Damage Progression and Accumulation

As to the damage progression during the application of a train or history of loads of equal amplitude cr, or Si the first hypothesis that comes to mind is that damage is related to the ratio between the number of applied cycles n,- and the allowable cycles Ni at that stress or strain amplitude, known as cycle ratio 

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Fig. 8.17 Linear damage progression and accumulation according to the Palmgren-Miner rule...

The summation is always greater that one. In the more general case of many load spectra or individual cycles of different amplitudes the procedure to assess damage progression and accumulation is basically the same. Figure 8.22 schematizes the most generic situation where different spectra of different stress... 

Fig. 8.20 Power low dependence of damage progression and accumulation for different load amplitudes...

Fig. 8.22 Damage progression and accumulation in a four-block load sequence...

Fig. 8.27 Damage evolution in Stage I and II of fatigue. In Stage I damage progression and accumulation is not linear and depends on stress amplitude. In Stage II damage progression is not linear, but accumulation is linear...

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