There are many rules to evaluate damage. The most accepted one is Miner´s rule. As a general rule, all the available methods are based on two main facts:
The additional damage produced by a group of cycles with a determined amplitude is a function of the level of this amplitude and the application sequence of the previous ones.
The additional damage produced by every solicitation level is accumulated to get one critical value that determines the crack initiation.
The Palmgren-Miner´s low (1945), says:
"If over a component are applied cycles of different amplitudes B1, B2,.....Bi,......Bm, where Bi can be the applied force, the nominal stress, the strain in the notch,..., then the additional damage produced by every amplitude level is equal to the relation between the number of cycles applied for this level, ni, and the number of cycles until fail, Ni, if this solicitation level was to remain constant throughout the component life."