Abstract
To clarify the interaction effect of stress changing on the crack propagation behavior, the fatigue tests of which stress sequence had high to low and low to high were carried out on high strength steel JIS SNCM439. The results obtained were as follows:
1) The acceleration behavior of crack propagation rate was observed just after changing the stress level from low to high, and the crack propagation rate became two times of that under the constant loading. This acceleration behavior continued for 500-1000 cycles of high stress loading.
2) The retardaton behavior of crack propagation rate was observed just after changing the stress level to low from high. The number of cycles of low stress loading needed for the crack propagation rate to return that of constant loading was about 10000-70000.
3) It was difficult to elucidate the acceleration and retardation behaviors only by the change of crack opening stress level.
4) It was necessary to consider the accerelation effect of crack propagation rate for the estimation of fatigue life under service loading conditions when a higher stress is intermittently loaded.
