— David Wagner 2009/11/30 03:38

A cyclic stress gradually breaks down the structure of the material at the crack tip as it absorbs the energy of plastic deformation. When the material in this small volume becomes weak enough, the next cycle is enough to cause failure and the crack propagates through the weakened material in subsequent cycles until it reaches a location where the material is strong enough to absorb the energy by plastic deformation again. Then this cycle repeats until the crack is long enough, and the remaining ligament narrow enough, that even unweakened material can no longer absorb the energy and catastrophic failure occurs.