On the Micromechanism of Fatigue Damage in an Interstitial-Free Steel Sheet

The micromechanism of fatigue damage in an interstitial-free (IF) steel sheet has been studied using fully reversed stress amplitudes (Δσ/2). The stress-life (S-N) curve of the steel sheet has been generated, together with a series of interrupted fatigue tests at each of the chosen Δσ/2, to study the progress of fatigue damage in terms of the initiation, growth, and coalescence of the fatigue cracks on the surfaces of the sheet specimens using scanning electron microscopy. The steel sheet possesses a higher endurance limit (0.98 times its yield strength (YS)), as compared to conventional low-carbon steel sheets. This is attributed to (1) the occurrence of nonpropagating microcracks initiating primarily at the inclusions below the endurance limit and (2) a significant delay in the spread of plastic deformation, until Δσ/2 is close to YS. Above the endurance limit, widespread plastic deformation through slip bands promotes the formation of fatigue cracks at the ferrite grain boundaries and occasionally within a ferrite grain body, as well as at inclusions. Fatigue failure is preceded by the significant growth of grain-boundary cracks over and above those at inclusions and the ferrite grain body. A series of grain-boundary cracks link up to form mesocracks, one of which grows to cause the final failure. The predominance of grain-boundary cracks in the process of fatigue failure is attributed to the lesser cohesive strength of the grain boundaries caused by the depletion of interstitials.