Abstract
Six kinds of metal (three ferrous and three non-ferrous) were tested under rotating and flexural bendings by applying n1 cycles of an overstress and n2 cycles of an understress alternately, to investigate the role of understress in fatigue damage accumulation.
Fatigue life calculation for such a case gives nonconservative side estimation when the original S-N curve determined by the constant stress amplitude tests is used. This is because an understress, when preceded by an overstress, contributes to damage, which otherwise does not cause damage.
The original S-N curve is to be modified to make an estimate coincide well with the test results. In case of ferrous metals downward extension of the inclined part of the original S-N curve fits the test data well so long as the value of n2 is large compared to n1. A steeper slope, however, seems necessary for a modified S-N curve as n2 becomes smaller.
As for non-ferrous metals downward translation of the original S-N curve, by keeping its shape and slope, leads to good agreement with test data.
The meaning of S-N curve modification is interpreted by the combination of increased damage accumulation brought about with the increased plastic strain range accompanying stress change and the counteracting phenomenon such as plastic strain concentration in crystals or the strengthening effect like strain aging especially under low stress amplitudes, which interferes with linear accumulation of fatigue damage at the higher and the lower stress amplitudes.
