EFFECT OF DISLOCATION SUBSTRUCTURE OF CRACK-TIP ON NEAR FATIGUE THRESHOLD IN DUAL-PHASE STEELS

The effects of stabilized subgrain cell and wall structures on the nea r-threshold of physical short cracks in dual-phase steels have been fo und for the first time in the work reported in this paper. These effec ts result from both the larger number of cycles in the original positi on and the higher stress-strain field of the crack tip. For the near-t hreshold of long cracks, there was a tendency for dislocation cells to form. These behaviours illustrate that the dislocation morphologies o f crack tips are the products of strain history. It is proposed for th e first time in this paper that threshold values computed by the subgr ain model should be related to effective fatigue threshold values: the dislocation cell is one of the intrinsic toughening factors and also has an effect on the microstructure.