|When you usually perform rotating bending fatigue test, the specimen is
fractured due to the crack occurred on the material surface.
But, in the giga-cycle regime more than 108 times, the specimen sometimes
fractures because “Fish-eye” which is generated inside of material (refer
to a figure below) causes progress of the crack.
Though there are still many unexplained facts regarding how and when “Fish-eye”
is generated, “Fish-eye” seems to be a factor which declines fatigue limit
further that is estimated by experiment result to 107 times.