Abstract
Two step interval tests have been carried out on notched specimens of four kinds of metals under rotating and flexural bending conditions. n1 cycles of overstress σ1 and n2 cycles of understress σ2 were alternately applied.
Both the crack initiation and final fracture lives were relatively longer compared with the fracture life of the unnotched specimen when n2 was larger than n1, though the lives become shorter as n2 decreases. In some cases specimens failed much earlier than expected from the Corten-Dolan modification line for a fairly small value of n2, and the regression line for the test data curved downward. The longer life of the notched specimen for a larger value of n2 seems due to retardation of crack initiation and propagation brought about by the compressive residual stress induced at the notch root and the crack tip after the stress amplitude was changed from σ1 to σ2.
For a material in which repeated plastic strain range is rather small, retardation of crack initiation and propagation was not observed. This seems due to small compressive residual stress produced at the time of stress change from σ1 to σ2.
