Fatigue load is defined by the stress range, number of stress cycles, the shape of the stress distribution, stress ratio, and the average stress. To define fatigue load, it is important to consider the time duration of the load and the minimum and maximum stress values. The stress range is the difference between the maximum stress and the minimum stress, and the stress amplitude is half of the stress range. The average stress is the mean value of the maximum stress and the minimum stress. The stress ratio, also called the R-value, is the minimum stress over the maximum stress. The shape of the stress distribution, which varies depending on loading conditions, is another important consideration in defining fatigue load.
σmax = maximum stress level during a load cycle
σmin = minimum stress level
σm = mean stress level
σa = stress amplitude
Δσ = stress range
σmin = minimum stress level
R = σmin/ σmax stress ratio
R < alternating load
R ≥ pulsating load