The two basic finite life approaches

Fatigue is the degradation of a material due to repeated cyclic loading. For metals, this typically means the initiation of small cracks from active slip bands in grains on the free surface of a specimen, component, or structure, which eventually link to form large cracks that either break or severely degrade the performance of a component.

For most machinery and ground vehicles, the length of time that it takes to initiate cracks is typically much longer than the length of time that it takes for the cracks to link and fracture the component. This means that the useful life of the component can be nearly the same as the length of time that it takes to initiate cracks.

The following approaches have been developed to estimate the fatigue life of the components:

• Stress-life approach

• Strain-life approach

• Fracture mechanics approaches

Stress-life and strain-life approaches estimate the crack initiation life of components whereas fracture mechanics approaches deal with the final stages of fatigue failure, in which cracks link and propagate. However, fracture mechanics will not be a main topic in this documentation. Interested readers are encouraged to examine the list of references for textbooks on this subject. From this point on, the term fatigue life will refer to the crack initiation life of a structure or specimen, unless otherwise specified.

The stress-life approach and the strain-life approach are similar in some respects, but quite different in others. The goal of both approaches is to estimate the crack initiation life of structures from a mechanics based analysis of the stresses or strains in the structure and basic material property tests.

The type of fatigue analysis approach to use will depend on the type of failure mode that is expected for the structure (see the figure below).

A static (non-fatigue) failure mode results in large structural deformation and is controlled primarily by the resistance of the net section.

Low cycle fatigue is usually the result of a large plastic zone at a notch, or stress concentration. Low cycle fatigue behavior depends on the notch severity and the inelastic material response.

High cycle fatigue is representative of situations in which there is little plasticity, and the notch severity, manufacturing processes, and residual stresses play an important role.

The identification of the failure mode is important both in testing and analysis. Testing at load levels that are too high will change the failure mode and give the incorrect safety factors or may identify incorrect critical locations.

The stress-life approach is typically more suitable in situations where the stresses are below (or not much above) the elastic limit of the material.

The strain-life approach is typically used in situations where yielding of the material may be present at certain locations on the structure, and has been developed based on knowledge of more detailed material behavior when subjected to stress levels above the yield strength of the material.

Failure Modes are Different for Static Failure, Low Cycle Fatigue, and High Cycle Fatigue.

Learn more

Concepts in common

Damage-based data reduction methods

Non-local and surface effects

The statistical nature of fatigue

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