Influence of microstructures on fatigue crack initiation and propagation in low-carbon steel

The influence of microstructures on fatigue crack initiation and threshold behavior of crack growth in large fatigue crack was investigated using notched specimens and CT specimens. Three different microstructure of low carbon steels simulated heat affected zone of welds were prepared. The crack initiation behavior for each materials were also discussed from fractographic examination and in situ observation tests in SEM with notched fatigue spesimens. Bainitic microstructures showed higher threshold stress intensity factor renge for fatigue crack growth than ferritic microstructure. The difference in threshold level between microstructures tested would be mainly resulted from the crack closure, but the influence of microstructures on large fatigue crack growth could not be described only in terms of crack closure mechanism. On the other hand, the small crack region which occupied the most of fatigue life was affected by microstructures. Particularly, the effects of microstructure on the shape and growth of Stage I fatigue crack. The Stage I fatigue crack size is almost correspondent with the microstructural unit size, i.e grain size or lath packet size.