Fatigue Strength under Variable Stress Amplitude

Fatigue strength under variable stress amplitude is a central concept in materials science and the calculated fatigue life of a component under real operating load conditions. It represents the ability of materials and structures to withstand static, quasi-static, and dynamic loads — whether cyclic or impact — without damage over the intended fatigue life, while accounting for relevant environmental factors.

Both numerical calculation methods and experimental procedures are used to evaluate fatigue strength under variable stress amplitude. This interdisciplinary field considers the interaction between mechanical loading, material behaviour, manufacturing processes, and design principles.

In practice, dynamic loading is often the cause of component failure—frequently occurring at load levels well below the static fracture load. An important subfield is fatigue strength under vibration, which describes the behavior of materials under cyclic loading.

The Wöhler curve (S–N curve) enables a statistically sound prediction of the number of load cycles that a component can withstand under operational loading before failing.

Endurance Limit as a Special Case

A specific application is the verification of the endurance limit, in which the characteristic point of deflection of the Wöhler curve (endurance limit) serves as the decisive parameter for sizing. The ratio between the maximum load occurring in operation and the endurance limit is referred to as the utilization factor.

For this calculation, no detailed load–time histories are required; knowledge of the maximum loads and their characteristics (fully reversed, pulsating, or constant) is sufficient. Assuming a worst-case scenario, the utilization factor can be determined. If this factor is low, the more complex operational fatigue strength assessment can often be omitted.

Guidelines and Standards

Numerous guidelines support the practical application of fatigue strength under variable amplitude:

  • FKM Guideline (general mechanical engineering)
  • Germanischer Lloyd (e.g., for shipbuilding and wind energy)
  • IIW Guideline (International Institute of Welding)

These standards help ensure a safe, reliable, and economically efficient design in accordance with the state of the art.

Benefits and Application with winLIFE

The application of fatigue strength under variable stress amplitude analysis leads to a deeper understanding of the product, an optimized component structure, weight reduction, and often cost savings. In addition to technical advantages, product liability also plays an important role.

Our winLIFE software was developed to make the application of established methods as user-friendly as possible. A prerequisite for successful use is a solid foundation in mechanics, which we provide through our two-day seminars (offered three times a year). These seminars form the basis for a more in-depth introduction to the theory and practice of operational fatigue strength under variable stress amplitude.

Upon request, we support our customers during their first projects to facilitate the initial learning curve and to provide confidence in the interpretation of the results. Our publications and video examples offer an initial insight into the wide range of application possibilities of winLIFE.

Component Types

With winLIFE, you can analyze the following types of components:

  • Non-welded components
  • Welded components
  • Gears and bearings

Analysis Methods

winLIFE supports the following analysis methods:

  • Crack initiation analysis
  • Crack propagation
  • Multiaxial problem analysis
  • Stochastic analysis
  • Gears and bearings