Stress Based Fatigue Analysis

Stress Based Fatigue Analysis

Stress based fatigue analysis is one of three approaches to assess fatigue (https://www.fea-solutions.co.uk/fatigue-analysis/). It is sometimes referred to as the stress-life approach.

A stress based fatigue analysis should be performed in the following scenarios:

  • High cycle fatigue problems, for structures that have to withstand more than 10,000 cycles during their intended lifetime.
  • Peak stresses (well) below the Yield Strength, hence no plasticity.
  • Components where crack initiation is not acceptable.
  • Problems where large amounts of pre-existing S-N data (https://www.fea-solutions.co.uk/s-n-curves/) exists.
  • Components which are required to be designed for fatigue by an engineering standard or regulatory body.
  • Spot weld analysis.
  • Fatigue problems induced by random vibrations.

The most simple stress based fatigue analyses can be conducted using an S-N curve. By using the peak stress from an existing stress analysis, the number of load cycles before the component fails can be read off the graph. However, if the mean stress of the load cycle is not 0, stress correction (https://www.fea-solutions.co.uk/mean-stress-correction/) must be done before the S-N curve is used. If several independent loads are involved, cumulative damage theory (https://www.fea-solutions.co.uk/cumulative-fatigue-damage/) has to be applied as well.

Stress based fatigue analysis assumes that the structure behaves fully elastically (https://www.fea-solutions.co.uk/elasticity-and-plasticity/), even in local areas of fatigue interest, such as notches or sharp corners, meaning that the peak stress stays below the Yield Strength. A stress based fatigue analysis does not differentiate between crack initiation and propagation, which is why it cannot be used where crack initiation is an important factor.

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