Damage Accumulation Model

Palmgren-Miner's linear damage summation rule is used.

Failure is predicted when:(1)(4)

\sum D_{i} = \sum \frac{n_{i}}{N_{i f}} \geq 1.0

Where:

$N_{i f}$ is the material’s fatigue life (number of cycles to failure) from its SN curve at a combination of stress amplitude and means stress level $i$
$n_{i}$ is the number of stress cycles at load level $i$
$D_{i}$ is the cumulative damage under the $n_{i}$ load cycle.

The linear damage summation rule does not take into account the effect of the load sequence on the accumulation of damage due to cyclic fatigue loading. However, it has been proved to work well for many applications.