What is Fatigue?

Metal fatigue is about the predominant cause of failure of structures. But what is fatigue? Fatigue occurs when a structure is subjected to cyclic loading. If the stress amplitude exceeds a threshold value, microscopic cracks will initiate at locations with high stresses (stress concentrations). At first, the cracks propagate very slowly and remain undetectable for the bare eye for most of the fatigue life. Gradually the crack propagation rate increases and the cracks will become visible. Eventually the crack will reach a critical size and the structure will fail. Due to the nature of the fatigue process, fatigue failure can lead to safety issues.

The stress levels that cause fatigue damage are much lower than the static strength of the material, i.e. ultimate tensile strength and yield strength. Decisive for fatigue damage propagation are stress amplitudes; it is cyclic loading that determines fatigue.

Many factors play a role in fatigue, such as incorrect choice of material, rough finish or damaged metal surface, poor maintenance, including failure to timely replace a part. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to high local stresses where fatigue cracks easily can initiate. Round holes and smooth transitions or fillets will increase the fatigue strength of the structure.

Some characteristics of fatigue are:

  • Fatigue is a structures issue, not just a material issue.
  • Stress concentrations (holes, keyways, fillets) and locations with secondary bending are common locations at which fatigue cracks initiate.
  • Fatigue often shows significant scatter.
  • The larger the stress amplitude, the shorter the fatigue life.
  • No large scale plastic deformation.
  • Damage is cumulative. Unlike humans, materials do not recuperate from fatigue.