Fatigue refers to the strength of a component under alternating stress.


The decisive factors here are not the maximum stresses, which are checked by means of a static verification, but the stress amplitudes and the position of the mean stress. A distinction is made between high cycle fatigue (HCF) for load cycles > 10,000 and low cycle fatigue (LCF) for low load cycles < 10,000. In the case of LCF, plastic deformation often occurs in the component.

The verification can be either time-fixed for a given number of load cycles or fatigue-fixed. Special cases are pulsating or alternating loads with constant load amplitude. Factors influencing fatigue strength include surface roughness, component size, temperature, support effect, etc. The stress amplitudes are usually determined by FE calculations and evaluated with special software tools such as FEMFAT. 

As load amplitudes and frequencies change, damage accumulation calculations can be performed, resulting in a specific utilization ratio. If the degree of utilization is > 1, failure of the component occurs mathematically. If the fatigue strength check can be performed, there are usually no failures at the assumed loads. The design is then "safe". 

The reverse conclusion, that a component fails exactly when load level 1 is reached, is not permissible, since these are conservative procedures. 

Fatigue calculations at a glance

The complexity of fatigue calculations ranges from constant uniaxial load amplitudes to the consideration of multiaxial stress states with measured load time series. 

  • Fatigue strength 
  • Force fracture 
  • Fatigue life 
  • Low Cycle Fatigue 
  • High Cycle Fatigue 
  • Fatigue strength 

Fatigue application areas

Verification of the fatigue or creep strength of: 

  • Housings 
  • pumps 
  • motors 
  • Gearboxes 
  • Machine components 
  • Components of any kind