The stress-life approach

The stress-life approach was the first well-developed approach to fatigue analysis. It was already developed in the 19th century primarily for use in the railroad industry. The loading of large, rotating axles in trains was such that bearing loads caused elastic bending stresses in the axles. As a location of the axle rotated, the stress state alternated from tension to compression, and fatigue failures occurred after a large number of cycles.

Although some full scale testing of the components was done, it was necessary that this type of loading be conducted on a smaller, laboratory scale. To accomplish this, a rotating-bending testing machine for specimens of materials was developed. Various bending loads led to crack initiation and failure of the specimens at different lengths of time. This test data is plotted on a stress amplitude vs. cycles to failure diagram, typically on logarithmic scales, along with a "best-fit" or other statistical line, called the SN-curve.

For fully-reversed tension-compression bending loading, the fatigue life of a component can be estimated by finding the life in cycles that corresponds to a calculated stress amplitude by the intersection on the SN-curve.

Data Flow in the Stress-Life Approach

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Defining SN-curves

Logarithmic nature of fatigue

Determination of SN-curves

How does the SN-curve definition via universal slope work?

Modifying factors of the SN-curve

Mean stress effects

Variable amplitude loading

Stress-life analysis in Specialist Durability

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