomogeneous stress distribution. At the edge of the notch there will be compressive stresses, which are in fact favourable for fatigue.

Damage Mechanism

The HCF mechanism is determined by cyclic elastic strains. Important parameters are the stress concentration factor (presence of a stress gradient), surface roughness and conditions and mean stress levels. The LCF mechanism is determined by cyclic plastic deformations. The parameters that are important for HCF have no impact on LCF.

Since the mechanisms are so different, different methods should be used for fatigue life estimation for HCF (using S-N data) and LCF (using e-N data). However, despite of the difference, it happens often that LCF methods are used for HCF analysis using local stresses (or strains). In a previous blog post (Fatigue Analysis using Local Stresses) and in the free eBook Common Mistakes in Fatigue Analysis it is explained why this approach is not correct.