Advanced Structural Analysis/Part I - Theory/Failure Modes/Fatigue/Crack Initiation/Loading/Load Spectra/Miner-Palmgren rule

Load histories in real applications are often complex and irregular. The total fatigue effect of such loads can be estimated by the Miner-Palmgren rule, see the equation below. Fatigue failure is assumed to occur when the Miner-Palmgren equation is true.

$D=\sum {\frac {n_{i}}{N_{i}}}$

Where:
$D=$ a constant, often ranging between 0.33 and 3
$n_{i}=$ the number of occurrences of a specific load cycle progression, $i$
$N_{i}=$ the fatigue life of a specific load cycle progression, $i$

Shortcomings of Miner's rule

The Miner-Palmgren rule is rather rough and neglects several important qualities of a general load history. Among these properties are the order and shape of the individual cycles.