Definitions for Fatigue

As the alternating stress gets smaller, the material can take more stress cycles before it fails due to fatigue. The endurance limit is the highest alternating stress that does not result in fatigue failure. In other words, if the alternating stress is equal to or lower than the endurance limit, the number of stress cycles to cause failure becomes very large (practically infinite). The endurance limit is usually defined for zero-mean alternating stresses. The endurance limit is also called the fatigue limit. Some metals do not have a measurable endurance limit.

The alternating stress is defined as |(σ_max - σ_min)| / 2 where σ_max and σ_min are the maximum and minimum stresses respectively.

In Simulation, you can set the stress component to calculate the alternating stress as: the Stress intensity (P1 - P3), the equivalent stress (von Mises), or the maximum absolute principal stress (P1).

Stress = (σ_max - σ_min)

Mean stress = Sm = (σ_max + σ_min)/2

Stress ratio R = σ_min/σ_max

Fatigue life, at a given alternating stress level and mean stress, is the number of cycles required to cause failure due to fatigue.

Number of cycles where the fatigue strength ceases to decrease. In other words, the number of stress cycles required to cause failure is practically infinite.

The stress at which fatigue failure occurs after a given number of loading cycles.