On the crystallographic, stage I-like, character of fine granular area formation in internal fish-eye fatigue cracks

- A constant plastic zone size was found at the end of the FGA for different temperatures.
- Different FGA diameters were found for different temperatures and the same max. stress.
- Different KFGA values were found for the different temperatures.
- The characteristic plastic zone corresponds to martensitic lath dimensions.
- FGA formation is similar to a stage I-like crystallographic crack propagation regime.

The crystallographic character of fine granular area (FGA) formation from internal particles in martensitic high strength steel has been revealed by an assessment of the plastic zone size at the FGA border. This plastic zone size corresponded to about 3 times the martensite lath width. Tests at different temperatures (20 °C, 200 °C and 400 °C) revealed a decreasing FGA size with increasing temperature at constant applied stress amplitude. As a consequence, the critical stress intensity factor varied as the FGA decreased with temperature. In contrast, the critical plastic zone size remained constant and equal to the sizes of microstructural features. This represents a strong similarity between crystallographic, stage I-like, crack propagation and FGA formation in a vacuum.

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