Definitions of Fatigue Quantities
In addition to the stress amplitude, the mean stress is also important when fatigue cracks appear. A tensile mean stress decreases the fatigue life, while a compressive stress increases it. If σmax is the maximum stress over the cycle and σmin is the minimum stress, the following definitions are used:
Stress amplitude:
Stress range:
Mean stress:
R-value:
The R-value is the most commonly used parameter to describe the mean stress level.
The most common fatigue test is where the loading is fully reversed. The mean stress is then zero and R = −1. The second fundamental test is the pulsating test, where the load varies between zero and a maximum value, giving R = 0.
Figure 3-3: Example of cyclic loading.
Fatigue data, like endurance limits and S-N curves can be defined either in terms of the stress amplitude or the maximum stress. Within COMSOL, it is always the amplitude which is used.
Cycle Cutoff
One of the simplest methods for fatigue evaluation is a relation between a fatigue controlling quantity, ξ, and the fatigue life, N, see Figure 3-4. This relation is the fatigue life curve. In most applications the fatigue controlling quantity is a stress amplitude or a strain amplitude.
Figure 3-4: Fatigue life curve
In COMSOL the fatigue life curve is used in the following models: Stress-Life Models, Strain-Life Models, Strain-Based Fatigue Models, Cumulative Damage Model, and Vibration Fatigue Model. The fatigue life curve goes asymptotically toward infinite life for low values of the fatigue controlling quantity. The objective of a fatigue analysis is to find the critical life that is found for the largest value of the fatigue controlling quantity. Since the spread in the results can be very large, a cycle cutoff, Ncut, controls how far on the life curve results are evaluated. When the fatigue controlling quantity is low, ξ < ξcut, the fatigue life is set to the cycle cutoff, N = Ncut.