過大応力と過小応力が交互に繰返される場合の切欠き試験片の疲労挙動

抄録

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.