It has long been recognized that the fatigue life of a component can be divided into a crack initiation phase and a crack propagation phase, but researchers have typically ignored one phase or the other in their analytical models. Even in the stress-life method of life prediction, which implicitly includes both phases, no distinction is made between the initiation and propagation phases. In this paper a methodology for generating initiation and propagation S-N curves will be outlined. It will be shown that for components like gears, both phases represent significant portions of total life. It will also be shown that gear bending fatigue lives for variable amplitude load tests are better predicted by a two-stage linear damage theory compared to the commonly used Miner's total life linear damage theory.
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