Constant Amplitude

All structures and mechanical components that are cyclically loaded can fail by fatigue. With limited input data, constant amplitude fatigue analysis is used to make a simple and quick estimate of the likely fatigue performance or durability.

There are three primary methods for estimating the fatigue resistance of components and structures. Stress-Life analysis assumes that the stresses always remain elastic even at the stress concentrators. Most of the live is consumed nucleating small microcracks. This is typical for long life situations (millions of cycles) where the fatigue resistance is controlled by nominal stresses and material strength. Strain-Life is used for situations where plastic deformation occurs around the stress concentrations. An example would be in a structure that has one major load cycle every day. Both stress-life and strain-life provide an estimate of how long it will take to form a crack about 1mm long. Crack growth analysis is then used to estimate how long it will take to grow a crack to final fracture. Fatigue of welds requires special considerations because of their complex shape and loading.

This section provides analysis for simple constant amplitude loading for all of the methods. It is typically found in power transmission applications such as shafts, gears etc. It is frequently used in the early stages of design to set the overall stress levels and to select appropriate materials. Many design and testing specifications are written in terms of constant amplitude loading.

Finders are provided to obtain the necessary input information for material properties and stress concentration or stress intensity factors.

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Fatigue Calculators

  • Stress-Life
    Use this method for long life situations where the strength of the material and the nominal stress control the fatigue life.
  • Strain-Life
    This method is used for finite fatigue lives where plasticity around stress concentrations is important.
  • Crack Growth
    Use this method to determine how long it will take a crack to grow to a critical size.

Finders

Technical Background