CAE Simulation & Solutions

Maschinenbau Ingenieurdienstleistungen GmbH

Technical Office for Engineering Services

# Infinite Life Fatigue Strength

In the case of constant amplitude fatigue loading most technically relevant materials and joining technologies show no damage in tests below a certain threshold value of the stress amplitude (s_{ai}), even for very high numbers of load cycles (n_{i} > 1000000). This amplitude value is often simply called fatigue strength or to be more precise infinite life fatigue strength. In the proof of infinite life fatigue strength the shape of the stress spectrum is not important anymore. If the largest occurring stress amplitude remains below the fatigue strength, then the proof is fulfilled for the complete spectrum.

The number of load cycles, which is considered “infinite” varies between approximately 1 to 100 million cycles, depending on material, joining technology and stress state as well as the specific code used for assessment.

The assessment of infinite life fatigue strength is commonly used for structures or machinery components experiencing very large numbers of load cycles or a long service life, such as railway vehicles with often more than 30 years of service time.

For stress spectra with at least one step (s_{ai}, n_{i}) above the infinite life fatigue strength a complete variable amplitude fatigue strength analysis must be performed, since damage will occur and even steps with stress amplitudes below the fatigue strength may cause further damage.

Infinite life fatigue strength can be seen as a simple and special case of variable amplitude fatigue strength.

The software LIMIT includes various design codes and guidelines for different types of proof of strength. All previously mentioned assessment types are available:

- - Finite life fatigue strength / variable amplitude fatigue strength
- - Static strength
- - Infinite life fatigue strength