LOW-CYCLE FATIGUE CRACK ADVANCE AND LIFE PREDICTION

A new concept for a fatigue process zone within which the actual degradation of the material takes place is proposed. This zone is described as the region in which the stress distribution of the HRR field approaches the maximum flow stress of the material, with the strain localization caused by a sliding-off process. In high-strain low-cycle fatigue conditions, this new concept is shown to be more realistic for the prediction of the fatigue life than that of previous work which has been based on a rough approximation. The proposed feature of the zone is experimentally supported by microhardness measurements. In particular, although this zone is formulated from continuum mechanics, it reflects microstructural factors such as precipitates and stacking-fault energy. Using the developed fatigue process zone and strain intensification in the zone near the crack tip, a modified analytical model for prediction of the continuous low-cycle fatigue life is proposed. The exponent of the Coffin-Manson law obtained from the present prediction is suggested to be 1 /(3n' + 1 ), which is different from the previously reported value of 1 / (2n' + 1 ), and is shown to be in good agreement with experimental results for five alloy systems.